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Sample search results for: zero-age main sequence

 

1

Theoretical Zero Age Main Sequences revisited

  Astrophysics (arXiv)

Summary: Zero Age Main Sequence (ZAMS) models with updated physical inputs are presented for selected assumptions about the chemical composition, covering the ranges 0.6 < M/Mo < 1.2, 0.0001 < Z < 0.04, 0.23 < Y < 0.34.The HR diagram location of the ZAMS as a function of Y and Z is discussed both in the theoretical and in the observational HR diagrams, showing that the V magnitude presents an increased dependence on Z to be taken into account when discussing observational evidences. Analytical relations quantifying both these dependences are derived. Implications for the galactic helium to heavier elements enrichment are finally discussed.

V. Castellani; S. Degl' Innocenti; M. Marconi
1999-08-26

2

The Zero Age Main Sequence of WIMP burners

  HEP - Phenomenology (arXiv)

Summary: We modify a stellar structure code to estimate the effect upon the main sequence of the accretion of weakly interacting dark matter onto stars and its subsequent annihilation. The effect upon the stars depends upon whether the energy generation rate from dark matter annihilation is large enough to shut off the nuclear burning in the star. Main sequence WIMP burners look much like protostars moving on the Hayashi track, although they are in principle completely stable. We make some brief comments about where such stars could be found, how they might be observed and more detailed simulations which are currently in progress. Finally we comment on whether or not it is possible to link the paradoxically young OB stars found at the galactic centre with WIMP burners.

Malcolm Fairbairn; Pat Scott; Joakim Edsjo
2008-03-03

3

Optically observable zero-age main-sequence O stars

  Astrophysics (arXiv)

Summary: A list of 50 optically observable O stars that are likely on or very near the ZAMS is presented. They have been selected on the basis of five distinct criteria, although some of them exhibit more than one. Three of the criteria are spectroscopic (He II \\lambda4686 absorption stronger than in normal luminosity class V spectra, abnormally broad or strong Balmer lines, weak UV wind profiles for their spectral types), one is environmental (association with dense, dusty nebular knots), and one is photometric (derived absolute magnitudes fainter than class V). Very few of these stars have been physically analyzed, and they have not been considered in the current framework of early massive stellar evolution. In particular, they may indicate that the earliest, embedded phases are not as large a fraction of the main-sequence lifetimes as is currently believed. Detailed analyses of these objects will likely prove essential to a complete understanding of the early evolution of massive stars.

Nolan R. Walborn
2007-02-15

4

Core-halo Structure of Chemically Homogeneous Massive Star and Bending of Zero-Age Main-Sequence

  Astrophysics (arXiv)

Summary: We have recalculated interior structure of very massive stars of uniform chemical composition with the OPAL opacity. Very massive stars are found to develop a core-halo structure with a extended radiative-envelope. With the core-halo structure, a more massive star has a more extended envelope, then the track of upper zero-age main-sequence (ZAMS) curves redward in the H-R diagram at > 100 M_sol (Z=0.02), >70 M_sol (Z=0.05), and > 15 M_sol for helium ZAMS (X=0., Z=0.02). Therefore, effective temperature of a very massive ZAMS star is rather low: e.g., for the 200 M_sol star, log T_eff=4.75 (Z=0.004), 4.60 (Z=0.02), 4.46 (Z=0.05), and 4.32 (Z=0.10). Effective temperatures of very luminous stars (> 120 M_sol) found in the LMC, the SMC, and the Galaxy are discussed in relation with this metal dependence of curving upper main-sequence.

Mie Ishii; Mariko Kato; Munetaka Ueno
1999-07-13

5

Adiabatic Mass Loss in Binary Stars. II. From Zero-Age Main Sequence to the Base of the Giant Branch

  CERN Preprints

Summary: In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We survey here adiabatic mass loss from Population I stars of mass 0.10 Msun to 100 Msun from the zero age main sequence to the base of the giant branch, or to central hydrogen exhaustion for lower main sequence stars. For intermediate- and high-mass stars, dynamical mass transfer is preceded by an extended phase of thermal time scale mass transfer as the star is stripped of most of its envelope mass. The critical mass ratio qad above which this delayed dynamical instability occurs increases with advancing evolutionary age of the donor star, by ever-increasing factors for more massive donors. Most intermediate- or high-mass binaries with nondegenerate accretors probably evolve into contact before manifesting this instability. As they approach the base of the giant branch, however, and begin developing a convective envelope, qad plummets dramatically among inte...

Ge, Hongwei; Chen, Xuefei; Han, Zhanwen
2015-01-01

6

Evaluating Gyrochronology on the Zero-Age-Main-Sequence: Rotation Periods in the Southern Open Cluster Blanco 1 from the KELT-South Survey

  CERN Preprints

Summary: We report periods for 33 members of Blanco 1 as measured from KELT-South light curves, the first reported rotation periods for this benchmark zero-age-main-sequence open cluster. The distribution of these stars spans from late-A or early-F dwarfs to mid-K with periods ranging from less than a day to ~8 days. The rotation period distribution has a morphology similar to the coeval Pleiades cluster, suggesting the universal nature of stellar rotation distributions. Employing two different gyrochronology methods, we find an age of 146+13-14 Myr for the cluster. Using the same techniques, we infer an age of 134+9-10 Myr for the Pleiades measured from existing literature rotation periods. These rotation-derived ages agree with independently determined cluster ages based on the lithium depletion boundary technique. Additionally, we evaluate different gyrochronology models, and quantify levels of agreement between the models and the Blanco 1/Pleiades rotation period distributions, including incorporating the rotation...

Cargile, P A; Pepper, J; Kuhn, R B; Siverd, R; Stassun, K G
2013-01-01

7

The evolution of massive stars and their spectra I. A non-rotating 60 Msun star from the zero-age main sequence to the pre-supernova stage

  CERN Preprints

Summary: For the first time, the interior and spectroscopic evolution of a massive star is analyzed from the zero-age main sequence (ZAMS) to the pre-supernova (SN) stage. For this purpose, we combined stellar evolution models using the Geneva code and atmospheric models using CMFGEN. With our approach, we were able to produce observables, such as a synthetic high-resolution spectrum and photometry, aiding the comparison between evolution models and observed data. Here we analyze the evolution of a non-rotating 60 Msun star and its spectrum throughout its lifetime. Interestingly, the star has a supergiant appearance (luminosity class I) even at the ZAMS. We find the following evolutionary sequence of spectral types: O3 I (at the ZAMS), O4 I (middle of the H-core burning phase), B supergiant (BSG), B hypergiant (BHG), hot luminous blue variable (LBV; end of H-core burning), cool LBV (H-shell burning through the beginning of the He-core burning phase), rapid evolution through late WN and early WN, early WC (middle of He...

Groh, Jose; Ekstrom, Sylvia; Georgy, Cyril
2014-01-01

8

The Blazar Main Sequence

  Astrophysics (arXiv)

Summary: We propose a sequence (the Blazar main sequence, BMS) that links the two main components of the Blazar class, namely, the Flat Spectrum Radio Quasars and the BL Lacertae objects, and yields all their distinctive features in a correlated way. In this view, both type of sources are centered on a supermassive Kerr hole close to maximal spin that produces a relativistic jet observed close to the axis, and on the associated accretion disk that emits more isotropically. But the FSRQs are energized by accretion at rates dm/dt ~ 1, and are dominated by the disk components (thermal and electrodynamic jet-like component) which provide outputs in excess of L ~ 10^46 erg/s. On the other hand, accretion levels dm/dt << 1 are enough to energize the BL Lacs; here the radiation is highly non-thermal and the power partly contributed by the rotational energy of the central Kerr hole, with the latter and the disk together sustaining L ~ 10^44 erg/s in the jet frame for several Gyrs. If so, we expect the BL Lacs to show quite different evolutionary signatures from the FSRQs, and in particular, number counts close to the Euclidean shape, or flatter if the sources make a transition to the BL Lac from a FSRQ mode. In addition, for lower dm/dt along the BMS, we expect the large scale electric fields to be less screened out, and to accelerate fewer particles to higher energies with non-thermal radiations at higher frequencies; so in moving from FSRQs to BL Lacs these non-thermal radiations will peak at frequencies inversely correlated with the disk output. For the BL Lacs such dependence implies increased scatter when one tries a correlation with the total ouput. At its endpoint, the BMS suggests widespread objects that are radiatively silent, but still efficient in accelerating cosmic rays to ultra high energies.

A. Cavaliere; V. D'Elia
2002-02-04

9

The Effect of Pre-Main Sequence Stars on Star Cluster Dynamics

  Astrophysics (arXiv)

Summary: We investigate the effects of the addition of pre-main sequence evolution to star cluster simulations. We allowed stars to follow pre-main sequence tracks that begin at the deuterium burning birthline and end at the zero age main sequence. We compared our simulations to ones in which the stars began their lives at the zero age main sequence, and also investigated the effects of particular choices for initial binary orbital parameters. We find that the inclusion of the pre-main sequence phase results in a slightly higher core concentration, lower binary fraction, and fewer hard binary systems. In general, the global properties of star clusters remain almost unchanged, but the properties of the binary star population in the cluster can be dramatically modified by the correct treatment of the pre-main sequence stage.

R. Wiersma; A. Sills; S. Portegies Zwart
2005-10-07

10

X-ray flares on zero-age- and pre-main sequence stars in Taurus-Auriga-Perseus

  Astrophysics (arXiv)

Summary: We present results of a systematic search for X-ray flares on young stars observed during ROSAT PSPC observations of the Taurus-Auriga-Perseus region. All pointed PSPC observations currently available from the ROSAT Public Data Archive with known T Tauri stars, Pleiads, or Hyads in the field are analyzed in order to study the evolution of coronal activity. 38 large flares can be attributed to late-type stars. A flare rate is derived for the different stellar samples making use of the flare duration and the observing time. We investigate the influence of stellar parameters such as age, rotation and multiplicity on individual flare parameters and the flare frequency.

B. Stelzer; R. Neuhaeuser; V. Hambaryan
2000-02-17

11

Age-Related Observations of Low Mass Pre-Main and Young Main Sequence Stars (Invited Review)

  Astrophysics (arXiv)

Summary: This overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsini and activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.

Lynne A. Hillenbrand
2008-12-06

12

6.1 THE STRUCTURE OF MAIN-SEQUENCE STARS (ZG: 16.2; CO 10.6, 13.1)

  Physics Websites

Summary: core burning phase zero-age main sequence (ZAMS): homogeneous composition Scaling relations for main.5 · mass­radius relationship central temperature determined by characteristic nuclear-burning temperature tempera- ture to produce the same pressure, but Tc is fixed (hydrogen burning (thermostat): Tc 107 K

Bureau, Martin

13

Pre-Main Sequence models for low-mass stars and brown dwarfs

  Astrophysics (arXiv)

Summary: We present evolutionary models for low mass stars and brown dwarfs ($m \\le 1.2 \\msol$) based on recent improvement of the theory: equation of state, atmosphere models, ... We concentrate on early evolutionary phases from the initial deuterium burning phase to the zero-age Main Sequence. Evolutionary models for young brown dwarfs are also presented. We discuss the uncertainties of the present models. We analyse the difficulties arising when comparing models with observations for very young objects, in particular concerning the problem of reddening.

I. Baraffe; G. Chabrier; F. Allard; P. Hauschildt
2000-07-12

14

The double main sequence of Omega Centauri

  CERN Preprints

Summary: Recent, high precision photometry of Omega Centauri, the biggest Galactic globular cluster, has been obtained with Hubble Space Telescope. The color magnitude diagram reveals an unexpected bifurcation of colors in the main sequence (MS). The newly found double MS, the multiple turnoffs and subgiant branches, and other sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. Among the possible explanations for the blue main sequence an anomalous overabundance of helium is suggested. The hypothesis will be tested with a set of FLAMES@VLT data we have recently obtained (ESO DDT program), and with forthcoming ACS@HST images.

Bedin, L R
2004-01-01

15

The double main sequence of Omega Centauri

  Astrophysics (arXiv)

Summary: Recent, high precision photometry of Omega Centauri, the biggest Galactic globular cluster, has been obtained with Hubble Space Telescope. The color magnitude diagram reveals an unexpected bifurcation of colors in the main sequence (MS). The newly found double MS, the multiple turnoffs and subgiant branches, and other sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. Among the possible explanations for the blue main sequence an anomalous overabundance of helium is suggested. The hypothesis will be tested with a set of FLAMES@VLT data we have recently obtained (ESO DDT program), and with forthcoming ACS@HST images.

L. R. Bedin; G. Piotto; J. Anderson; I. R. King; S. Cassisi; Y. Momany; .
2004-06-03

16

Blue Stragglers After the Main Sequence

  Astrophysics (arXiv)

Summary: We study the post-main sequence evolution of products of collisions between main sequence stars (blue stragglers), with particular interest paid to the horizontal branch and asymptotic giant branch phases. We found that the blue straggler progeny populate the colour-magnitude diagram slightly blueward of the red giant branch and between 0.2 and 1 magnitudes brighter than the horizontal branch. We also found that the lifetimes of collision products on the horizontal branch is consistent with the numbers of so-called "evolved blue straggler stars" (E-BSS) identified by various authors in a number of globular clusters, and is almost independent of mass or initial composition profile. The observed ratio of the number of E-BSS to blue stragglers points to a main sequence lifetime for blue stragglers of approximately 1-2 Gyr on average.

Alison Sills; Amanda Karakas; John Lattanzio
2008-11-18

17

Nitrogen chronology of massive main sequence stars

  CERN Preprints

Summary: Rotational mixing in massive main sequence stars is predicted to monotonically increase their surface nitrogen abundance with time. We use this effect to design a method for constraining the age and the inclination angle of massive main sequence stars, given their observed luminosity, effective temperature, projected rotational velocity and surface nitrogen abundance. This method relies on stellar evolution models for different metallicities, masses and rotation rates. We use the population synthesis code STARMAKER to show the range of applicability of our method. We apply this method to 79 early B-type main sequence stars near the LMC clusters NGC 2004 and N 11 and the SMC clusters NGC 330 and NGC 346. From all stars within the sample, 17 were found to be suitable for an age analysis. For ten of them, which are rapidly rotating stars without a strong nitrogen enhancement, it has been previously concluded that they did not evolve as rotationally mixed single stars. This is confirmed by our analysis, which fla...

Köhler, K; Brott, I; Langer, N; de Koter, A
2012-01-01

18

Differential rotation on the lower main sequence

  Astrophysics (arXiv)

Summary: We compute the differential rotation of main sequence stars of the spectral types F, G, K, and M by solving the equation of motion and the equation of convective heat transport in a mean-field formulation. For each spectral type the rotation rate is varied to study the dependence of the surface shear on this parameter. The resulting rotation patterns are all solar-type. The horizontal shear turns out to depend strongly on the effective temperature and only weakly on the rotation rate. The meridional flow depends more strongly on the rotation rate and has different directions in the cases of very slow and very fast rotation, respectively.

M. Kueker; G. Ruediger
2005-04-19

19

Debris disks in main sequence binary systems

  CERN Preprints

Summary: We observed 69 A3-F8 main sequence binary star systems using the Multiband Imaging Photometer for Spitzer onboard the Spitzer Space Telescope. We find emission significantly in excess of predicted photospheric flux levels for 9(+4/-3)% and 40(+7/-6)% of these systems at 24 and 70 microns, respectively. Twenty two systems total have excess emission, including four systems that show excess emission at both wavelengths. A very large fraction (nearly 60%) of observed binary systems with small (<3 AU) separations have excess thermal mission. We interpret the observed infrared excesses as thermal emission from dust produced by collisions in planetesimal belts. The incidence of debris disks around main sequence A3-F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to a...

Trilling, D E; Stapelfeldt, K R; Rieke, G H; Su, K Y L; Gray, R O; Corbally, C J; Bryden, G; Chen, C H; Boden, A; Beichman, C A
2006-01-01

20

Differential rotation of main sequence F stars

  Astrophysics (arXiv)

Summary: The differential rotation of a 1.2 $M_\\odot$ zero age MS star (spectral type F8) is computed and the results are compared with those from a similar model of the Sun. The rotation pattern is determined by solving the Reynolds equation including the convective energy transport. The latter is anisotropic due to the Coriolis force causing a horizontal temperature gradient of ~ 7 K between the poles and the equator. Comparison of the transport mechanisms of angular momentum (the eddy viscosity, the $\\Lambda$-effect and the meridional flow) shows that for the F star the $\\Lambda$-effect is the most powerful transporter for rotation periods of 7 d or less. In the limit of very fast rotation the $\\Lambda$-effect is balanced by the meridional flow alone and the rotation is nearly rigid. The rotation pattern found for the F star is very similar to the solar rotation law, but the horizontal shear is about twice the solar value. As a function of the rotation period, the total equator-pole difference of the angular velocity has a (slight) maximum at a period of 7 d and (slowly) vanishes in both the limiting cases of very fast and very slow rotation. A comparison of the solar models with those for the F-type star shows a much stronger dependence of the differential surface rotation on the stellar luminosity rather than on the rotation rate.

M. Kueker; G. Ruediger
2004-09-10

21

Main sequence stars with asymmetric dark matter

  HEP - Phenomenology (arXiv)

Summary: We study the effects of feebly or non-annihilating weakly interacting Dark Matter (DM) particles on stars that live in DM environments denser than that of our Sun. We find that the energy transport mechanism induced by DM particles can produce unusual conditions in the core of Main Sequence stars, with effects which can potentially be used to probe DM properties. We find that solar mass stars placed in DM densities of rhochi>= e2 GeV/cm3 are sensitive to Spin-Dependent scattering cross-section sigmsd >= e-37 cm2 and a DM particle mass as low as mchi=5 GeV, accessing a parameter range weakly constrained by current direct detection experiments.

Fabio Iocco; Marco Taoso; Florent Leclercq; Georges Meynet
2012-01-25

22

Main Sequence Evolution with Layered Semiconvection

  CERN Preprints

Summary: Semiconvection - mixing that occurs in regions that are stable when considering compositional gradients, but unstable when ignoring them - is shown to have the greatest potential impact on main sequence stars with masses in the range 1.2 - 1.7 solar masses. We present the first stellar evolution calculations using a prescription for semiconvection derived from extrapolation of direct numerical simulations of double-diffusive mixing down to stellar parameters. The dominant mode of semiconvection in stars is layered semiconvection, where the layer height is an adjustable parameter analogous to the mixing length in convection. The rate of mixing across the semiconvective region is sensitively dependent on the layer height. We find that there is a critical layer height that separates weak semiconvective mixing (where evolution is well-approximated by using the Ledoux criterion) from strong semiconvective mixing (where evolution is well-approximated by using the Schwarzschild criterion). This critical layer height...

Moore, Kevin
2015-01-01

23

Debris disks in main sequence binary systems

  Astrophysics (arXiv)

Summary: We observed 69 A3-F8 main sequence binary star systems using the Multiband Imaging Photometer for Spitzer onboard the Spitzer Space Telescope. We find emission significantly in excess of predicted photospheric flux levels for 9(+4/-3)% and 40(+7/-6)% of these systems at 24 and 70 microns, respectively. Twenty two systems total have excess emission, including four systems that show excess emission at both wavelengths. A very large fraction (nearly 60%) of observed binary systems with small (<3 AU) separations have excess thermal mission. We interpret the observed infrared excesses as thermal emission from dust produced by collisions in planetesimal belts. The incidence of debris disks around main sequence A3-F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to any great degree. We model these dust disks through fitting the spectral energy distributions and derive typical dust temperatures in the range 100--200 K and typical fractional luminosities around 10^-5, with both parameters similar to other Spitzer-discovered debris disks. Our calculated dust temperatures suggest that about half the excesses we observe are derived from circumbinary planetesimal belts and around one third of the excesses clearly suggest circumstellar material. Three systems with excesses have dust in dynamically unstable regions, and we discuss possible scenarios for the origin of this short-lived dust.

D. E. Trilling; J. A. Stansberry; K. R. Stapelfeldt; G. H. Rieke; K. Y. L. Su; R. O. Gray; C. J. Corbally; G. Bryden; C. H. Chen; A. Boden; C. A. Beichman
2006-12-01

24

VINCI / VLTI observations of Main Sequence stars

  Astrophysics (arXiv)

Summary: Main Sequence (MS) stars are by far the most numerous class in the Universe. They are often somewhat neglected as they are relatively quiet objects (but exceptions exist), though they bear testimony of the past and future of our Sun. An important characteristic of the MS stars, particularly the solar-type ones, is that they host the large majority of the known extrasolar planets. Moreover, at the bottom of the MS, the red M dwarfs pave the way to understanding the physics of brown dwarfs and giant planets. We have measured very precise angular diameters from recent VINCI/VLTI interferometric observations of a number of MS stars in the K band, with spectral types between A1V and M5.5V. They already cover a wide range of effective temperatures and radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as well as the asteroseismic information available for some of these stars, the angular diameters put strong constraints on the detailed models of these stars, and therefore on the physical processes at play.

P. Kervella; F. Thevenin; P. Morel; J. Provost; G. Berthomieu; D. Segransan; D. Queloz; P. Borde; E. Di Folco; T. Forveille
2003-09-29

25

Post-Main-Sequence Evolution 16 November 2011

  Geosciences Websites

Summary: Post-Main-Sequence Evolution 16 November 2011 Goals · Review the post-main-sequence evolution of a 1 M star · Review the Chandrasehkar mass Post-Main-Sequence Evolution 1. We're going to do the same HR diagram exercise as before, except now we're going to include the phases of stellar evolution

Militzer, Burkhard

26

Simulations of Stellar Collisions Involving Pre-Main Sequence Stars

  Astrophysics (arXiv)

Summary: In this paper, we present the results of smoothed particle hydrodynamic (SPH) simulations of collisions between pre-main sequence stars and a variety of other kinds of stars. Simulations over a range of impact parameters and velocities were performed. We find that pre-main sequence stars tend to ``wrap themselves'' around their impactor. We discuss the probable evolutionary state of products of collisions between pre-main sequence stars and pre-main sequence, main sequence, giant branch, and compact stars. The nature of the collision product does not depend strongly on the impact parameter or the velocity of the collision.

Daniel Laycock; Alison Sills
2005-03-21

27

Pulsating pre-main sequence stars in IC 4996 and NGC 6530

  Astrophysics (arXiv)

Summary: Asteroseismology of pulsating pre-main sequence (PMS) stars has the potential of testing the validity of current models of PMS structure and evolution. As a first step, a sufficiently large sample of pulsating PMS stars has to be established, which allows to select candidates optimally suited for a detailed asteroseismological analysis based on photometry from space or ground based network data. A search for pulsating PMS members in the young open clusters IC 4996 and NGC 6530 has been performed to improve the sample of known PMS pulsators. As both clusters are younger than 10 million years, all members with spectral types later than A0 have not reached the zero-age main sequence yet. Hence, IC 4996 and NGC 6530 are most suitable to search for PMS pulsation among their A- and F-type cluster stars. CCD time series photometry in Johnson B and V filters has been obtained for IC 4996 and NGC 6530. The resulting light curves for 113 stars in IC 4996 and 194 stars in NGC 6530 have been subject to detailed frequency analyses. 2 delta Scuti-like PMS stars have been discovered in IC 4996 and 6 in NGC 6530. For another PMS star in each cluster, pulsation can only be suspected. According to the computed pulsation constants, the newly detected PMS stars seem to prefer to pulsate in a similar fashion to the classical delta Scuti stars, and with higher overtone modes.

Konstanze Zwintz; Werner W. Weiss
2006-07-26

28

The Main Sequence of Star Clusters

  Astrophysics (arXiv)

Summary: A novel way of looking at the evolution of star clusters is presented. With a dynamical temperature, given by the mean kinetic energy of the cluster stars, and a dynamical luminosity, which is defined as the kinetic energy of the stars leaving the cluster in analogy to the energy of photons emitted by a star, the dissolution of star clusters is studied using a new dynamical temperature-luminosity diagram for star clusters. The investigation contains a parameter-space study of open clusters of up to N = 32768 single-mass stars with different initial density distributions, half-mass radii, tidal conditions and binary fractions. The clusters show a strong correlation between dynamical temperature and dynamical luminosity and most of the investigated cluster families share a common sequence in such a dynamical temperature-luminosity diagram. Deviations from this sequence are analyzed and discussed. After core collapse, the position of a cluster within this diagram can be defined by three parameters: the mass, the tidal conditions and the binary fraction. Due to core collapse all initial conditions are lost and the remaining stars adjust to the given tidal conditions. Binaries as internal energy sources influence this adjustment. A further finding concerns the Lagrange radii of star clusters: Throughout the investigated parameter space nearly all clusters show a constant half-mass radius for the time after core collapse until dissolution. Furthermore, the ratio of half-mass radius to tidal radius evolves onto a common sequence which only depends on the mass left in the cluster.

Andreas H. W. Kuepper; Pavel Kroupa; Holger Baumgardt
2008-06-24

29

Older and Colder: The impact of starspots on pre-main sequence stellar evolution

  CERN Preprints

Summary: We assess the impact of starspots on the evolution of late-type stars during the pre-main sequence (pre-MS) using a modified stellar evolution code. We find that heavily spotted models of mass 0.1-1.2\\msun\\ are inflated by up to $10$% during the pre-MS, and up to 4% and 9% for fully- and partially-convective stars at the zero-age MS, consistent with measurements from active eclipsing binary systems. Spots similarly decrease stellar luminosity and $T_{\\rm eff}$, causing isochrone-derived masses to be under-estimated by up to a factor of $2 \\times$, and ages to be under-estimated by a factor of 2-10$\\times$, at 3 Myr. Consequently, pre-MS clusters and their active stars are systematically older and more massive than often reported. Cluster ages derived with the lithium depletion boundary technique are erroneously young by $\\sim 15$% and $10$% at $30$ and $100$ Myr respectively, if 50% spotted stars are interpreted with un-spotted models. Finally, lithium depletion is suppressed in spotted stars with radiative c...

Somers, Garrett
2015-01-01

30

The Lithium Depletion Boundary in NGC 2547 as a test of pre-main-sequence evolutionary models

  Astrophysics (arXiv)

Summary: Intermediate resolution spectroscopy from the ESO Very Large Telescope is analysed for 63 photometrically selected low-mass (0.08-0.30 Msun) candidates of the open cluster NGC 2547. We have confirmed membership for most of these stars using radial velocities, and found that lithium remains undepleted for cluster stars with I>17.54+/-0.14 and K_{s}>14.86+/-0.12. From these results, several pre-main-sequence evolutionary models give almost model independent ages of 34-36 Myr, with a precision of 10 per cent. These ages are only slightly larger than the ages of 25-35 (+/-5) Myr obtained using the same models to fit isochrones to higher mass stars descending towards the zero age main sequence (ZAMS), both in empirically calibrated and theoretical colour-magnitude diagrams. This agreement between age determinations in different mass ranges is an excellent test of the current generation of low-mass pre-main sequence stellar models and lends confidence to ages determined with either method between 30 and 120 Myr.

R. D. Jeffries; J. M. Oliveira
2004-11-04

31

CN abundance variations on the main sequence of 47 Tuc

  Astrophysics (arXiv)

Summary: We report on a deep spectroscopic survey for star-to-star CN variations along the main sequence (MS) of the globular cluster 47 Tuc with ESO's VLT. We find a significant bimodal distribution in the S(3839) index for main-sequence stars in the mass range of ~0.85 to 0.65 M_sun, or from the main-sequence turn-off down to ~2.5 mag below the main sequence turn-off. An anti-correlation of CN and CH is evident on the MS. The result is discussed in the context of the ability of faint MS stars to alter their surface composition through internal evolutionary effects. We argue against internal stellar evolution as the only origin for the abundance spread in 47 Tuc; an external origin such as pollution seems to be more likely.

Daniel Harbeck; Graeme H. Smith; Eva K. Grebel
2002-10-16

32

Lithium Depletion in Fully Convective Pre-Main Sequence Stars

  CERN Preprints

Summary: We present an analytic calculation of the thermonuclear depletion of lithium in contracting, fully convective, pre-main sequence stars of mass M 0.08 M_sun) and for constraining the masses of lithium depleted stars.

Bildsten, L; Matzner, C D; Ushomirsky, G; Bildsten, Lars; Brown, Edward F.; Matzner, Christopher D.; Ushomirsky, Greg
1996-01-01

33

Structure and Evolution of Pre-Main Sequence Stars

  MIT - DSpace

Summary: Low-mass pre-main sequence (PMS) stars are strong and variable X-ray emitters, as has been well established by EINSTEIN and ROSAT observatories. It was originally believed that this emission was of thermal nature and ...

Schulz, Norbert S.
2009-01-01

34

Main Sequence Masses and Radii from Gravitational Redshifts

  Astrophysics (arXiv)

Summary: Modern instrumentation makes it possible to measure the mass to radius ratio for main sequence stars in open clusters from gravitational redshifts. For stars where independent information is available for either the mass or the radius, this application of general relativity directly determines the other quantity. Applicable examples are: 1) measuring the radii of solar metallicity main sequence stars for which the mass - luminosity relation is well known, 2) measuring the radii for stars where model atmospheres can be used to determine the surface gravity (the mass to radius squared ratio), 3) refining the mass - radius relation for main sequence stars, and 4) measuring the change in radius as stars evolve off the main sequence and up the giant branch.

Ted von Hippel
1995-12-02

35

Preface: Planetary Systems Beyond the Main Sequence 2010

  CERN Preprints

Summary: Preface of Planetary Systems Beyond the Main Sequence including conference scope and summary, short overview of programme, acknowledgements of patronage, sponsors, the scientific organising committee, and the local organising committee.

Heber, Ulrich; Schuh, Sonja
2010-01-01

36

Circumstellar Disks in pre-Main Sequence Stars

  Astrophysics (arXiv)

Summary: This review covers the properties of disks around pre-main--sequence stars. It is at this time in the evolution that planets form, and it is important to understand the properties of these disks to understand planet formation. I discuss disk shapes, masses and temperatures, the properties of the host stars, disk lifetimes and dissipation processes. Finally, evidence of grain growth during pre-main-sequence evolution is briefly summarized.

Antonella Natta
2003-04-10

37

The Main Sequence of three Red Supergiant Clusters

  CERN Preprints

Summary: Massive clusters in our Galaxy are an ideal testbed to investigate the properties and evolution of high mass stars. They provide statistically significant samples of massive stars of uniform ages. To accurately determine the intrinsic physical properties of these stars we need to establish the distances, ages and reddening of the clusters. One avenue to achieve this is the identification and characterisation of the main sequence members of red supergiant rich clusters. Here we utilise publicly available data from the UKIDSS galactic plane survey. We show that point spread function photometry in conjunction with standard photometric decontamination techniques allows us to identify the most likely main sequence members in the 10-20Myr old clusters RSGC1, 2, and 3. We confirm the previous detection of the main sequence in RSGC2 and provide the first main sequence detection in RSGC1 and RSGC3. There are in excess of 100 stars with more than 8Msun identified in each cluster. These main sequence members are concent...

Froebrich, Dirk
2013-01-01

38

The Habitable Zones of Pre-Main-Sequence Stars

  CERN Preprints

Summary: We calculate the pre-main-sequence HZ for stars of spectral classes F to M. The spatial distribution of liquid water and its change during the pre-main-sequence phase of protoplanetary systems is important in understanding how planets become habitable. Such worlds are interesting targets for future missions because the coolest stars could provide habitable conditions for up to 2.5 billion years post-accretion. Moreover, for a given star type, planetary systems are more easily resolved because of higher pre-main-sequence stellar luminosities, resulting in larger planet to star separation for cool stars than is the case for the traditional main-sequence (MS) habitable zone (HZ). We use 1D radiative-convective climate and stellar evolutionary models to calculate pre-main-sequence HZ distances for F1 to M8 stellar types. We also show that accreting planets that are later located in the traditional MS HZ orbiting stars cooler than a K5 (including the full range of M-stars) receive stellar fluxes that exceed the ru...

Ramirez, Ramses M
2014-01-01

39

Magnetic flaring in pre-main sequence stars

  Astrophysics (arXiv)

Summary: Observations of nearby star forming clouds with imaging X-ray telescopes have revealed that X-ray emission is elevated $10^1-10^4$ above main sequence levels in low-mass pre-main sequence (PMS) stars. The variability and spectral X-ray evidence, together with circularly polarized radio continuum flares seen in a few cases, strongly argues for an origin in magnetic reconnection flares. These high levels of magnetic activity are present from the earliest protostellar phase to the main sequence. After a brief review of past observations, three astrophysical issues are raised: the location of the flaring magnetic fields, the origin of these fields, and the effects of flare high-energy photons and particles on the environs. New results from Chandra observations of a well-defined samples of PMS solar analogues are presented, giving an improved view of magnetic flaring in the early Sun.

Eric D. Feigelson
2001-02-19

40

The Multiplicity of Main Sequence Turnoffs in Globular Clusters

  Astrophysics (arXiv)

Summary: We present color-magnitude diagrams of globular clusters for models with self-enrichment and pre-enrichment. The models with self-enrichment turn out to have two or more main sequence turnoff points in the color-magnitude diagram if the fraction of mass lost by the globular cluster under supernova explosions does not exceed 95-97%. The models with pre-enrichment can have only one main sequence turnoff point. We argue that the cluster wCen evolved according to a self-enrichment scenario.

M. V. Ryabova; Yu. A. Shchekinov
2008-08-06

41

Recent results on Pre-main sequence delta Scuti stars

  CERN Preprints

Summary: Intermediate mass Pre-main sequence stars (1.5 $M_{\\odot}< M < 5M_{\\odot}$) cross the instability strip on their way to the main sequence. They are therefore expected to be pulsating in a similar way as the $\\delta$ Scuti stars. In this contribution we present the status of the observational studies of pulsations in these stars with special emphasis on recent results from our group. The prospects for future investigations of these objects from the ground and from space are discussed.

Ripepi, V; Palla, F; Bernabei, S; Ruoppo, A; Cusano, F; Alcalá, J M
2005-01-01

42

Recent results on Pre-main sequence delta Scuti stars

  Astrophysics (arXiv)

Summary: Intermediate mass Pre-main sequence stars (1.5 $M_{\\odot}< M < 5M_{\\odot}$) cross the instability strip on their way to the main sequence. They are therefore expected to be pulsating in a similar way as the $\\delta$ Scuti stars. In this contribution we present the status of the observational studies of pulsations in these stars with special emphasis on recent results from our group. The prospects for future investigations of these objects from the ground and from space are discussed.

V. Ripepi; M. Marconi; F. Palla; S. Bernabei; A. Ruoppo; F. Cusano; J. M. Alcala'
2005-10-03

43

Magnetic fields along the pre-main-sequence phase

  CERN Preprints

Summary: In this contribution I review some of the major work that have been undertaken in the last decade to study the magnetic fields in intermediate-mass pre-main sequence stars, with the goal of understanding the origin of the magnetic fields detected in the chemically peculiar Ap/Bp stars, and testing the fossil field theory.

Alecian, E
2013-01-01

44

SPH Calculations of Collisions between Main-Sequence Stars

  Astrophysics (arXiv)

Summary: The hydrodynamics of collisions and mergers of main-sequence stars is discussed in the light of recent 3-D calculations using the smoothed particle hydrodynamics (SPH) method. Theoretical models for the formation of blue stragglers are reviewed in the context of recent comparisons between the observed properties of blue stragglers in dense globular clusters and the predictions of those models.

Frederic A. Rasio
1995-12-12

45

A non-main-sequence secondary in SY Cancri

  Astrophysics (arXiv)

Summary: Simultaneous spectroscopic and photometric observations of the Z Cam type dwarf nova SY Cancri were used to obtain absolute flux calibrations. A comparison of the photometric calibration with a wide slit spectrophotometric calibration showed that either method is equally satisfactory. A radial velocity study of the secondary star, made using the far red NaI doublet, yielded a mass ratio q = 0.68; this is very different from the value of 1.13 quoted in the literature. Using the new lower mass ratio, and constraining the mass of the white dwarf to be within reasonable limits, then leads to a mass for the secondary star that is substantially less than would be expected for its orbital period if it satisfied a main-sequence mass-radius relationship. We find a spectral type of M0 that is consistent with that expected for a main-sequence star of the low mass we have found. However, in order to fill its Roche lobe, the secondary must be significantly larger than a main sequence star of that mass and spectral type. The secondary is definitely not a normal main-sequence star.

R. C. Smith; O. Mehes; D. Vande Putte; N. A. Hawkins
2005-06-03

46

Asteroseismology of stars on the upper main sequence

  CERN Preprints

Summary: I review the properties of pulsators located on the upper main sequence in the HR diagram and discuss asteroseismic inferences on the internal structure of stars of spectral type A and B. Special attention is given to the problem of uncertainties in stellar opacities in modelling.

Lenz, P
2012-01-01

47

A Double Main Sequence in the Globular Cluster NGC 6397

  CERN Preprints

Summary: High-precision multi-band HST photometry reveals that the main sequence (MS) of the globular cluster NGC 6397 splits into two components, containing ~30% and ~70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (i) a primordial population that has a composition similar to field stars, and containing ~30% of the stars, and (ii) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m_F336W. Theoretical isochrones indicate that this could be due to NH depletion.

Milone, A P; Piotto, G; Bedin, L R; Anderson, J; Aparicio, A; Cassisi, S; Rich, R M
2011-01-01

48

HST Observations of the Main Sequence of M4

  Astrophysics (arXiv)

Summary: This paper presents new results from a photometric study of the main-sequence stars in M4, the closest globular cluster to the sun. Multi-field, multi-epoch observations at approximately 1, 2, and 6 core-radii were obtained with the WFPC2 camera on the HST. The multi-epoch observations allowed us to clean the data on the basis of proper motion. In all the fields the cluster main sequence can be traced to at least V = 27.0 but there remains a trail of stars to the limit of the data near V = 30 in the deepest outer field. There is no evidence that we have reached the end of the hydrogen burning main sequence in any of our fields, however, there is some indication that very few stars remain to be detected in the deepest data. The scatter about the cluster main sequence yields a surprisingly small and variable binary fraction; f_b = 2% in the inner parts of the cluster falling to the 1% range outside. For the currently visible main sequence stars, the cluster mass function is very flat (alpha =0.1) in the outer field and flattens further in the inner fields suggesting well developed mass segregation. The observed variation in the mass function is broadly consistent with isotropic, multi-mass Michie-King models. Because we have a large sample of white dwarfs in the outer field we are able to show that the cluster IMF above 0.8Msun was considerably steeper than the present day mass function for low mass stars. Two appendicies are included in this contribution; a detailed discussion of the techniques used to reduce the data and a direct comparison between the cluster stars and those belonging to the inner spheroid of the Galaxy yielding a Galactocentric distance of 7.5+/-0.6 kpc.

H. B. Richer; G. G. Fahlman; J. Brewer; S. Davis; J. Kalirai; P. B. Stetson; B. M. S. Hansen; R. M. Rich; R. A. Ibata; B. K. Gibson; M. Shara
2004-01-21

49

Structure and Evolution of Pre-Main Sequence Stars

  CERN Preprints

Summary: Low-mass pre-main sequence (PMS) stars are strong and variable X-ray emitters, as has been well established by EINSTEIN and ROSAT observatories. It was originally believed that this emission was of thermal nature and primarily originated from coronal activity (magnetically confined loops, in analogy with Solar activity) on contracting young stars. Broadband spectral analysis showed that the emission was not isothermal and that elemental abundances were non-Solar. The resolving power of the Chandra and XMM X-ray gratings spectrometers have provided the first, tantalizing details concerning the physical conditions such as temperatures, densities, and abundances that characterize the X-ray emitting regions of young star. These existing high resolution spectrometers, however, simply do not have the effective area to measure diagnostic lines for a large number of PMS stars over required to answer global questions such as: how does magnetic activity in PMS stars differ from that of main sequence stars, how do they ...

Schulz, Norbert S; Bautz, Mark W; Canizares, Claude C; Davis, John; Dewey, Dan; Huenemoerder, David P; Heilmann, Ralf; Houck, John; Marshall, Herman L; Nowak, Mike; Schattenburg, Mark; Audard, Marc; Drake, Jeremy; Gagne, Marc; Kastner, Joel; Kallman, Tim; Lautenegger, Maurice; Lee, Julia; Miller, Jon; Montmerle, Thierry; Mukai, Koji; Osten, Rachel; Parerels, Frits; Pollock, Andy; Preibisch, Thomas; Raymond, John; Reale, Fabio; Smith, Randall; Testa, Paola; Weintraub, David
2009-01-01

50

Amplitudes of stochastically excited oscillations in main-sequence stars

  Astrophysics (arXiv)

Summary: We present estimates of the amplitudes of intrinsically stable stochastically excited radial oscillations in stars near the main sequence. The amplitudes are determined by the balance between acoustical energy generation by turbulent convection (the Lighthill mechanism) and linear damping. Convection is treated with a time-dependent, nonlocal, mixing-length model, which includes both convective heat flux and turbulent pressure in both the equilibrium model and the pulsations. Velocity and luminosity amplitudes are computed for stars with masses between 0.9 M_\\sun and 2.0 M_\\sun in the vicinity of the main sequence, for various metallicities and convection parameters. As in previous studies, the amplitudes are found to increase with stellar mass, and therefore with luminosity. Amongst those stars that are pulsationally stable, the largest amplitudes are predicted for a 1.6 M_\\sun model of spectral type F2; the values are approximately 15 times larger than those measured in the Sun.

G. Houdek; N. J. Balmforth; J. Christensen-Dalsgaard; D. O. Gough
1999-09-06

51

Magnetic coronae of active main-sequence stars

  Astrophysics (arXiv)

Summary: The coronal structure of main sequence stars continues to puzzle us. While the solar corona is relatively well understood, it has become clear that even stars of the same mass as the Sun can display very non-solar coronal behaviour, particularly if they are rapid rotators or in a binary system. At masses greater than and also less than that of the Sun, the non-solar internal structure appears to affect both the geometry and dynamics of the stellar corona and the nature of the X-ray and radio emission. In this talk I will describe some recent advances in our understanding of the structure of the coronae of some of the most active (and interesting) main sequence stars.

M. Jardine; J. -F. Donati
2008-11-12

52

Amplitudes of stochastically excited oscillations in main-sequence stars

  CERN Preprints

Summary: We present estimates of the amplitudes of intrinsically stable stochastically excited radial oscillations in stars near the main sequence. The amplitudes are determined by the balance between acoustical energy generation by turbulent convection (the Lighthill mechanism) and linear damping. Convection is treated with a time-dependent, nonlocal, mixing-length model, which includes both convective heat flux and turbulent pressure in both the equilibrium model and the pulsations. Velocity and luminosity amplitudes are computed for stars with masses between 0.9 M_\\sun and 2.0 M_\\sun in the vicinity of the main sequence, for various metallicities and convection parameters. As in previous studies, the amplitudes are found to increase with stellar mass, and therefore with luminosity. Amongst those stars that are pulsationally stable, the largest amplitudes are predicted for a 1.6 M_\\sun model of spectral type F2; the values are approximately 15 times larger than those measured in the Sun.

Houdek, G; Christensen-Dalsgaard, J; Gough, D O
1999-01-01

53

The Pre--Main-Sequence of A-type stars

  Astrophysics (arXiv)

Summary: Young A-type stars in the pre--main-sequence (PMS) evolutionary phase are particularly interesting objects since they cover the mass range (1.5-4 solar ma sses) which is most sensitive to the internal conditions inherited during the protostellar phase. In particular, they undergo a process of thermal relaxation from which they emerge as fully radiative objects contracting towards the main sequence. A-type stars also show intense surface activity (including winds, accretion, pulsations) whose origin is still not completely understood, and infrared excesses related to the presence of circumstellar disks and envelopes. Disks display significant evolution in the dust properties, likely signalling the occurrence of protoplanetary growth. Finally, A-type stars are generally found in multiple systems and small aggregates of lower mass companions.

Marcella Marconi; Francesco Palla
2004-10-06

54

Orbital Motion in Pre-Main Sequence Binaries

  CERN Preprints

Summary: We present results from our ongoing program to map the visual orbits of pre-main sequence binaries in the Taurus star forming region using adaptive optics imaging at the Keck Observatory. We combine our results with measurements reported in the literature to analyze the orbital motion for each binary. We present preliminary orbits for DF Tau, T Tau S, ZZ Tau, and the Pleiades binary HBC 351. Seven additional binaries show curvature in their relative motion. Currently, we can place lower limits on the orbital periods for these systems; full solutions will be possible with more orbital coverage. Five other binaries show motion that is indistinguishable from linear motion. We suspect that these systems are bound and might show curvature with additional measurements in the future. The observations reported herein lay critical groundwork toward the goal of measuring precise masses for low-mass pre-main sequence stars.

Schaefer, G H; Simon, M; Patience, J
2014-01-01

55

The Ages of Pre-main-sequence Stars

  Astrophysics (arXiv)

Summary: The position of pre-main-sequence or protostars in the Hertzsprung--Russell diagram is often used to determine their mass and age by comparison with pre-main-sequence evolution tracks. On the assumption that the stellar models are accurate, we demonstrate that, if the metallicity is known, the mass obtained is a good estimate. However, the age determination can be very misleading because it is significantly (generally different by a factor of two to five) dependent on the accretion rate and, for ages less than about one million years, the initial state of the star. We present a number of accreting protostellar tracks that can be used to determine age if the initial conditions can be determined and the underlying accretion rate has been constant in the past. Because of the balance established between the Kelvin-Helmholtz, contraction timescale and the accretion timescale a pre-main-sequence star remembers its accretion history. Knowledge of the current accretion rate, together with an H--R-diagram position gives information about the rate of accretion in the past but does not necessarily improve any age estimate. We do not claim that ages obtained by comparison with these particular accreting tracks are likely to be any more reliable than those from comparisons with non-accreting tracks. Instead we stress the unreliability of any such comparisons and use the disparities between various tracks to estimate the likely errors in age and mass estimates. We also show how a set of coeval accreting objects do not appear coeval when compared with non-accreting tracks. Instead accreting pre-main-sequence stars of around a solar mass are likely to appear older than those of either smaller or larger mass.

Christopher A. Tout; Mario Livio; Ian A. Bonnell
1999-07-30

56

A Triple Main Sequence in the Globular Cluster NGC 2808

  Astrophysics (arXiv)

Summary: Accurate photometry with HST/ACS shows that the main sequence of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with complexities of the cluster's horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

G. Piotto; L. R. Bedin; J. Anderson; I. R. King; S. Cassisi; A. P. Milone; S. Villanova; A. Pietrinferni; A. Renzini
2007-03-29

57

Mass loss in main-sequence B stars

  CERN Preprints

Summary: We calculate radiatively driven wind models of main-sequence B stars and provide the wind mass-loss rates and terminal velocities. The main-sequence mass-loss rate strongly depends on the stellar effective temperature. For the hottest B stars the mass-loss rate amounts to $10^{-9}\\,\\text{M}_\\odot\\,\\text{year}^{-1}$, while for the cooler ones the mass-loss rate is by more than three orders of magnitude lower. Main sequence B stars with solar abundance and effective temperatures lower than about $15\\,000\\,\\text{K}$ (later than the spectral type B5) do not have any homogeneous line-driven wind. We predict the wind mass-loss rates for the solar chemical composition and for modified abundance of heavier elements to study the winds of chemically peculiar stars. The mass-loss rate may both increase or decrease with increasing abundance depending on the importance of the induced emergent flux redistribution. Stars with overabundant silicon may have homogeneous winds even below the solar abundance wind limit at $15\\,0...

Krticka, Jiri
2014-01-01

58

Rotational Evolution of Solar-Like Stars in Clusters from Pre-Main Sequence to Main Sequence: Empirical Results

  Astrophysics (arXiv)

Summary: Rotation periods are now available for ~500 pre-main sequence and recently arrived main sequence stars of solar-like mass (0.4-1.2 M_sun) in five nearby young clusters: the Orion Nebula Cluster, NGC 2264, alpha Per, IC 2602 and the Pleiades. In combination with estimates of stellar radii these data allow us to construct distributions of surface angular momentum per unit mass at three different epochs: nominally, 1, 2 and 50 My. Our main result is illustrated in Fig. 18 and may be summarized as follows: (1) 50-60% of the stars on convective tracks in this mass range are released from any locking mechanism very early on and are free to conserve angular momentum throughout most of their PMS evolution, i.e. to spin up and account for the rapidly rotating young main sequence stars. (2) The other 40-50% lose substantial amounts of angular momentum during the first few million years, and end up as slowly rotating main sequence stars. The duration of the rapid angular momentum loss phase is ~5-6 My, which is roughly consistent with the lifetimes of disks estimated from infrared surveys of young clusters. The rapid rotators of Orion age lose less than 10% of their (surface) specific angular momentum during the next 50 My while the slow rotators lose about two-thirds of theirs. A detectable part of this loss occurs even during the ~1 My interval between the ONC and NGC 2264. The data support the view that interaction between an accretion disk and star is the primary mechanism for evolving the broad, bimodal distribution of rotation rates seen for solar-like stars in the ONC into the even broader distributions seen in the young MS clusters.

W. Herbst; R. Mundt
2005-07-30

59

Rotation and activity of pre-main-sequence stars

  Astrophysics (arXiv)

Summary: We present a study of rotation (vsini) and chromospheric activity (Halpha EW) based on an extensive set of high-resolution optical spectra obtained with MIKE on the 6.5m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young stellar associations Eta Cha, TW Hydrae, Beta Pic, and Tuc-Hor, spanning ages from 6 to 30 Myr. While the Halpha EW for most F and G stars are consistent with pure photospheric absorption, most K and M stars show chromospheric emission. By comparing Halpha EW in our sample to results in the literature, we see a clear evolutionary sequence: Chromospheric activity declines steadily from the T Tauri phase to the main sequence. Using activity as an age indicator, we find a plausible age range for the Tuc-Hor association of 10-40 Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the total sample, thus angular momentum is conserved, in contrast to younger stars. This difference indicates a change in the rotational regulation at 5-10 Myr, possibly because disk braking cannot operate longer than typical disk lifetimes, allowing the objects to spin up. The rotation-activity relation is flat in our sample; in contrast to main-sequence stars, there is no linear correlation for slow rotators. We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars, and not just the result of a saturated solar-type dynamo. By comparing our rotational velocities with published rotation periods for a subset of stars, we determine ages of 13 (7-20) Myr and 9 (7-17} Myr for the Eta Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by Baraffe et al. (1998) are in agreement with the observed radii within +-15%. (abridged)

Aleks Scholz; Jaime Coffey; Alexis Brandeker; Ray Jayawardhana
2007-04-24

60

The Solar System's Post-Main Sequence Escape Boundary

  CERN Preprints

Summary: The Sun will eventually lose about half of its current mass nonlinearly over several phases of post-main sequence evolution. This mass loss will cause any surviving orbiting body to increase its semimajor axis and perhaps vary its eccentricity. Here, we use a range of Solar models spanning plausible evolutionary sequences and assume isotropic mass loss to assess the possibility of escape from the Solar System. We find that the critical semimajor axis in the Solar System within which an orbiting body is guaranteed to remain bound to the dying Sun due to perturbations from stellar mass loss alone is approximately 1,000 AU - 10,000 AU. The fate of objects near or beyond this critical semimajor axis, such as the Oort Cloud, outer scattered disc and specific bodies such as Sedna, will significantly depend on their locations along their orbits when the Sun turns off of the main sequence. These results are applicable to any exoplanetary system containing a single star with a mass, metallicity and age which are appro...

Veras, Dimitri
2012-01-01

61

Theoretical mass loss rates of cool main-sequence stars

  Astrophysics (arXiv)

Summary: We develop a model for the wind properties of cool main-sequence stars, which comprises their wind ram pressures, mass fluxes, and terminal wind velocities. The wind properties are determined through a polytropic magnetised wind model, assuming power laws for the dependence of the thermal and magnetic wind parameters on the stellar rotation rate. We use empirical data to constrain theoretical wind scenarios, which are characterised by different rates of increase of the wind temperature, wind density, and magnetic field strength. Scenarios based on moderate rates of increase yield wind ram pressures in agreement with most empirical constraints, but cannot account for some moderately rotating targets, whose high apparent mass loss rates are inconsistent with observed coronal X-ray and magnetic properties. For fast magnetic rotators, the magneto-centrifugal driving of the outflow can produce terminal wind velocities far in excess of the surface escape velocity. Disregarding this aspect in the analyses of wind ram pressures leads to overestimations of stellar mass loss rates. The predicted mass loss rates of cool main-sequence stars do not exceed about ten times the solar value. Our results are in contrast with previous investigations, which found a strong increase of the stellar mass loss rates with the coronal X-ray flux. Owing to the weaker dependence, we expect the impact of stellar winds on planetary atmospheres to be less severe and the detectability of magnetospheric radio emission to be lower then previously suggested. Considering the rotational evolution of a one solar-mass star, the mass loss rates and the wind ram pressures are highest during the pre-main sequence phase.

V. Holzwarth; M. Jardine
2006-11-14

62

Stochastically excited oscillations on the upper main sequence

  CERN Preprints

Summary: Convective envelopes in stars on the main sequence are usually connected only with stars of spectral types F5 or later. However, observations as well as theory indicate that the convective outer layers in earlier stars, despite being shallow, are still effective and turbulent enough to stochastically excite oscillations. Because of the low amplitudes, exploring stochastically excited pulsations became possible only with space missions such as Kepler and CoRoT. Here I review the recent results and discuss among others, pulsators such as delta Scuti, gamma Doradus, roAp, beta Cephei, Slowly Pulsating B and Be stars, all in the context of solar-like oscillations.

Antoci, Victoria
2013-01-01

63

Pre-Main Sequence Evolutions of Solar Abundance Lowmass Stars

  Astrophysics (arXiv)

Summary: We present the Pre-Main-Sequence (PMS) evolutionary tracks of stars with 0.0065~5.0Ms. The models were evolved from the PMS stellar birthline to the onset of hydrogen burning in the core. The convective turnover timescales which enables an observational test of theoretical model, particulary in the stellar dynamic activity, are also calculated. All models have Sun-like metal abundances, typically considered as the stars in the Galactic disk and the star formation region of Population I star. The convection phenomenon is treated by the usual mixing length approximation. All evolutionary tracks are available upon request.

Youn Kil Jung; Yong-Cheol Kim
2007-02-12

64

Massive main sequence stars evolving at the Eddington limit

  CERN Preprints

Summary: The evolution of massive stars even on the main sequence is not yet well understood. Due to the steep mass-luminosity relation, massive main sequence stars become very luminous. This brings their envelopes very close to the Eddington limit. We are analysing stellar evolutionary models in which the Eddington limit is reached and exceeded, and explore the rich diversity of physical phenomena which take place in their envelopes, and investigate their observational consequences. We use the grids of detailed stellar models by Brott et al. (2011) and Koehler et al. (2015), to investigate the envelope properties of core hydrogen burning massive stars. We find that at the stellar surface, the Eddington limit is almost never reached, even for stars up to 500 Msun. When an appropriate Eddington limit is defined locally in the stellar envelope, most stars more massive than 40 Msun actually exceed this limit, in particular in the partial ionization zones of iron, helium or hydrogen. While most models adjust their structu...

Sanyal, Debashis; Langer, Norbert; Bestenlehner, Joachim M
2015-01-01

65

Pre-main-sequence isochrones -- I. The Pleiades benchmark

  CERN Preprints

Summary: We present a critical assessment of commonly used pre-main-sequence isochrones by comparing their predictions to a set of well-calibrated colour-magnitude diagrams of the Pleiades in the wavelength range 0.4 to 2.5 microns. Our analysis shows that for temperatures less than 4000 K the models systematically overestimate the flux by a factor two at 0.5 microns, though this decreases with wavelength, becoming negligible at 2.2 microns. In optical colours this will result in the ages for stars younger than 10 Myr being underestimated by factors between two and three. We show that using observations of standard stars to transform the data into a standard system can introduce significant errors in the positioning of pre-main-sequences in colour-magnitude diagrams. Therefore we have compared the models to the data in the natural photometric system in which the observations were taken. Thus we have constructed and tested a model of the system responses for the Wide-Field Camera on the Isaac Newton Telescope. As a ben...

Bell, Cameron P M; Mayne, N J; Jeffries, R D; Littlefair, S P
2012-01-01

66

Probing the circumstellar structure of pre-main sequence stars

  Astrophysics (arXiv)

Summary: We present Halpha spectropolarimetry of a large sample of pre-main sequence (PMS) stars of low and intermediate mass, and argue that the technique is a powerful tool in studying the circumstellar geometry around these objects. For the intermediate mass (2 -- 15 Msun) Herbig Ae/Be stars we find that 16 out of 23 show a line effect, which immediately implies that flattening is common among these objects. Furthermore, we find a significant difference in Halpha spectropolarimetry behaviour between the Herbig Be and Ae groups. For the Herbig Be stars, the concept of an electron scattering disc is shown to be a useful concept to explain the depolarizations seen in this spectral range. At lower masses, more complex Halpha polarimetry behaviour starts to appear. The concept of a compact source of Halpha emission that is formed close to the stellar surface, for instance by hot spots due to magnetospheric accretion, is postulated as a working hypothesis to qualitatively explain the Halpha spectropolarimetry behaviour around Herbig Ae and lower mass (M < 2 Msun) T Tauri stars. The striking resemblance in spectropolarimetric behaviour between the T Tauri star RY Tau and the Herbig Ae stars suggests a common origin of the polarized line photons, and hints that low and higher mass pre-main sequence stars may have more in common than had hitherto been suspected.

Jorick S. Vink; Janet E. Drew; Tim J. Harries; Rene D. Oudmaijer
2003-06-04

67

Lithium Depletion in Fully Convective Pre-Main Sequence Stars

  Astrophysics (arXiv)

Summary: We present an analytic calculation of the thermonuclear depletion of lithium in contracting, fully convective, pre-main sequence stars of mass M < 0.5 M_sun. Previous numerical work relies on still-uncertain physics (atmospheric opacities and convection, in particular) to calculate the effective temperature as a unique function of stellar mass. We assume that the star's effective temperature, T_eff, is fixed during Hayashi contraction and allow its actual value to be a free parameter constrained by observation. Using this approximation, we compute lithium burning analytically and explore the dependence of lithium depletion on T_eff, M, and composition. Our calculations yield the radius, age, and luminosity of a pre-main sequence star as a function of lithium depletion. This allows for more direct comparisons to observations of lithium depleted stars. Our results agree with those numerical calculations that explicitly determine stellar structure during Hayashi contraction. In agreement with Basri, Marcy, and Graham (1996), we show that the absence of lithium in the Pleiades star HHJ 3 implies that it is older than 100 Myr. We also suggest a generalized method for dating galactic clusters younger than 100 Myr (i.e., those with contracting stars of M > 0.08 M_sun) and for constraining the masses of lithium depleted stars.

Lars Bildsten; Edward F. Brown; Christopher D. Matzner; Greg Ushomirsky
1996-12-16

68

Revisiting Hipparcos data for pre-main sequence stars

  CERN Preprints

Summary: We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the relevant associations and using Hipparcos intermediate astrometric data to derive new parallaxes of the pre-main sequence stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecular cloud distance, although Hipparcos distances of some faint Taurus-Auriga stars must be viewed with caution. We then improve some of the solutions for the binary and multiple pre-main sequence stars. In particular, we confirm three new astrometric young binaries discovered by Hippa...

Bertout, C; Arenou, F; Bertout, Claude; Robichon, Noel; Arenou, Frederic
1999-01-01

69

Revisiting Hipparcos data for pre-main sequence stars

  Astrophysics (arXiv)

Summary: We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the relevant associations and using Hipparcos intermediate astrometric data to derive new parallaxes of the pre-main sequence stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecular cloud distance, although Hipparcos distances of some faint Taurus-Auriga stars must be viewed with caution. We then improve some of the solutions for the binary and multiple pre-main sequence stars. In particular, we confirm three new astrometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX Oph.

Claude Bertout; Noel Robichon; Frederic Arenou
1999-09-27

70

Dissecting the quasar main sequence: insight from host galaxy properties

  CERN Preprints

Summary: The diverse properties of broad-line quasars appear to follow a well-defined main sequence along which the optical FeII strength increases. It has been suggested that this sequence is mainly driven by the Eddington ratio (L/L_Edd) of the black hole (BH) accretion. Shen & Ho demonstrated with quasar clustering analysis that the average BH mass decreases with increasing FeII strength when quasar luminosity is fixed, consistent with this suggestion. Here we perform an independent test by measuring the stellar velocity dispersion sigma* (hence the BH mass via the M-sigma* relation) from decomposed host spectra in low-redshift Sloan Digital Sky Survey quasars. We found that at fixed quasar luminosity, sigma* systematically decreases with increasing FeII strength, confirming that Eddington ratio increases with FeII strength. We also found that at fixed luminosity and FeII strength, there is little dependence of sigma* on the broad Hbeta FWHM. These new results reinforce the framework put forward by Shen & H...

Sun, Jiayi
2015-01-01

71

HII region variability and pre-main-sequence evolution

  CERN Preprints

Summary: Recent observations and simulations have suggested that HII regions around massive stars may vary in their size and emitted flux on timescales short enough to be observed. This variability can have a number of causes, ranging from environmental causes to variability of the ionizing source itself. We explore the latter possibility by considering the pre-main-sequence evolution of massive protostars and conducting numerical simulations with ionizing radiation feedback using the FLASH AMR hydrodynamics code. We investigate three different models: a simple ZAMS model, a self-consistent one-zone model by Offner et al. (2009), and a model fit to the tracks computed by Hosokawa & Omukai (2009). The protostellar models show that hypercompact HII regions around massive, isolated protostars collapse or shrink from diameters of 80 or 300 AU, depending on the model choice, down to near absence during the swelling of stellar radius that accompanies the protostar's transition from a convective to a radiative internal s...

Klassen, Mikhail; Pudritz, Ralph E
2012-01-01

72

Pre-main sequence binaries with aligned disks ?

  Astrophysics (arXiv)

Summary: We present the results of a study performed with the goal to investigate whether low-mass pre-main sequence binary stars are formed by multiple fragmentation or via stellar capture. If binaries form preferentially by fragmentation, we expect their disks to be co-planar. On the other hand, the capture scenario will lead to a random distribution of disk orientations. We performed near-infrared polarization measurements of 49 young close binary stars in the K band with SOFI at the NTT. The near-infrared excess radiation of the targets mostly point to the presence of disks. For a major fraction of the sample, evidence for disks is also obvious from other features (outflows, jets, Herbig-Haro objects). We derived the disk orientation from the orientation of the polarization vector of both components of each binary. This statistical study allows to test which hypothesis (co-planarity, random orientation) is consistent with the observed distribution of polarimetric

Sebastian Wolf; Bringfried Stecklum; Thomas Henning
2000-06-21

73

Old Main-Sequence Turnoff Photometry in the SMC

  Astrophysics (arXiv)

Summary: We present ground-based {\\it B} and {\\it R}-band color-magnitude diagrams (CMDs) of unprecedented depth for twelve fields in the Small Magellanic Cloud (SMC). They reach the oldest main-sequence turnoffs and cover a wide range of galactocentric distances up to $\\sim4\\arcdeg$ from the SMC center, and are located at different position angles. A picture of the stellar content in our SMC fields is presented, through the comparison with theoretical isochrones. Our study confirms the existence of strong population gradients and spatial variation in the SMC stellar content. None of the SMC fields presented here are dominated by old stellar populations which proves that at $\\sim4\\arcdeg$ from the SMC center we do not reach an old stellar halo similar to that of the Milky Way.

Noelia E. D. Noel; Carme Gallart; Edgardo Costa; Rene A. Mendez
2006-03-14

74

Abundances on the Main Sequence of Omega Centauri

  Astrophysics (arXiv)

Summary: Abundance ratios of carbon, nitrogen and strontium relative to iron, calculated using spectrum synthesis techniques, are given for a sample of main sequence and turnoff stars that belong to the globular cluster omega Centauri. The variations of carbon, nitrogen and/or strontium show several different abundance patterns as a function of [Fe/H]. The source of the enhancements/depletions in carbon, nitrogen and/or strontium may be enrichment from asymptotic giant branch stars of low (1--3 solar masses) and intermediate (3--8 solar masses) mass. Massive rotating stars which produce excess nitrogen without carbon and oxygen overabundances may also play a role. These abundances enable different contributors to be considered and incorporated into the evolutionary picture of omega Cen.

Laura M. Stanford; G. S. Da Costa; John E. Norris
2007-08-01

75

The Instability Strip for Pre--Main-Sequence Stars

  Astrophysics (arXiv)

Summary: We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars by means of linear and nonlinear calculations. The equilibrium models were taken from models evolved from the protostellar birthline to the ZAMS for masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to define the instability strip of PMS stars in the HR diagram. These models are used to constrain the internal structure of young stars and to test evolutionary models. We compare our results with observations of the best case of a pulsating young star, HR~5999, and we also identify possible candidates for pulsational variability among known Herbig Ae/Be stars which are located within or close to the instability strip boundaries.

M. Marconi; F. Palla
1998-09-21

76

Main-sequence stellar eruption model for V838 Mon

  Astrophysics (arXiv)

Summary: We propose that the energy source of the outburst of V838 Mon and similar objects is an accretion event, i.e., gravitational energy rather than thermonuclear runaway. We show that the merger of two main sequence stars, of masses 1.5 Mo and 0.1-0.5 Mo can account for the luminosity, large radius, and low effective temperture of V838 Mon and similar objects. Subsequent cooling and gravitational contraction lead such objects to move along the Hayashi limit, as observed. By varying the masses and types of the merging stars, and by considering slowly expanding, rather than hydrostatic, envelopes, this model can account for a large range in luminosities and radii of such outburst events.

Noam Soker; Romuald Tylenda
2002-10-21

77

A timing formula for main-sequence star binary pulsars

  Astrophysics (arXiv)

Summary: In binary radio pulsars with a main-sequence star companion, the spin-induced quadrupole moment of the companion gives rise to a precession of the binary orbit. As a first approximation one can model the secular evolution caused by this classical spin-orbit coupling by linear-in-time changes of the longitude of periastron and the projected semi-major axis of the pulsar orbit. This simple representation of the precession of the orbit neglects two important aspects of the orbital dynamics of a binary pulsar with an oblate companion. First, the quasiperiodic effects along the orbit, due to the anisotropic $1/r^3$ nature of the quadrupole potential. Secondly, the long-term secular evolution of the binary orbit which leads to an evolution of the longitude of periastron and the projected semi-major axis which is non-linear in time. In this paper a simple timing formula for binary radio pulsars with a main-sequence star companion is presented which models the short-term secular and most of the short-term periodic effects caused by the classical spin-orbit coupling. I also give extensions of the timing formula which account for long-term secular changes in the binary pulsar motion. It is shown that the short-term periodic effects are important for the timing observations of the binary pulsar PSR B1259--63. The long-term secular effects are likely to become important in the next few years of timing observations of the binary pulsar PSR J0045--7319. They could help to restrict or even determine the moments of inertia of the companion star and thus probe its internal structure. Finally, I reinvestigate the spin-orbit precession of the binary pulsar PSR J0045--7319 since the analysis given in the literature is based on an incorrect expression for the precession of the longitude of periastron.

N. Wex
1998-02-12

78

ROTATIONAL EVOLUTION OF SOLAR-LIKE STARS IN CLUSTERS FROM PREMAIN SEQUENCE TO MAIN SEQUENCE: EMPIRICAL RESULTS

  Geosciences Websites

Summary: ­main-sequence (PMS) and recently arrived main-sequence stars of solar-like mass (0.4­1.2 M) in five nearby young young main-sequence stars; (2) the other 40%­50% lose substantial amounts of angular momentum during from infrared surveys of young clusters. The rapid rotators of Orion age lose less than 10

Royer, Dana

79

Main Sequence Star Populations in the Virgo Overdensity Region

  CERN Preprints

Summary: We present deep CMDs for two Subaru Suprime-Cam fields in the Virgo Stellar Stream(VSS)/Virgo Overdensity(VOD) and compare them to a field centred on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of MS stars is detected in all three fields and can be traced as faint as g~24 mag. Using theoretical isochrone fitting we derive an age of 9.1(+1.0;-1.1)Gyr, a median abundance of [Fe/H]=-0.70 (+0.15; -0.20)dex and d_helio of 30.9+-3.0kpc for the MS of the Sgr Stream Branch A. The dominant main sequence populations in the two VSS/VOD fields (Lsun ~265 deg, Bsun ~13 deg) are located at a mean distance of 23.3+-1.6kpc and have an age ~8.2Gyr and an abundance [Fe/H]=-0.67(+0.16;-0.12)dex similar to the Sgr Stream stars. These parameters are also in good agreement with the age of the main population in the Sagittarius dwarf (8.0+-1.5Gyr). They also agree with the peak in the metallicity distribution of 2-3Gyr old M-giants, [Fe/H]...

Jerjen, H; Willman, B; Tisserand, P; Arimoto, N; Okamoto, S; Mateo, M; Saviane, I; Walsh, S; Geha, M; Jordán, A; Olszewski, E; Walker, M; Zoccali, M; Kroupa, P
2013-01-01

80

IRAC Observations of Taurus Pre-Main Sequence Stars

  Astrophysics (arXiv)

Summary: We present infrared photometry obtained with the IRAC camera on the Spitzer Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in the Taurus star-forming region. We find a clear separation in some IRAC color-color diagrams between objects with and without disks. A few ``transition'' objects are noted, which correspond to systems in which the inner disk has been evacuated of small dust. Separating pure disk systems from objects with remnant protostellar envelopes is more difficult at IRAC wavelengths, especially for objects with infall at low rates and large angular momenta. Our results generally confirm the IRAC color classification scheme used in previous papers by Allen et al. and Megeath et al. to distinguish between protostars, T Tauri stars with disks, and young stars without (inner) disks. The observed IRAC colors are in good agreement with recent improved disk models, and in general accord with models for protostellar envelopes derived from analyzing a larger wavelength region. We also comment on a few Taurus objects of special interest. Our results should be useful for interpreting IRAC results in other, less well-studied star-forming regions.

Lee Hartmann; S. T. Megeath; Lori Allen; Kevin Luhman; Nuria Calvet; Paola D'Alessio; Ramiro Franco-Hernandez; Giovanni Fazio
2005-05-16

81

Stellar Winds on the Main-Sequence I: Wind Model

  CERN Preprints

Summary: Aims: We develop a method for estimating the properties of stellar winds for low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a range of distances from the star. Methods: We use 1D thermal pressure driven hydrodynamic wind models run using the Versatile Advection Code. Using in situ measurements of the solar wind, we produce models for the slow and fast components of the solar wind. We consider two radically different methods for scaling the base temperature of the wind to other stars: in Model A, we assume that wind temperatures are fundamentally linked to coronal temperatures, and in Model B, we assume that the sound speed at the base of the wind is a fixed fraction of the escape velocity. In Paper II of this series, we use observationally constrained rotational evolution models to derive wind mass loss rates. Results: Our model for the solar wind provides an excellent description of the real solar wind far from the solar surface, but is unrealistic within the solar corona. We run ...

Johnstone, C P; Lüftinger, T; Toth, G; Brott, I
2015-01-01

82

The Main Sequence Luminosity Function of Palomar 5 from HST

  Astrophysics (arXiv)

Summary: A low mass, large core radius, and strong tidal tails suggest that the globular cluster Palomar 5 has lost a large fraction of its initial mass over time. If the dynamical evolution of Palomar 5 has been dominated by the effects of mass loss, then the luminosity function should be deficient in low-mass stars. Using deep WFPC2 F555W and F814W photometry, we determine the main sequence luminosity functions both near the cluster center and in a field near the half-light radius. There is no compelling evidence for mass segregation within the cluster, but a comparison of the global mass function of Palomar 5 with those of Omega Cen and M55 indicates an increasing deficiency of stars with progressively lower masses. A fit of the observed luminosity function to theoretical models yields a power-law mass function with an exponent of -0.5. The flatness of the mass function is consistent with models of the dynamical evolution of globular clusters that have lost on the order of 90% of their original stellar mass. We suggest that Palomar 5 has lost a large percentage of its original stellar content as a result of tidal shocking.

Carl J. Grillmair; Graeme H. Smith
2001-10-18

83

Photospheric temperature measurements in young main sequence stars

  Astrophysics (arXiv)

Summary: As part of our program to study stellar photospheric and chromospheric activity, we have examined several young solar type stars with activity levels intermediate between the Sun and the very active RS CVn binaries. We have analysed contemporaneous spectroscopic data obtained at Catania Observatory (Serra La Nave station, Mt. Etna) and photometric data acquired in the Stromgren bands with an automatic photometric telescope (APT) at Fairborn Observatory (Arizona, USA). Surface inhomogeneities have been detected from the rotational modulation of stellar brightness as well as from the modulation of several photospheric line-depth ratios (LDRs). The presence of chromospheric plage-like regions has been inferred from the rotational modulation of the Halpha line equivalent width (EW_Halpha) evaluated with the spectral synthesis method. The most relevant results are the strong correlation between the brightness and temperature curves derived respectively from photometry and the LDRs as well as the striking anti-correlation between brightness and Halpha emission. This suggests a close spatial association of spots and plages, as frequently observed for the largest sunspot groups (e.g., Catalano et al. (1998)) and for some very active RS CVn systems (Catalano et al. 2002). Moreover, a simple spot/plage model applied to the observed flux curves allows a rough reconstruction of photospheric and chromospheric features of young main sequence stars.

K. Biazzo; A. Frasca; G. W. Henry; S. Catalano; E. Marilli
2006-10-19

84

Acoustic Glitches in Main-Sequence Stellar models

  CERN Preprints

Summary: There are regions in stars, such as ionization zones and the interface between radiative and convective regions, that cause a localized sharp variation in the sound speed. These are known as "acoustic glitches". Acoustic glitches leave their signatures on the oscillation frequencies of stars, and hence these signature can be used as diagnostics of these regions. In particular, the signature of these glitches can be used as diagnostics of the position of the second helium ionization zone and that of the base of the envelope convection zone. With the help of stellar models we study the properties of these acoustic glitches in main-sequence stars. We find that the acoustic glitch due to the helium ionization zone does not correspond to the dip in the adiabatic index \\Gamma_1 caused by the ionization of HeII, but to the peak in \\Gamma_1 between the HeI and HeII ionization zones. We find that it is easiest to study the acoustic glitch due to the helium ionization zone in stars with masses in the range 0.9--1.2 M_\\...

Verma, Kuldeep; Basu, Sarbani; Mazumdar, Anwesh
2014-01-01

85

Stellar Diameters and Temperatures II. Main Sequence K & M Stars

  CERN Preprints

Summary: We present interferometric diameter measurements of 21 K- and M- dwarfs made with the CHARA Array. This sample is enhanced by literature radii measurements to form a data set of 33 K-M dwarfs with diameters measured to better than 5%. For all 33 stars, we compute absolute luminosities, linear radii, and effective temperatures (Teff). We develop empirical relations for \\simK0 to M4 main- sequence stars between the stellar Teff, radius, and luminosity to broad-band color indices and metallicity. These relations are valid for metallicities between [Fe/H] = -0.5 and +0.1 dex, and are accurate to \\sim2%, \\sim5%, and \\sim4% for Teff, radius, and luminosity, respectively. Our results show that it is necessary to use metallicity dependent transformations to convert colors into stellar Teff's, radii, and luminosities. We find no sensitivity to metallicity on relations between global stellar properties, e.g., TEFF-radius and TEFF-luminosity. Robust examinations of single star TEFF's and radii compared to evolutionary m...

Boyajian, Tabetha S; van Belle, Gerard; McAlister, Harold A; Brummelaar, Theo A ten; Kane, Stephen R; Muirhead, Phil; Jones, Jeremy; White, Russel; Schaefer, Gail; Ciardi, David; Henry, Todd; López-Morales, Mercedes; Ridgway, Stephen; Gies, Douglas; Jao, Wei-Chun; Rojas-Ayala, Bárbara; Parks, J Robert; Sturmann, Laszlo; Sturmann, Judit; Turner, Nils H; Farrington, Chris; Goldfinger, P J; Berger, David H
2012-01-01

86

Spectroscopy of Main Sequence Stars in Globular Clusters

  Astrophysics (arXiv)

Summary: Although globular clusters are generally chemically homogeneous, substantial abundance variations are sometimes seen even among unevolved main sequence stars, especially for the CNO group of elements. Multi-object intermediate-dispersion spectroscopic systems are now being used to determine the patterns of abundance variations for large samples of stars, while high dispersion spectrographs on 8m-class telescopes are providing good spectra of individual faint stars. In some circumstances the spectra of many similar stars can be combined to yield extremely high S/N spectra. The sample of clusters which has been studied remains small, but it seems that many of the more metal-rich clusters must have experienced some sort of self-enrichment, either in a proto-cluster stage, or through successive episodes of star formation or some other processes early in their lives. The metal-poor clusters provide equally clear evidence for internal mixing and dredge-up of processed material within evolving red giant stars.

Russell Cannon; Gary Da Costa; John Norris; Laura Stanford; Barry Croke
2002-10-15

87

Collisions and close encounters involving massive main-sequence stars

  Astrophysics (arXiv)

Summary: We study close encounters involving massive main sequence stars and the evolution of the exotic products of these encounters as common--envelope systems or possible hypernova progenitors. We show that parabolic encounters between low-- and high--mass stars and between two high--mass stars with small periastrons result in mergers on timescales of a few tens of stellar freefall times (a few tens of hours). We show that such mergers of unevolved low--mass stars with evolved high--mass stars result in little mass loss ($\\sim0.01$ M$_{\\odot}$) and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common--envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of $\\sim100$. If these remnants exist in very densely-populated environments ($n\\gtrsim10^{7}$ pc$^{-3}$), they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma--ray burst progenitors and that $\\sim0.1%$ of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma--ray bursts.

James E. Dale; Melvyn B. Davies
2006-02-02

88

Exo--Zodiacal Dust Levels for Nearby Main Sequence Stars

  CERN Preprints

Summary: The Keck Interferometer Nuller (KIN) was used to survey 25 nearby main sequence stars in the mid-infrared, in order to assess the prevalence of warm circumstellar (exozodiacal) dust around nearby solar-type stars. The KIN measures circumstellar emission by spatially blocking the star but transmitting the circumstellar flux in a region typically 0.1 - 4 AU from the star. We find one significant detection (eta Crv), two marginal detections (gamma Oph and alpha Aql), and 22 clear non-detections. Using a model of our own Solar System's zodiacal cloud, scaled to the luminosity of each target star, we estimate the equivalent number of target zodis needed to match our observations. Our three zodi detections are eta Crv (1250 +/- 260), gamma Oph (200 +/- 80) and alpha Aql (600 +/- 200), where the uncertainties are 1-sigma. The 22 non-detected targets have an ensemble weighted average consistent with zero, with an average individual uncertainty of 160 zodis (1-sigma). These measurements represent the best limits to da...

Millan-Gabet, R; Mennesson, B; Traub, W A; Barry, R K; Danchi, W C; Kuchner, M; Ragland, S; Hrynevych, M; Woillez, J; Stapelfeldt, K; Bryden, G; Colavita, M M; Booth, A J
2011-01-01

89

Dust and Gas Debris Around Main Sequence Stars

  Astrophysics (arXiv)

Summary: Debris disks are dusty, gas-poor disks around main sequence stars (Backman & Paresce 1993; Lagrange, Backman & Artymowicz 2000; Zuckerman 2001). Micron-sized dust grains are inferred to exist in these systems from measurements of their thermal emission at infrared through millimeter wavelengths. The estimated lifetimes for circumstellar dust grains due to sublimation, radiation and corpuscular stellar wind effects are typically significantly smaller than the estimated ages for the stellar systems, suggesting that the grains are replenished from a reservoir, such as sublimation of comets or collisions between parent bodies. Since the color temperature for the excess emission is typically Tgr ~ 110 - 120 K, similar to that expected for small grains in the Kuiper Belt, these objects are believe to be generated by collisions between parent bodies analogous to Kuiper Belt objects in our solar system; however, a handful of systems possess warm dust, with Tgr > 300 K, at temperatures similar to the terrestrial planets. We describe the physical characteristics of debris disks, the processes that remove dust from disks, and the evidence for the presence of planets in debris disks. We also summarize observations of infalling comets toward beta Pictoris and measurements of bulk gas in debris disks.

Christine H. Chen
2005-11-22

90

Main-sequence fitting and the Hipparcos open cluster distance scale

  Astrophysics (arXiv)

Summary: The difference between the Hipparcos open cluster distance scale and that obtained from main sequence fitting is examined. The two color main sequence fitting technique of Pinsonneault et al. (1998) is extended to NGC 2516, NGC 6475, and NGC 6633. The error sources for main sequence fitting are examined, and possible evidence for systematic errors in the Hipparcos parallaxes is discussed.

M. H. Pinsonneault; D. M. Terndrup; Y. Yuan
1999-11-30

91

Main Sequence Evolution As the hydrogen in its core is slowly used up, a star's

  Physics Websites

Summary: Main Sequence Evolution As the hydrogen in its core is slowly used up, a star's `thermostat photosphere hydrogen- `burning' core A Main-Sequence Star... photosphere hydrogen- `burning' shell inert the Main Sequence Once hydrogen starts burning in a shell, a star becomes larger, redder, and much more

Barnes, Joshua Edward

92

Detached white dwarf main-sequence star binaries

  Astrophysics (arXiv)

Summary: We considered the formation of detached white dwarf main-sequence star (WDMS) binaries through seven evolutionary channels subdivided according to the evolutionary process that gives rise to the formation of the white dwarf or its helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type evolution), dynamically unstable Roche-lobe overflow (common-envelope evolution), or stellar winds (single star evolution). We examine the sensitivity of the population to changes in the amount of mass lost from the system during stable Roche-lobe overflow, the common-envelope ejection efficiency, and the initial mass ratio or initial secondary mass distribution. In the case of a flat initial mass ratio distribution, the local space density of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to 10^{-4}/pc^3 when the initial mass ratio distribution is approximately proportional to the inverse of the initial mass ratio. More than 75% of the WDMS binary population stems from wide systems in which both components evolve as if they were single stars. The remaining part of the population is dominated by systems in which the white dwarf is formed in a common-envelope phase. The birthrate of WDMS binaries forming through a common-envelope phase is about 10 times larger than the birthrate of WDMS binaries forming through a stable Roche-lobe overflow phase. The ratio of the number of helium white dwarf systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf systems derived from large samples of observed WDMS binaries by, e.g., future planet-search missions such as SuperWASP, COROT, and Kepler may furthermore constrain the common-envelope ejection efficiency.

B. Willems; U. Kolb
2004-03-03

93

Coronal structure geometries on pre-main sequence stars

  Astrophysics (arXiv)

Summary: We have re-analyzed using a hydrodynamic model large flaring events on three different categories of pre-main sequence (PMS) stars: the young stellar object (YSO) YLW 15, the classical T Tauri star (CTTS) LkHalpha92, the weak-line T Tauri star (WTTS) V773 Tau, and the WTTS HD 283572 (the first three objects were observed by ASCA, the last by ROSAT; all have been previously reported in the literature). The first three flares were previously analyzed on the basis of the quasi-static model mostly used up to now, consistently yielding large loops (L >= R*) and no evidence of sustained heating. Our hydrodynamic modeling approach, however, shows that the size of the flaring regions must be much smaller (L <=R*) and moreover this method shows in all cases evidence of vigorous sustained heating during the flare decay, so that the decay of the observed light curve actually reflects the temporal profile of the heating rather than that of the free decay of the heated loop(s). The events on the protostar YLW 15 have durations comparable to the stellar rotation period, so that their limited size and their lack of self-eclipses give evidence of a polar location on the star. This is in contrast with the recently advanced hypothesis that these flares are due to long loops spanning the region between the star and the accretion disk. In general, the present analysis shows that flaring coronae on PMS stars have a structure similar to the coronae on older active stars.

F. Favata; G. Micela; F. Reale
2001-04-19

94

Pre-main sequence spectroscopic binaries suitable for VLTI observations

  Astrophysics (arXiv)

Summary: A severe problem of the research in star-formation is that the masses of young stars are almost always estimated only from evolutionary tracks. Since the tracks published by different groups differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, with the VLTI it is now possible to set constrains on the tracks by determining the masses of many young binary stars precisely. In order to use the VLTI efficiently, a first step is to find suitable targets, which is the purpose of this work. Given the distance of nearby star-forming regions, suitable VLTI targets are binaries with orbital periods between at least 50 days, and few years. Although a number of surveys for detecting spectroscopic binaries have been carried out, most of the binaries found so far have periods which are too short. We thus surveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, rho Ophiuci star-forming regions in order to search for spectroscopic binaries with periods longer than 50 days, which are suitable for the VLTI observations. As a result of the 8 years campaign we discovered 8 binaries with orbital periods longer than 50 days. Amongst the newly discovered long period binaries is CS Cha, which is one of the few classical T Tauri stars with a circumbinary disk. The survey is limited to objects with masses higher than 0.1 to 0.2 Modot for periods between 1 and 8 years. We find that the frequency of binaries with orbital periods < 3000 days is of 20+/-5 percent. The frequency of long and short period pre-main sequence spectroscopic binaries is about the same as for stars in the solar neighbourhood. In total 14 young binaries are now known which are suitable for mass determination with the VLTI.

E. W. Guenther; M. Esposito; R. Mundt; E. Covino; J. M. Alcala; F. Cusano; B. Stecklum
2007-02-09

95

Dark stars at the Galactic centre - the main sequence

  HEP - Phenomenology (arXiv)

Summary: In regions of very high dark matter density such as the Galactic centre, the capture and annihilation of WIMP dark matter by stars has the potential to significantly alter their evolution. We describe the dark stellar evolution code DarkStars, and present a series of grids of WIMP-influenced stellar models for main sequence stars. We describe changes in which occur as a function of the rate of energy injection by WIMPs, for stars of 0.3-2.0 solar masses and metallicities Z = 0.0003-0.02. We show what rates of energy injection can be obtained using realistic orbital parameters for stars at the Galactic centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. If there is a spike of dark matter induced by the supermassive black hole at the Galactic centre, single solar-mass stars following orbits with periods as long as 50 years and eccentricities as low as 0.9 could be significantly affected. Binary systems with similar periods about the Galactic centre could be affected on even less eccentric orbits. The most striking evidence of this scenario would be the existence of a binary consisting of a low-mass protostar and a higher-mass evolved star. The observation of low-mass stars and/or binaries on such orbits would either provide a detection of WIMP dark matter, or place stringent limits on the combination of the WIMP mass, spin-dependent nuclear-scattering cross-section, halo density and velocity distribution near the Galactic centre. In some cases, the limits on the WIMP mass and spin-dependent nuclear-scattering cross-section would be of comparable sensitivity to current direct-detection experiments.

Pat Scott; Malcolm Fairbairn; Joakim Edsjö
2009-09-15

96

A Tale of Two Anomalies: Depletion, Dispersion, and the Connection Between the Stellar Lithium Spread and Inflated Radii on the Pre-Main Sequence

  CERN Preprints

Summary: We investigate lithium depletion in standard stellar models (SSMs) and main sequence (MS) open clusters, and explore the origin of the Li dispersion in young, cool stars of equal mass, age and composition. We first demonstrate that SSMs accurately predict the Li abundances of solar analogs at the zero-age main sequence (ZAMS) within theoretical uncertainties. We then measure the rate of MS Li depletion by removing the [Fe/H]-dependent ZAMS Li pattern from three well-studied clusters, and comparing the detrended data. MS depletion is found to be mass dependent, in the sense of more depletion at low mass. A dispersion in Li abundance at fixed $T_{\\rm eff}$ is nearly universal, and sets in by $\\sim$200 Myr. We discuss mass and age dispersion trends, and the pattern is mixed. We argue that metallicity impacts the ZAMS Li pattern, in agreement with theoretical expectations but contrary to the findings of some previous studies, and suggest Li as a test of cluster metallicity. Finally, we argue that a radius dispers...

Somers, Garrett
2014-01-01

97

Habitability of Super-Earth Planets around Main-Sequence Stars including Red Giant Branch Evolution: Models based on the Integrated System Approach

  CERN Preprints

Summary: In a previous study published in Astrobiology, we focused on the evolution of habitability of a 10 M_E super-Earth planet orbiting a star akin to the Sun. This study was based on a concept of planetary habitability in accordance to the integrated system approach that describes the photosynthetic biomass production taking into account a variety of climatological, biogeochemical, and geodynamical processes. In the present study, we pursue a significant augmentation of our previous work by considering stars with zero-age main sequence masses between 0.5 and 2.0 M_sun with special emphasis on models of 0.8, 0.9, 1.2 and 1.5 M_sun. Our models of habitability consider again geodynamical processes during the main-sequence stage of these stars as well as during their red giant branch evolution. Pertaining to the different types of stars, we identify so-called photosynthesis-sustaining habitable zones (pHZ) determined by the limits of biological productivity on the planetary surface. We obtain various sets of solution...

Cuntz, M; Schroeder, K -P; Bounama, C; Franck, S
2011-01-01

98

Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution

  Astrophysics (arXiv)

Summary: Boron abundances have been derived for seven main sequence B-type stars from HST STIS spectra around the B III 2066 A line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but it is clearly depleted. In the other four stars, boron is undetectable implying depletions of 1 to 2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. Also, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally-induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near solar iron-group abundances, as expected for young stars in the solar neighborhood. We have also analysed the halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H]<2.5. These and other published abundances are used to infer the star's evolutionary status as a post-AGB star.

K. A. Venn; A. M. Brooks; D. L. Lambert; M. Lemke; N. Langer; D. J. Lennon; F. P. Keenan
2001-08-15

99

Repetitive elements underestimated? Repeat sequences --mainly transposable elements --have

  Biology and Medicine Websites

Summary: sequence evolution. Cell 147, 1537­1550 (2011) EVOLUTION Paths of antibiotic resistance These authors developed a system to study the gradual evolution of antibiotic resistance. In their `morbidostat' system growth inhibition (that is, as resistance in the culture increases, so does the antibiotic concentration

Castoe, Todd A.

100

Evolution of a 3 \\msun star from the main sequence to the ZZ Ceti stage: the role played by element diffusion

  Astrophysics (arXiv)

Summary: The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to the white dwarf formation. In particular, we follow the evolution of a 3 \\msun model from the zero-age main sequence (the adopted metallicity is Z=0.02) all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. Emphasis is placed on the evolution of the chemical abundance distribution due to diffusion processes and the role played by hydrogen burning during the white dwarf evolution. Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother as compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt-Vaisala frequency.

L. G. Althaus; A. M. Serenelli; A. H. Corsico; O. G. Benvenuto
2001-10-29

101

Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement

  CERN Preprints

Summary: Having shown in a recent paper that the main sequence of omega Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is_less_ metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

Piotto, G; Bedin, L R; Gratton, R; Cassisi, S; Momany, Y; Recio-Blanco, A; Lucatello, S; Anderson, J; King, I R; Pietrinferni, A; Carraro, G; Piotto, Giampaolo; Villanova, Sandro; Bedin, Luigi R.; Gratton, Raffaele; Cassisi, Santi; Momany, Yazan; Recio-Blanco, Alejandra; Lucatello, Sara; Anderson, Jay; King, Ivan R.; Pietrinferni, Adriano; Carraro, Giovanni
2004-01-01

102

Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement

  Astrophysics (arXiv)

Summary: Having shown in a recent paper that the main sequence of omega Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is_less_ metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

Giampaolo Piotto; Sandro Villanova; Luigi R. Bedin; Raffaele Gratton; Santi Cassisi; Yazan Momany; Alejandra Recio-Blanco; Sara Lucatello; Jay Anderson; Ivan R. King; Adriano Pietrinferni; Giovanni Carraro; .
2004-12-01

103

Rotational mixing in tidally locked massive main-sequence binaries

  CERN Preprints

Summary: One of the main uncertainties in evolutionary calculations of massive stars is the efficiency of internal mixing. It changes the chemical profile inside the star and can therefore affect the structure and further evolution. We demonstrate that eclipsing binaries, in which the tides synchronize the rotation period of the stars and the orbital period, constitute a potentially strong test for the efficiency of rotational mixing. We present detailed stellar evolutionary models of massive binaries assuming the composition of the Small Magellanic Cloud. In these models we find enhancements in the surface nitrogen abundance of up to 0.6 dex.

de Mink, S E; Langer, N; Pols, O R
2008-01-01

104

Rotational mixing in tidally locked massive main-sequence binaries

  Astrophysics (arXiv)

Summary: One of the main uncertainties in evolutionary calculations of massive stars is the efficiency of internal mixing. It changes the chemical profile inside the star and can therefore affect the structure and further evolution. We demonstrate that eclipsing binaries, in which the tides synchronize the rotation period of the stars and the orbital period, constitute a potentially strong test for the efficiency of rotational mixing. We present detailed stellar evolutionary models of massive binaries assuming the composition of the Small Magellanic Cloud. In these models we find enhancements in the surface nitrogen abundance of up to 0.6 dex.

S. E. de Mink; M. Cantiello; N. Langer; O. R. Pols
2008-11-24

105

ROSAT and Hipparcos Observations of Isolated Pre--MainSequence Stars near HD 98800

  Physics Websites

Summary: ROSAT and Hipparcos Observations of Isolated Pre--Main­Sequence Stars near HD 98800 Eric L. N young stars HD 98800 and CD \\Gamma33 ffi 7795. Pointed ROSAT observations show that their X­ray properties, including X­ray luminosity and variability, are consistent with those of pre--main­ sequence (PMS

Jensen, Eric L. N.

106

Comparative seismology of pre- and main sequence stars in the instability strip

  Astrophysics (arXiv)

Summary: Pulsational properties of 1.8 M$_{\\odot}$ stellar models covering the latest stages of contraction toward the main sequence up to early hydrogen burning phases are investigated by means of linear nonadiabatic analyses. Results confirm that pre-main sequence stars (pms) which cross the classical instability strip on their way toward the main sequence are pulsationally unstable with respect to the classical opacity mechanisms. For both pms and main sequence types of models in the lower part of the instability strip, the unstable frequency range is found to be roughly the same. Some non-radial unstable modes are very sensitive to the deep internal structure of the star. It is shown that discrimination between pms and main sequence stages is possible using differences in their oscillation frequency distributions in the low frequency range.

Marian Suran; MarieJo Goupil; Annie Baglin; Yveline Lebreton; Claude Catala
2001-04-19

107

Tracing early stellar evolution with asteroseismology: pre-main sequence stars in NGC 2264

  CERN Preprints

Summary: Asteroseismology has been proven to be a successful tool to unravel details of the internal structure for different types of stars in various stages of their main sequence and post-main sequence evolution. Recently, we found a relation between the detected pulsation properties in a sample of 34 pre-main sequence (pre-MS) delta Scuti stars and the relative phase in their pre-MS evolution. With this we are able to demonstrate that asteroseismology is similarly powerful if applied to stars in the earliest stages of evolution before the onset of hydrogen core burning.

Zwintz, Konstanze; Ryabchikova, Tatiana; Guenther, David; Aerts, Conny
2014-01-01

108

Low mass X-ray binaries with pre-main sequence companions

  Astrophysics (arXiv)

Summary: In this Letter we examine the idea that a subset of short-period black-hole low-mass X-ray binaries could be powered by the mass transfer from pre-main sequence donors. As the star contracts towards the main sequence, the strong magnetic fields operate the magnetic braking which dissipates the orbital angular momentum, driving the binary to contact. We show that the periods and apparent donor spectral classes of the X-ray binaries with a pre-main sequence donor agree better with the available observations of black hole X-ray binaries than those of binaries with a main-sequence donor. This mechanism also explains, without the need for additional hypotheses, the roughly primordial abundance of Li detected in donor companions of black hole X-ray candidates in our Galaxy.

Natalia Ivanova
2006-11-07

109

YSOVAR: SIX PRE-MAIN-SEQUENCE ECLIPSING BINARIES IN THE ORION NEBULA CLUSTER

  MIT - DSpace

Summary: Eclipsing binaries (EBs) provide critical laboratories for empirically testing predictions of theoretical models of stellar structure and evolution. Pre-main-sequence (PMS) EBs are particularly valuable, both due to their ...

Morales-Calderón, M.

110

Comparing the observational instability regions for pulsating pre-main sequence and classical $?$ Scuti stars

  Astrophysics (arXiv)

Summary: A comparison of the hot and cool boundaries of the classical instability strip with observations has been an important test for stellar structure and evolution models of post- and main sequence stars. Over the last few years, the number of pulsating pre-main sequence (PMS) stars has increased significantly: 36 PMS pulsators and candidates are known as of June 2007. This number allows to investigate the location of the empirical PMS instability region and to compare its boundaries to those of the classical (post- and main sequence) instability strip. Due to the structural differences of PMS and (post-)main sequence stars, the frequency spacings for nonradial modes will be measurably different, thus challenging asteroseismology as a diagnostic tool.

K. Zwintz
2007-11-16

111

The main sequence from F to K stars of the solar neighbourhood in SDSS colours

  Astrophysics (arXiv)

Summary: For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour-magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g',r',i' show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems especially in (r-i)-colour.

A. Just; H. Jahreiss
2008-08-15

112

The main sequence from F to K stars of the solar neighbourhood in SDSS colours

  CERN Preprints

Summary: For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour-magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g',r',i' show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems especially in (r-i)-colour.

Just, A
2008-01-01

113

EXPORT optical photometry and polarimetry of Vega-type and pre-main sequence stars

  CERN Preprints

Summary: This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5m Nordic Optical Telescope. The database consists of multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars. An investigation of the polarization variability indicates that 22 objects are variable at the 3sigma level in our data. All these objects are pre-main sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the main sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostly indicative of the UXOR-type behaviour; the objects show highest polarization when the brightness is at minimum. We add seven new objects to the class of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkHa 234, KK Oph and RY Ori). The main reason for their discovery is the fact that our data-set is the largest in its kind, indicating that many more young UXOR-type pre-main sequence stars remain to be discovered. The set ...

Oudmaijer, R D; Eiroa, C
2001-01-01

114

Lithium evolution in metal-poor stars: from Pre-Main Sequence to the Spite plateau

  CERN Preprints

Summary: Lithium abundance derived in metal-poor main sequence stars is about three times lower than the value of primordial Li predicted by the standard Big Bang nucleosynthesis when the baryon density is taken from the CMB or the deuterium measurements. This disagreement is generally referred as the lithium problem. We here reconsider the stellar Li evolution from the pre-main sequence to the end of the main sequence phase by introducing the effects of convective overshooting and residual mass accretion. We show that $^7$Li could be significantly depleted by convective overshooting in the pre-main sequence phase and then partially restored in the stellar atmosphere by a tail of matter accretion which follows the Li depletion phase and that could be regulated by EUV photo-evaporation. By considering the conventional nuclear burning and microscopic diffusion along the main sequence we can reproduce the Spite plateau for stars with initial mass $m_0=0.62 - 0.80 M_{\\odot}$, and the Li declining branch for lower mass dwa...

Fu, Xiaoting; Molaro, Paolo; Marigo, Paola
2015-01-01

115

An Absence of Gaps in the Main Sequence Population of Field Stars

  Astrophysics (arXiv)

Summary: Using high precision parallaxes from the Hipparcos catalog, we construct H-R diagrams for two samples of bright stars. The first is a magnitude-limited sample that is over 90% complete and uses uniform photometry from the Catalog of WBVR Magnitudes of Northern Sky Bright Stars (declination above -14 deg). This sample shows a smooth distribution of stars along the main sequence, with no detectable gaps. The second contains all of the stars closer than 100 parsecs in the Hipparcos catalog with declination less than -12 deg. Uniform spectroscopy from the Michigan Spectral Survey shows that some stars which appear on the main sequence in the H-R diagram, particularly those in the 0.2 < B-V < 0.3 region that has been labeled the Bohm-Vitense gap, are classified as giants by the MK system of spectral classification. Other gaps that have been identified in the main sequence are also affected by such classification criteria. This analysis casts doubt on the existence of the Bohm-Vitense gap, which is thought to result from the sudden onset of convection in stars. The standard identification of main sequence stars with luminosity class V, and giants with luminosity class III, must be reconsidered for some spectral types. The true nature of the stars that lie on the main sequence in the H-R diagram, but which do not have luminosity class V designations, remains to be investigated.

Heidi Jo Newberg; Brian Yanny
1998-02-14

116

The theoretical calculation of the Rossby number and the `non-local' convective overturn time for pre-main sequence and early post-main sequence stars

  Astrophysics (arXiv)

Summary: This paper provides estimates of convective turnover time scales for Sun-like stars in the pre-main sequence and early post-main sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory which were previously impossible with semi-empirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution.

Yong -Cheol Kim; Pierre Demarque
1995-07-24

117

Isolating the pre-main sequence in Collinder34, NGC3293, NGC3766 and NG6231

  CERN Preprints

Summary: We employed field star decontaminated 2MASS photometry to study four nearby optical embedded clusters -- Collinder34, NGC3293, NGC3766 and NGC6231 -- obtaining deep colour-magnitude diagrams and stellar radial density profiles. We found what seems to be pre-main sequences detached in different amounts from main sequences in these diagrams. The structural analysis of each cluster revealed different radial distributions for these two sequences. We argued that the detached evolutionary sequences in our sample may be evidence of sequential star formation. Finally, we compared the sample cluster parameters with those of other young clusters in the literature and point out evidence that NGC3766 and NGC6231 might be evolving to OB associations.

Saurin, T A; Bonatto, C
2015-01-01

118

Rotating Stellar Models Can Account for the Extended Main Sequence Turnoffs in Intermediate Age Clusters

  CERN Preprints

Summary: We show that the extended main sequence turnoffs seen in intermediate age Large Magellanic Cloud (LMC) clusters, often attributed to age spreads of several hundred Myr, may be easily accounted for by variable stellar rotation in a coeval population. We compute synthetic photometry for grids of rotating stellar evolution models and interpolate them to produce isochrones at a variety of rotation rates and orientations. An extended main sequence turnoff naturally appears in color-magnitude diagrams at ages just under 1 Gyr, peaks in extent between ~1 and 1.5 Gyr, and gradually disappears at around 2 Gyr in age. We then fit our interpolated isochrones by eye to four LMC clusters with very extended main sequence turnoffs: NGC 1783, 1806, 1846, and 1987. In each case, stellar populations with a single age and metallicity can comfortably account for the observed extent of the turnoff region.

Brandt, Timothy D
2015-01-01

119

An Assessment of Dynamical Mass Constraints on Pre-Main Sequence Evolutionary Tracks

  Astrophysics (arXiv)

Summary: [abridged] We have assembled a database of stars having both masses determined from measured orbital dynamics and sufficient spectral and photometric information for their placement on a theoretical HR diagram. Our sample consists of 115 low mass (M < 2.0 Msun) stars, 27 pre-main sequence and 88 main sequence. We use a variety of available pre-main sequence evolutionary calculations to test the consistency of predicted stellar masses with dynamically determined masses. Despite substantial improvements in model physics over the past decade, large systematic discrepancies still exist between empirical and theoretically derived masses. For main-sequence stars, all models considered predict masses consistent with dynamical values above 1.2 Msun, some models predict consistent masses at solar or slightly lower masses, and no models predict consistent masses below 0.5 Msun but rather all models systematically under-predict such low masses by 5-20%. The failure at low masses stems from the poor match of most models to the empirical main-sequence below temperatures of 3800 K where molecules become the dominant source of opacity and convection is the dominant mode of energy transport. For the pre-main sequence sample we find similar trends. There is generally good agreement between predicted and dynamical masses above 1.2 Msun for all models. Below 1.2 Msun and down to 0.3 Msun (the lowest mass testable) most evolutionary models systematically under-predict the dynamically determined masses by 10-30% on average with the Lyon group models (e.g. Baraffe et al. 1998) predicting marginally consistent masses *in the mean* though with large scatter.

Lynne A. Hillenbrand; Russel J. White
2003-12-06

120

Angular Momentum Transport by Internal Gravity Waves. I - Pop I Main Sequence Stars

  Astrophysics (arXiv)

Summary: We examine the generation of gravity waves by the surface convection zone of low-mass main sequence stars with solar metallicity. It is found that the total momentum luminosity in waves rises with stellar mass, up to the quasi-disappearance of the convection zone around 6500K (corresponding to a mass of about 1.4 Msun for solar metallicity) where the luminosity drastically drops. We calculate the net momentum extraction associated with these waves and explain how the calculated mass dependence helps resolve the enigma of the Li dip in terms of rotational mixing, forming a coherent picture of mixing in all main sequence stars.

Suzanne Talon; Corinne Charbonnel
2003-05-09

121

Lithium abundances in main-sequence F stars and sub-giants

  Astrophysics (arXiv)

Summary: The application to main-sequence stars of the rotation-induced mixing theory in the presence of mu-gradients leads to partial mixing in the lithium destruction region, not visible in the atmosphere. The induced lithium depletion becomes visible in the sub-giant phase as soon as the convective zone deepens enough. This may explain why the observed " lithium dilution " is smoother and the final dilution factor larger than obtained in standard models, while the lithium abundance variations are very small on the main sequence.

Jose Dias do Nascimento Jr; Sylvie Theado; Sylvie Vauclair
2000-02-17

122

EXPORT: optical photometry and polarimetry of Vega-type and pre-main sequence stars

  Astrophysics (arXiv)

Summary: This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5m Nordic Optical Telescope. The database consists of multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars. An investigation of the polarization variability indicates that 22 objects are variable at the 3sigma level in our data. All these objects are pre-main sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the main sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostly indicative of the UXOR-type behaviour; the objects show highest polarization when the brightness is at minimum. We add seven new objects to the class of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkHa 234, KK Oph and RY Ori). The main reason for their discovery is the fact that our data-set is the largest in its kind, indicating that many more young UXOR-type pre-main sequence stars remain to be discovered. The set of Vega-like systems has been investigated for the presence of intrinsic polarization. As they lack variability, this was done using indirect methods, and apart from the known case of BD +31.643, the following stars were found to be strong candidates to exhibit polarization due to the presence of circumstellar disks: 51 Oph, BD +31.643C, HD 58647 and HD 233517.

R. D. Oudmaijer; J. Palacios; C. Eiroa; the EXPORT collaboration
2001-10-30

123

Energy transport, overshoot, and mixing in the atmospheres of M-type main- and pre-main-sequence objects

  Astrophysics (arXiv)

Summary: We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence (PMS) objects. Despite the complex chemistry encountered in these cool atmospheres a reasonably accurate representation of the radiative transfer is possible, even in the context of time-dependent and three-dimensional models. The models provide detailed information about the morphology of M-type granulation and statistical properties of the convective surface flows. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency was expressed in terms of an equivalent mixing-length parameter alpha in the formulation of mixing-length theory (MLT) given by Mihalas (1978). Alpha amounts to values around 2 for matching the entropy of the deep, adiabatically stratified regions of the convective envelope, and lies between 2.5 and 3.0 for matching the thermal structure of the deep photosphere. For current spectral analysis of PMS objects this implies that MLT models based on alpha=2.0 overestimate the effective temperature by 100 K and surface gravities by 0.25 dex. The average thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e., stays close to radiative equilibrium conditions. Our models suggest that the rate of mixing by convective overshoot declines exponentially with geometrical distance to the Schwarzschild stability boundary. It increases at given effective temperature with decreasing gravitational acceleration.

H. -G. Ludwig; F. Allard; P. H. Hauschildt
2006-08-12

124

Are the red dwarfs in cataclysmic variables main-sequence stars?

  Astrophysics (arXiv)

Summary: We show that the secondaries in short-period cataclysmic variables with orbital periods $P < 3$ hr are close to the solar-abundance main sequence defined by single field stars. In cataclysmic variables with $P > 3$ hr, the earliest spectral types at a given period correspond to main sequence stars, while the majority of secondaries have later spectral types. Possible causes are nuclear evolution prior to mass transfer and lack of thermal equilibrium due to mass transfer. A comparison with evolutionary sequences obtained with up--to--date stellar models implies unusually high transfer rates and a large fraction of systems with evolved donors. There is no evidence for a secondary of low metallicity in any of the well-studied cataclysmic variables.

K. Beuermann; I. Baraffe; U. Kolb; M. Weichhold
1998-09-18

125

Stellar Models of Multiple Populations in Globular Clusters. I. The Main Sequence of NGC 6752

  CERN Preprints

Summary: We present stellar atmosphere and evolution models of main sequence stars in two stellar populations of the Galactic globular cluster NGC 6752. These populations represent the two extremes of light-element abundance variations in the cluster. NGC 6752 is a benchmark cluster in the study of multiple stellar populations because of the rich array of spectroscopic abundances and panchromatic Hubble Space Telescope photometry. The spectroscopic abundances are used to compute stellar atmosphere and evolution models. The synthetic spectra for the two populations show significant differences in the ultraviolet and, for the coolest temperatures, in the near-infrared. The stellar evolution models exhibit insignificant differences in the H-R diagram except on the lower main sequence. The appearance of multiple sequences in the colour-magnitude diagrams (CMDs) of NGC 6752 is almost exclusively due to spectral effects caused by the abundance variations. The models reproduce the observed splitting and/or broadening of sequ...

Dotter, Aaron; Conroy, Charlie; Milone, A P; Marino, A F; Yong, David
2014-01-01

126

THE STARS OF THE LOW PART OF MAIN SEQUENCE T.V. Mishenina 1

  Computer Technologies and Information Sciences Websites

Summary: THE STARS OF THE LOW PART OF MAIN SEQUENCE T.V. Mishenina 1 , S.I. Belik 1 , I.A. Usenko 1 , O in 133 stars belonging to the low part of MS have been determined. Among them about 30 stars are the variable stars of BY Dra type, for which the determination of the chemical composition was made

Boyer, Edmond

127

Fourteen new eclipsing white dwarf plus main-sequence binaries from the SDSS and Catalina surveys

  CERN Preprints

Summary: We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) binaries in the light curves of the Catalina surveys. We use a colour selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M type main-sequence stars. We identify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find 3 candidate eclipsing systems, 2 main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar periods, near 4 hours, and spectral types M2--3, which are very close to Roche-lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower ...

Parsons, S G; Gaensicke, B T; Rebassa-Mansergas, A; Schreiber, M R; Marsh, T R; Dhillon, V S; Littlefair, S P; Drake, A J; Bours, M C P; Breedt, E; Copperwheat, C M; Hardy, L K; Buisset, C; Prasit, P; Ren, J J
2015-01-01

128

Convection, Protostellar Collapse, Deuterium Burning and PreMain Sequence Tracks

  Physics Websites

Summary: formation and early stellar evolution Present stellar evolution theory mostly deals with stars are usually neglected. The starting point of stellar evolution calculations is the pre­main­sequence. One for stellar evolution, i.e. the masses, radii and the internal structure of young stars at the moment when

Wuchterl, Günther

129

Main-sequence stars masquerading as Young Stellar Objects in the central molecular zone

  CERN Preprints

Summary: In contrast to most other galaxies, star-formation rates in the Milky Way can be estimated directly from Young Stellar Objects (YSOs). In the Central Molecular Zone (CMZ) the star-formation rate calculated from the number of YSOs with 24 microns emission is up to order of magnitude higher than the value estimated from methods based on diffuse emission (such as free-free emission). Whether this effect is real or whether it indicates problems with either or both star formation rate measures is not currently known. In this paper, we investigate whether estimates based on YSOs could be heavily contaminated by more evolved objects such as main-sequence stars. We present radiative transfer models of YSOs and of main-sequence stars in a constant ambient medium which show that the main-sequence objects can indeed mimic YSOs at 24 microns. However, we show that in some cases the main-sequence models can be marginally resolved at 24 microns, whereas the YSO models are always unresolved. Based on the fraction of resolve...

Koepferl, Christine M; Morales, Esteban F E; Johnston, Katharine G
2014-01-01

130

IDENTIFICATION OF MAIN-SEQUENCE STARS WITH MID-INFRARED EXCESSES USING GLIMPSE: PICTORIS ANALOGS?

  Physics Websites

Summary: IDENTIFICATION OF MAIN-SEQUENCE STARS WITH MID-INFRARED EXCESSES USING GLIMPSE: #12; PICTORIS as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks

Clemens, Dan

131

Incidence and survival of remnant disks around main-sequence stars

  CERN Preprints

Summary: We present photometric ISO 60 and 170um measurements, complemented by some IRAS data at 60um, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Of the stars younger than 400 Myr one in two has a disk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their dis...

Habing, H J; De Muizon, M J; Laureijs, R J; Kessler, M F; Leech, K J; Metcalfe, L; Salama, A; Siebenmorgen, R; Trams, N R; Bouchet, P
2000-01-01

132

Incidence and survival of remnant disks around main-sequence stars

  Astrophysics (arXiv)

Summary: We present photometric ISO 60 and 170um measurements, complemented by some IRAS data at 60um, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Of the stars younger than 400 Myr one in two has a disk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their disk in a similar way: some stars as old as the Sun still have significant disks.

H. J. Habing; C. Dominik; M. Jourdain de Muizon; R. J. Laureijs; M. F. Kessler; K. Leech; L. Metcalfe; A. Salama; R. Siebenmorgen; N. Trams; P. Bouchet
2000-11-07

133

Convection and light element evolution from pre--main sequence to the upper asymptotic giant branch

  Physics Websites

Summary: Convection and light element evolution from pre--main sequence to the upper asymptotic giant branch aspects of the physics of mixing in stars: convection model, mixing timescales, mixing at the convective interest for this conference, namely: 1) the convection model: how do we compute fluxes and define

D'Antona, Francesca

134

Microwave emission from Xray bright solarlike stars: The FG main sequence and beyond

  Physics Websites

Summary: Journal 1 #12; Abstract. A sample of F and G main sequence stars and slightly evolved F and G stars (RASS), has been observed in microwaves to search for coronae with strong heating and populations modulations agree with the optically measured rotation. On the other hand, three very old stars

Guedel, Manuel

135

An Assessment of Dynamical Mass Constraints on Pre-Main Sequence Evolutionary Tracks

  CERN Preprints

Summary: [abridged] We have assembled a database of stars having both masses determined from measured orbital dynamics and sufficient spectral and photometric information for their placement on a theoretical HR diagram. Our sample consists of 115 low mass (M < 2.0 Msun) stars, 27 pre-main sequence and 88 main sequence. We use a variety of available pre-main sequence evolutionary calculations to test the consistency of predicted stellar masses with dynamically determined masses. Despite substantial improvements in model physics over the past decade, large systematic discrepancies still exist between empirical and theoretically derived masses. For main-sequence stars, all models considered predict masses consistent with dynamical values above 1.2 Msun, some models predict consistent masses at solar or slightly lower masses, and no models predict consistent masses below 0.5 Msun but rather all models systematically under-predict such low masses by 5-20%. The failure at low masses stems from the poor match of most mode...

Hillenbrand, L A; Hillenbrand, Lynne A.; White, Russel J.
2004-01-01

136

Biases in the Main Sequence Fitting Distances to Globular Clusters based on the Hipparcos Catalogue

  Astrophysics (arXiv)

Summary: We discuss the different biases affecting the sample of field subdwarfs selected from the Hipparcos Catalogue, and used in the Main Sequence Fitting technique to derive distances to Galactic Globular Clusters. The adopted average corrections significantly affect the derived distance moduli, explaining the differences among various groups using this technique.

Eugenio Carretta; Raffaele G. Gratton; Gisella Clementini; Flavio Fusi Pecci
1998-11-04

137

Physical Properties of 90 AU to 250 AU Pre-Main-Sequence Binaries

  Astrophysics (arXiv)

Summary: We have analyzed photometric and spectroscopic data of a sample of 14 spatially resolved pre-main-sequence binaries (separations 0.6" to 1.7") in the nearby (150 pc) low-mass star-forming regions of Chamaeleon, Lupus, and rho Ophiuchi. The spectroscopic data have been obtained with the ESO New Technology Telescope (NTT) at La Silla under subarcsec seeing conditions. All binaries (originally unresolved) were identified as pre-main-sequence stars based on their strong H-alpha emission -- which classifies them as classical T Tauri stars -- and their association with dark clouds. One of the presumed binaries turned out to be a likely chance projection with the `primary' showing neither H-alpha emission nor Li absorption. Using the spectral A index (as defined by Kirkpatrick et al. 1991), which measures the strength of the CaH band at 697.5nm relative to the nearby continuum, as a luminosity class indicator, we could show that the classical T Tauri stars in our sample tend to be close to luminosity class V. Eight out of the 14 pairs could be placed on an H-R diagram. A comparison with theoretical pre-main-sequence evolutionary tracks yields that for all pairs the individual components appear to be coeval within the observational errors. This finding is similar to Hartigan et al. (1994) who detected that two third of the wider pairs with separations from 400 AU to 6000 AU are coeval. However, unlike Hartigan et al. for the wider pairs, we find no non-coeval pairs among our sample. Thus, the formation mechanism for a significant fraction of the wider pre-main-sequence binaries might be different from that of closer pre-main-sequence binaries. All of the latter appear to have formed simultaneously.

Wolfgang Brandner; Hans Zinnecker
1996-10-16

138

The Stability of Massive Main Sequence Stars as a Function of Metallicity

  CERN Preprints

Summary: We investigate the pulsational stability of massive (M >~ 120 Msun) main sequence stars of a range of metallicities, including primordial, Population III stars. We include a formulation of convective damping motivated by numerical simulations of the interaction between convection and periodic shear flows. We find that convective viscosity is likely strong enough to stabilize radial pulsations whenever nuclear-burning (the epsilon-mechanism) is the dominant source of driving. This suggests that massive main sequence stars with Z ~ 2 x 10^-3), the dominant pulsational driving arises due to the kappa-mechanism arising from the iron-bump in opacity and is strong enough to overcome convective damping. Our results highlight that even for oscillations with periods a few orders of magnitude shorter than the outer convective turnover time, the "frozen-in" approximation for the convection-oscillation interaction is inappropriate, and convective damping should be taken into account when assessing mode stability.

Shiode, Joshua H; Arras, Phil
2012-01-01

139

Nearby candidate dust-disk pre-main-sequence solar-mass stars

  Astrophysics (arXiv)

Summary: I have isolated a population of numerous F stars that appear to be pre-main-sequence (PMS). The candidate PMS stars have been identified using CM diagram, reddening, flux excess in the UV and near-infrared, and luminosity anomaly. Negative luminosity anomaly and excessive UV flux for many of these stars is suggestive of accretion disks, while the NIR excess is indicative in many cases of the presence of dust disk thermal emission. Observed overluminosity of many PMS candidates is consistent with their pre-main-sequence status. The bulk of the PMS candidates is located within 200 pc, exhibiting strong association with regions of star formation that are numerous between ~130 to 180 pc. The number of PMS candidates plummets redward of the spectral type ~ F5. This effect may provide important clues for understanding the evolution of PMS stars in the solar-mass range.

A. A. Suchkov
2002-10-25

140

Evolution of magnetic fields in stars across the upper main sequence

  Astrophysics (arXiv)

Summary: To properly understand the physics of upper main sequence stars it is particularly important to identify the origin of their magnetic fields. Recently, we confirmed that magnetic fields appear in Ap stars of mass below 3 M_sun only if they have already completed at least approximately 30% of their main-sequence lifetime. The absence of stars with strong magnetic fields close to the ZAMS might be seen as an argument against the fossil field theories. Here we present the results of our recent magnetic survey with FORS1 at the VLT in polarimetric mode of a sample of A, B and Herbig Ae stars with previously undetected magnetic fields and briefly discuss their significance for our understanding of the origin of the magnetic fields in intermediate mass stars.

S. Hubrig; M. Schoeller; P. North
2005-10-06

141

Inferring properties of small convective cores in main-sequence solar-like pulsators

  CERN Preprints

Summary: This work concerns the study of the properties of convective cores in main-sequence models of solar-like pulsators and what information they may hold about stellar ages. We verified that the maximum absolute frequency derivative of particular combinations of frequencies, which we name "the slopes", provides information on the relative size of the discontinuity in the sound-speed profile at the border of the convectively mixed region. Since the latter is related to the evolutionary state of stars, we show that for models with masses above $1.3\\,\\rm M_\\odot$, it may be possible to estimate the fraction of stellar main-sequence evolution from the slopes. Moreover, for models with masses below $1.2\\,\\rm M_\\odot$ we verified that it may be possible to use the slopes to discriminate against models with small amounts of core overshoot.

Brandão, I M; Christensen-Dalsgaard, J
2014-01-01

142

Discovery of ZZ Cetis in detached white dwarf plus main-sequence binaries

  CERN Preprints

Summary: We present the first results of a dedicated search for pulsating white dwarfs (WDs) in detached white dwarf plus main-sequence binaries. Candidate systems were selected from a catalogue of WD+MS binaries, based on the surface gravities and effective temperatures of the WDs. We observed a total of 26 systems using ULTRACAM mounted on ESO's 3.5m New Technology Telescope (NTT) at La Silla. Our photometric observations reveal pulsations in seven WDs of our sample, including the first pulsating white dwarf with a main-sequence companion in a post common envelope binary, SDSSJ1136+0409. Asteroseismology of these new pulsating systems will provide crucial insight into how binary interactions, particularly the common envelope phase, affect the internal structure and evolution of WDs. In addition, our observations have revealed the partially eclipsing nature of one of our targets, SDSSJ1223-0056.

Pyrzas, S; Hermes, J J; Copperwheat, C M; Rebassa-Mansergas, A; Dhillon, V S; Littlefair, S P; Marsh, T R; Parsons, S G; Savoury, C D J; Schreiber, M R; Barros, S C C; Bento, J; Breedt, E; Kerry, P
2014-01-01

143

Main-sequence progenitor configurations of the NN Ser candidate circumbinary planetary system are dynamically unstable

  CERN Preprints

Summary: Recent observations of the NN Serpentis post-common envelope binary system have revealed eclipse timing variations that have been attributed to the presence of two Jovian-mass exoplanets. Under the assumption that these planets are real and survived from the binary's Main Sequence state, we reconstruct initial binaries that give rise to the present NN Ser configuration and test the dynamical stability of the original system. Under standard assumptions about binary evolution, we find that survival of the planets through the entire Main Sequence life-time is very unlikely. Hence, we conclude that the planets are not survivors from before the Common Envelope phase, implying that either they formed recently out of material ejected from the primary, or that the observed signals are of non-planetary origin.

Mustill, Alexander J; Villaver, Eva; Veras, Dimitri; Davis, Philip J; Horner, Jonathan; Wittenmyer, Robert A
2013-01-01

144

ISO Observations of Pre-Main Sequence and Vega-type Stars

  CERN Preprints

Summary: I present an overview of the results obtained by the Infrared Space Observatory (ISO) on circumstellar material in pre-main sequence (PMS) stars. Results obtained for embedded YSOs, Herbig Ae/Be systems and T Tauri stars are reviewed and their connection to the disks around Vega-type systems is discussed. Although the gas contents of the PMS environment will also be discussed briefly, this review will mainly focus on the composition, mineralogy and evolution of dust in these systems, and the results will be compared to those found in other classes of objects, including solar system comets.

Van den Ancker, M E
2000-01-01

145

ISO Observations of Pre-Main Sequence and Vega-type Stars

  Astrophysics (arXiv)

Summary: I present an overview of the results obtained by the Infrared Space Observatory (ISO) on circumstellar material in pre-main sequence (PMS) stars. Results obtained for embedded YSOs, Herbig Ae/Be systems and T Tauri stars are reviewed and their connection to the disks around Vega-type systems is discussed. Although the gas contents of the PMS environment will also be discussed briefly, this review will mainly focus on the composition, mineralogy and evolution of dust in these systems, and the results will be compared to those found in other classes of objects, including solar system comets.

M. E. van den Ancker
2000-05-03

146

High Resolution Imaging of PHIBSS z~2 Main Sequence Galaxies in CO J=1-0

  CERN Preprints

Summary: We present Karl G. Jansky Very Large Array observations of the CO J=1-0 transition in a sample of four $z\\sim2$ main sequence galaxies. These galaxies are in the blue sequence of star-forming galaxies at their redshift, and are part of the IRAM Plateau de Bure HIgh-$z$ Blue Sequence Survey (PHIBSS) which imaged them in CO J=3-2. Two galaxies are imaged here at high signal-to-noise, allowing determinations of their disk sizes, line profiles, molecular surface densities, and excitation. Using these and published measurements, we show that the CO and optical disks have similar sizes in main-sequence galaxies, and in the galaxy where we can compare CO J=1-0 and J=3-2 sizes we find these are also very similar. Assuming a Galactic CO-to-H$_2$ conversion, we measure surface densities of $\\Sigma_{mol}\\sim1200$ M$_\\odot$pc$^{-2}$ in projection and estimate $\\Sigma_{mol}\\sim500-900$ M$_\\odot$pc$^{-2}$ deprojected. Finally, our data yields velocity-integrated Rayleigh-Jeans brightness temperature line ratios $r_{31}$ th...

Bolatto, A D; Leroy, A K; Tacconi, L J; Bouché, N; Schreiber, N M Förster; Genzel, R; Cooper, M C; Fisher, D B; Combes, F; García-Burillo, S; Burkert, A; Bournaud, F; Weiss, A; Saintonge, A; Wuyts, S; Sternberg, A
2015-01-01

147

Possible Evidence for Metal Accretion onto the Surfaces of Metal-Poor Main-Sequence Stars

  CERN Preprints

Summary: The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parametrized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the ...

Hattori, Kohei; Beers, Timothy C; Carollo, Daniela; Lee, Young Sun
2014-01-01

148

The main sequences of NGC2808: constraints on the early disc accretion scenario

  CERN Preprints

Summary: [Abridged] A new scenario --early disc accretion-- has been proposed very recently to explain the origin of the multiple population phenomenon in Galactic globular clusters. It envisages the possibility that a fraction of low- and very low-mass cluster stars may accrete the ejecta of interacting massive binary (and possibly also fast rotating massive) stars during the fully convective, pre-main sequence stage, to reproduce the CN and ONa anticorrelations observed among stars in individual clusters. This scenario is assumed to be able to explain the presence (and properties) of the multiple populations in the majority of globular clusters in the Milky Way. Here we have considered the well studied cluster NGC 2808, which displays a triple main sequence with well defined and separate He abundances. Knowledge of these abundances allowed us to put strong constraints on the He mass fraction and amount of matter to be accreted by low-mass pre-main sequence stars. We find that the minimum He mass fraction in the accr...

Cassisi, Santi
2013-01-01

149

Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars?

  Astrophysics (arXiv)

Summary: We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT, of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe star. Remarkably, evidence of surface abundance spots is detected in the spectra of HD 72106A. The magnetic fields of these objects display intensities of order 1 kG at the photospheric level, are ordered on global scales, and appear in approximately 10% of the stars studied. Based on these properties, the detected stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and 2.4 solar masses) represent additional contradictions to the hypothesis of Hubrig et al. (2000), who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. Finally, we fail to confirm claims by Hubrig et al. (2004) of magnetic fields in the Herbig Ae star HD 139614.

G. A. Wade; D. Drouin; S. Bagnulo; J. D. Landstreet; E. Mason; J. Silvester; E. Alecian; T. Bohm; J. -C. Bouret; C. Catala; J. -F. Donati
2005-09-12

150

A Distributed Population of Low Mass Pre-Main Sequence Stars near the Taurus Molecular Clouds

  Astrophysics (arXiv)

Summary: We present a drift scan survey covering a ~5 deg by 50 deg region toward the southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I filters with the Quest-2 camera on the Palomar 48-inch telescope were combined with 2MASS near-infrared photometry to select candidate young stars. Follow-up optical spectroscopy of 190 candidates led to identification of 42 new low mass pre-main sequence stars with spectral types M4-M8, of which approximately half exhibit surface gravity signatures similar to known Taurus stars while the other half exhibit surface gravity signatures similar to members of the somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main sequence stars are spread over ~35 deg, and many are located well outside of previously explored regions. From assessment of the spatial and proper motion distributions, we argue that the new pre-main sequence stars identified far from the clouds cannot have originated from the vicinity of the 1-2 Myr-old subclusters which contain the bulk of the identified Taurus members, but instead represent a newly-identified area of recent star-formation near the clouds.

Catherine L. Slesnick; John M. Carpenter; Lynne A. Hillenbrand; Eric E. Mamajek
2006-09-01

151

Close Encounters between a Neutron Star and a Main-Sequence Star

  Astrophysics (arXiv)

Summary: We have examined consequences of strong tidal encounters between a neutron star and a normal star using SPH as a possible formation mechanism of isolated recycled pulsars in globular clusters. We have made a number of SPH simulations for close encounters between a main-sequence star of mass ranging from 0.2 to 0.7 Solar masses represented by an n=3/2 polytrope and a neutron star represented by a point mass. The outcomes of the first encounters are found to be dependent only on the dimensionless parameter eta' = (m/(m+M))^(1/2) (r_min/R_MS)^(3/2) (m/M)^(1/6), where m and M are the masses of the main-sequence star and the neutron star, respectively, r_min the minimum separation between two stars, and R_MS the size of the main-sequence star. The material from the (at least partially) disrupted star forms a disk around the neutron star. If all material in the disk is to be acctreted onto the neutron star's surface, the mass of the disk is enough to spin up the neutron star to spin period of 1 ms.

Hyung Mok Lee; Sungsoo S. Kim; Hyesung Kang
1996-03-27

152

Abundances on the Main Sequence of the Globular Cluster Omega Centauri

  Astrophysics (arXiv)

Summary: Using the 2dF multi-fibre instrument on the AAT, moderate resolution spectra have been obtained for a large sample of stars on the main sequence and at the turnoff in the unusual globular cluster omega Centauri. We investigate the behaviour of CH, CN and SrII line strength indices as a function of overall abundance for the main sequence sample. A number of stars do not follow the relations defined by the majority. These anomalous objects can be categorised into (at least) three types. (1) Carbon enhanced stars, which represent about 5% of the sample, and which are found at all metallicities. Spectrum synthesis calculations show that the atmospheres of these stars are typically enhanced in C by factors of between 3 and 10. (2) Nitrogen enhanced stars, revealed for [Fe/H] > -1.3 by strong CN indices, which make up ~40% of the cluster main sequence population above this metallicity. These stars are enhanced in N by factors of up to 100. Our data, however, provide no constraints of their relative numbers at lower [Fe/H]. (3) Stars with enhancements of the s-process element Sr by factors of 30 to 60. The possible origins for these abundance anomalies are discussed.

G. S. Da Costa; Laura Stanford; John E. Norris; R. D. Cannon
2005-07-07

153

Main-Sequence Effective Temperatures from a Revised Mass-Luminosity Relation Based on Accurate Properties

  CERN Preprints

Summary: The mass-luminosity (M-L), mass-radius (M-R) and mass-effective temperature ($M-T_{eff}$) diagrams for a subset of galactic nearby main-sequence stars with masses and radii accurate to $\\leq 3\\%$ and luminosities accurate to $\\leq 30\\%$ (268 stars) has led to a putative discovery. Four distinct mass domains have been identified, which we have tentatively associated with low, intermediate, high, and very high mass main-sequence stars, but which nevertheless are clearly separated by three distinct break points at 1.05, 2.4, and 7$M_{\\odot}$ within the mass range studied of $0.38-32M_{\\odot}$. Further, a revised mass-luminosity relation (MLR) is found based on linear fits for each of the mass domains identified. The revised, mass-domain based MLRs, which are classical ($L \\propto M^{\\alpha}$), are shown to be preferable to a single linear, quadratic or cubic equation representing as an alternative MLR. Stellar radius evolution within the main-sequence for stars with $M>1M_{\\odot}$ is clearly evident on the M-R d...

Eker, Z; Soydugan, E; Bilir, S; Gokce, E Yaz; Steer, I; Tuysuz, M; Senyuz, T; Demircan, O
2015-01-01

154

The distance to NGC 6397 by M-subdwarf main-sequence fitting

  Astrophysics (arXiv)

Summary: Recent years have seen a substantial improvement both in photometry of low luminosity stars in globular clusters and in modelling the stellar atmospheres of late-type dwarfs. We build on these observational and theoretical advances in undertaking the first determination of the distance to a globular cluster by main-sequence fitting using stars on the lower main sequence. The calibrating stars are extreme M subdwarfs, as classified by Gizis (1997), with parallaxes measured to a precision of better than 10%. Matching against King et al's (1998) deep (V, (V-I)) photometry of NGC 6397, and adopting E_{B-V}=0.18 mag, we derive a true distance modulus of 12.13 +- 0.15 mag for the cluster. This compares with (m-M)_0=12.24 +- 0.1 derived through conventional main-sequence fitting in the (V, (B-V)) plane. Allowing for intrinsic differences due to chemical composition, we derive a relative distance modulus of delta (m-M)_0=2.58 mag between NGC 6397 and the fiducial metal-poor cluster M92. We extend this calibration to other metal-poor clusters, and examine the resulting RR Lyrae (M_V, [Fe/H]) relation.

I. Neill Reid; John E. Gizis
1998-09-02

155

The Coronal Temperatures of Low-Mass Main-Sequence Stars

  CERN Preprints

Summary: Aims. We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. Methods. We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. Results. We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, Fx, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to Fx by Tcor=0.11 Fx^0.26, where Tcor is in MK and Fx is in erg/s/cm^2.

Johnstone, Colin P
2015-01-01

156

Understanding white dwarf binary evolution with white dwarf/main sequence binaries: first results from SEGUE

  Astrophysics (arXiv)

Summary: Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries, which are possible SN Ia progenitors, cataclysmic variables, super soft sources, or AM CVn stars. The evolution and formation of close white dwarf binaries crucially depends on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes are so far poorly understood. Observational population studies of white dwarf/main sequence binaries provide the potential to significantly progress with this situation and to clearly constrain magnetic braking and the CE-phase. However, the current population of white dwarf/main sequence binaries is highly incomplete and heavily biased towards young systems containing hot white dwarfs. The SDSSII/SEGUE collaboration awarded us with 5 fibers per plate pair in order to fill this gap and to identify the required unbiased sample of old white dwarf/main sequence binaries. The success rate of our selection criteria exceeds 65% and during the first 10 months we have identified 41 new systems, most of them belonging to the missed old population.

M. R. Schreiber; A. Nebot Gomez-Moran; A. D. Schwope
2006-11-15

157

On the evolutionary status of chemically peculiar stars of the upper main sequence

  Astrophysics (arXiv)

Summary: We present further evidence that the magnetic chemically peculiar stars (CP2) of the upper main sequence already occur at very early stages of the stellar evolution, significantly before they reach 30% of their life-time on the main sequence. This result is especially important for models dealing with dynamo theories, angular momentum loss during the pre- as well as main sequence and evolutionary calculations for CP2 stars. Results from the literature either derived for objects in the Hyades and the UMa cluster or from the Hipparcos mission contradict each other. A way out of this dilemma is to investigate young open clusters with known ages and accurate distances (error < 10%), including CP2 members. Up to now, four open clusters fulfill these requirements: IC 2391, IC 2602, NGC 2451A and NGC 2516. In total, 13 CP2 stars can be found within these clusters. We have used the measurements and calibrations of the Geneva 7-color photometric system to derive effective temperatures and luminosities. Taking into account the overall metallicity of the individual clusters, isochrones and evolutionary tracks were used to estimate ages and masses for the individual objects. The derived ages (between 10 and 140 Myr) are well in line with those of the corresponding clusters and further strengthen the membership of the investigated CP2 stars.

H. Poehnl; H. M. Maitzen; E. Paunzen
2003-03-19

158

Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss

  CERN Preprints

Summary: The effects of atomic diffusion on internal and surface abundances of A and F pre-main-sequence stars with mass loss are studied in order to determine at what age the effects materialize, as well as to further understand the processes at play in HAeBe and young ApBp stars. Self-consistent stellar evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative accelerations) for all species within the OPAL opacity database were computed and compared to observations of HAeBe stars. Atomic diffusion in the presence of weak mass loss can explain the observed abundance anomalies of pre-main-sequence stars, as well as the presence of binary systems with metal rich primaries and chemically normal secondaries such as V380 Ori and HD72106. This is in contrast to turbulence models which do not allow for abundance anomalies to develop on the pre-main-sequence. The age at which anomalies can appear depends on stellar mass. For A and F stars, the effects of atomic diffusion can modify both the internal and s...

Vick, M; Richer, J; Richard, O
2010-01-01

159

Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds

  CERN Preprints

Summary: Aims: We study the evolution of stellar rotation and wind properties for low-mass main-sequence stars. Our aim is to use rotational evolution models to constrain the mass loss rates in stellar winds and to predict how their properties evolve with time on the main-sequence. Methods: We construct a rotational evolution model that is driven by observed rotational distributions of young stellar clusters. Fitting the free parameters in our model allows us to predict how wind mass loss rate depends on stellar mass, radius, and rotation. We couple the results to the wind model developed in Paper I of this series to predict how wind properties evolve on the main-sequence. Results: We estimate that wind mass loss rate scales with stellar parameters as $\\dot{M}_\\star \\propto R_\\star^2 \\Omega_\\star^{1.33} M_\\star^{-3.36}$. We estimate that at young ages, the solar wind likely had a mass loss rate that is an order of magnitude higher than that of the current solar wind. This leads to the wind having a higher density at y...

Johnstone, C P; Brott, I; Lüftinger, T
2015-01-01

160

Magnetic flaring in the pre-main sequence Sun and implications for the early solar system

  Astrophysics (arXiv)

Summary: To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main sequence Sun based on 23 hours observations of 43 analogs of the young Sun in the Orion Nebula Cluster obtained with the Chandra X-ray Observatory. We find the X-ray luminosities are strongly elevated over main sequence levels with average = 30.3 erg/s and = -3.9 (0.5-8 keV), and dozens of flares are present. Analogs of the <= 1 My old pre-main sequence Sun exhibit X-ray flares 10^{1.5} times more powerful and 10^{2.5} times more frequent than the most powerful flares seen on the contemporary Sun. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 10^5-fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash melted meteoritic chondrules and produced excess 21-Ne seen in meteoritic grains.

Eric D. Feigelson; Gordon P. Garmire; Steven H. Pravdo
2002-02-01

161

A helium spread among the main sequence stars in NGC 2808

  Astrophysics (arXiv)

Summary: We studied the color distribution of the main sequence of the Globular Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The color distribution of main sequence stars is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. We estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may have suffered self-enrichment, with different stellar populations born from the ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the first generation. In addition to the Y=0.40 stars, roughly 30% of the stars should have Y distributed between 0.26-0.29 while 50% have primordial Y, to explain also the peculiar horizontal branch morphology. Three main stages of star formation are identified, the first with primordial helium content Y ~ 0.24, the second one born from the winds of the most massive AGBs of the first stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a long hiatus of time (several 10^7yr) between the second and third generation, star formation might have been inhibited by the explosion of late Supernovae II deriving from binary evolution.

F. D'Antona; M. Bellazzini; V. Caloi; F. Fusi Pecci; S. Galleti; R. T. Rood
2005-05-17

162

Radial Color Gradient and Main Sequence Mass Segregation in M30 (NGC 7099)

  Astrophysics (arXiv)

Summary: It has long been known that the post-core-collapse globular cluster M30 has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giants does not fully account for this gradient. This study uses Hubble Space Telescope WFPC2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the non-cluster background contribution, and finds \\Delta(B-V) ~ +0.3 mag for the overall cluster starlight over the range r = 2" to >~ 1'. The slope of the color profile is: \\Delta(B-V)/\\Delta\\log(r) = 0.20 +/- 0.07 mag dex^{-1}. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus the color gradient in M30's central region appears to be due entirely to post-main-sequence stars. Two classes of plausible dynamical models, Fokker-Planck and multimass King models, are combined with theoretical stellar isochrones from Bergbusch & VandenBerg (1992) and from D'Antona et al. to quantify the effect of mass segregation of main sequence stars. In all cases, mass segregation of main sequence stars results in \\Delta(B-V) ~ -0.06 to +0.02 mag over the range r = 20" - 80"; this is consistent with M30's residual color gradient within measurement error. The observed fraction of evolved star light in the B and V bands agrees with the corresponding model predictions at small radii but drops below it for r >~ 20".

Justin H. Howell; Puragra Guhathakurta; Amy Tan
1999-12-01

163

Light Element Depletion in Contracting Brown Dwarfs and Pre--Main-Sequence Stars

  Astrophysics (arXiv)

Summary: We present an analytic calculation of the thermonuclear depletion of the light elements lithium, beryllium, and boron in fully convective, low-mass stars. Under the presumption that the pre--main-sequence star is always fully mixed during contraction, we find that the burning of these rare light elements can be computed analytically, even when the star is degenerate. Using the effective temperature as a free parameter, we constrain the properties of low-mass stars from observational data, independently of the uncertainties associated with modeling their atmospheres and convection. Our analytic solution explains the dependence of the age at a given level of elemental depletion on the stellar effective temperature, nuclear cross sections, and chemical composition. Most importantly, our results allow observers to translate lithium non-detections in young cluster members into a model-independent minimum age for that cluster. Using this procedure, we have found lower limits to the ages of the Pleiades (100 Myr) and Alpha Persei (60 Myr) clusters. Recent experimental work on the low energy resonance in the ^10B(p,\\alpha)^7Be reaction has greatly enhanced estimates of the destruction rate of ^10B, making it possible for stars with M>0.1 M_sun to deplete both ^10B and ^11B before reaching the main sequence. Moreover, there is an interesting range of masses, 0.085 M_sun < M < 0.13 M_sun, where boron depletion occurs on the main sequence in less than a Hubble time, providing a potential ``clock'' for dating low-mass stars.

Greg Ushomirsky; Christopher D. Matzner; Edward F. Brown; Lars Bildsten; Vadim G. Hilliard; Peter C. Schroeder
1997-11-10

164

Two Boehm-Vitense gaps in the main sequence of the Hyades

  Astrophysics (arXiv)

Summary: Hipparcos proper motions and trigonometric parallaxes allow the derivation of secular parallaxes which fix the distances to individual stars in the Hyades cluster to an accuracy of \\sim 2 percent. The resulting color-absolute magnitude diagram for 92 high-fidelity single members of the cluster displays a very narrow main sequence, with two turn-offs and associated gaps. These occur at the locations where the onset of surface convection affects the B-V colors, as predicted by Boehm-Vitense thirty years ago. The new distances provide stringent constraints on the transformations of colors and absolute magnitudes to effective temperatures and luminosities, and on models of stellar interiors.

Jos H. J. de Bruijne; Ronnie Hoogerwerf; P. Tim de Zeeuw
2000-11-08

165

Astrobiological Effects of F, G, K and M Main-Sequence Stars

  Astrophysics (arXiv)

Summary: We focus on the astrobiological effects of photospheric radiation produced by main-sequence stars of spectral types F, G, K, and M. The photospheric radiation is represented by using realistic spectra, taking into account millions or hundred of millions of lines for atoms and molecules. DNA is taken as a proxy for carbon-based macromolecules, assumed to be the chemical centerpiece of extraterrestrial life forms. Emphasis is placed on the investigation of the radiative environment in conservative as well as generalized habitable zones.

M. Cuntz; L. Gurdemir; E. F. Guinan; R. L. Kurucz
2008-02-15

166

Astrobiology in the Environments of Main-Sequence Stars: Effects of Photospheric Radiation

  Astrophysics (arXiv)

Summary: We explore if carbon-based macromolecules (such as DNA) in the environments of stars other than the Sun are able to survive the effects of photospheric stellar radiation, such as UV-C. Therefore, we focus on main-sequence stars of spectral types F, G, K, and M. Emphasis is placed on investigating the radiative environment in the stellar habitable zones. Stellar habitable zones are relevant to astrobiology because they constitute circumstellar regions in which a planet of suitable size can maintain surface temperatures for water to exist in fluid form, thus increasing the likelihood of Earth-type life.

M. Cuntz; L. Gurdemir; E. F. Guinan; R. L. Kurucz
2007-12-19

167

On Chromospheric Variations Modeling for Main-Sequence Stars of G and K Spectral Classes

  CERN Preprints

Summary: We present a method of chromospheric flux simulation for 13 late-type main-sequence stars. These Sun-like stars have well-determined cyclic flux variations similar to 11 yr solar activity cycle. Our flux prediction is based on chromospheric HK emission time series measurements from Mount Wilson Observatory and comparable solar data. We show that solar three - component modeling explains well the stellar observations. We find that the 10 - 20% of K - stars disc surfaces are occupied by bright active regions.

Bruevich, E A
2010-01-01

168

DY/DZ from fine structure in the Main Sequence based on Hipparcos parallaxes

  Astrophysics (arXiv)

Summary: The slope Delta Y/Delta Z is a quantity of interest in relation to stellar evolution, the initial mass function and the determination of the primordial helium abundance. In this paper we estimate Delta Y/Delta Z from fine structure in the Main Sequence of nearby stars from Hipparcos data for stars with Z \\leq Zsun and find a value of about 3, which is consistent with what has been found in extragalactic H II regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the IMF slope adopted.

B. E. J. Pagel; L. Portinari
1997-11-27

169

Old main-sequence turnoff photometry in the SMC: Star Formation History and Chemical Enrichment Law

  Astrophysics (arXiv)

Summary: We present deep ground-based {\\it B} and {\\it R} observations of 12 fields in the Small Magellanic Cloud (SMC). The resulting color-magnitude diagrams (CMDs) reach the oldest main-sequence (MS) turnoff at M$_{R}$$\\thicksim$3.5 and reveal the stellar population differences between the part of the galaxy facing the Large Magellanic Cloud (LMC) and an area on the opposite side. In the Southern part of the galaxy, we found that there are still intermediate-age stars as far as 4 kpc from the SMC center. The Chemical Enrichment History (CEH) in one of our SMC fields is also presented.

Noelia E. D. Noel; Antonio Aparicio; Carme Gallart; Sebastian L. Hidalgo; Ricardo Carrera; Edgardo Costa; Rene A. Mendez
2007-03-09

170

A Sr-Rich Star on the Main Sequence of Omega Centauri

  Astrophysics (arXiv)

Summary: Abundance ratios relative to iron for carbon, nitrogen, strontium and barium are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the globular cluster omega Centauri. This star, designated 2015448, shows depleted carbon and solar nitrogen, but more interestingly, shows an enhanced abundance ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is [Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium abundance ratios that are roughly equal. Possible formation scenarios of this peculiar object are considered.

Laura M. Stanford; G. S. Da Costa; John E. Norris; Russell D. Cannon
2006-11-16

171

Astrobiology in the Environments of Main-Sequence Stars: Effects of Photospheric Radiation

  CERN Preprints

Summary: We explore if carbon-based macromolecules (such as DNA) in the environments of stars other than the Sun are able to survive the effects of photospheric stellar radiation, such as UV-C. Therefore, we focus on main-sequence stars of spectral types F, G, K, and M. Emphasis is placed on investigating the radiative environment in the stellar habitable zones. Stellar habitable zones are relevant to astrobiology because they constitute circumstellar regions in which a planet of suitable size can maintain surface temperatures for water to exist in fluid form, thus increasing the likelihood of Earth-type life.

Cuntz, M; Guinan, E F; Kurucz, R L
2007-01-01

172

Discovery of fossil magnetic fields in the intermediate-mass pre-main sequence stars

  Astrophysics (arXiv)

Summary: It is now well-known that the surface magnetic fields observed in cool, lower-mass stars on the main sequence (MS) are generated by dynamos operating in their convective envelopes. However, higher-mass stars (above 1.5 Msun) pass their MS lives with a small convective core and a largely radiative envelope. Remarkably, notwithstanding the absence of energetically-important envelope convection, we observe very strong (from 300 G to 30 kG) and organised (mainly dipolar) magnetic fields in a few percent of the A and B-type stars on the MS, the origin of which is not well understood. In this poster we propose that these magnetic fields could be of fossil origin, and we present very strong observational results in favour of this proposal.

E. Alecian; G. A. Wade; C. Catala
2008-12-19

173

Super-solar metallicity in G0-G3 main sequence stars with V$<$15

  CERN Preprints

Summary: The basic stellar atmospheric parameters (effective temperature, surface gravity and global metallicity) were simultaneously determined for a sample of 233 stars, limited in magnitude ($V<15$) with spectral types between G0 and G3 and luminosity class V (main sequence). The analysis was based on spectroscopic observations collected at the Observatorio Astrof\\'isico Guillermo Haro and using a set of Lick-like indices defined in the spectral range of 3800-4800 \\AA. An extensive set of indices computed in a grid of theoretical spectra was used as a comparison tool in order to determine the photospheric parameters. The method was validated by matching the results from spectra of the asteroids Vesta and Ceres with the Sun parameters. The main results were: i) the photospheric parameters were determined for the first time for 213 objects in our sample; ii) a sample of 20 new super metal-rich stars candidates was found.

López-Valdivia, R; Chávez, M; Tapia-Schiavon, C; Hernández-Águila, J B; Valdés, J R; Chavushyan, V
2014-01-01

174

Discovery of fossil magnetic fields in the intermediate-mass pre-main sequence stars

  CERN Preprints

Summary: It is now well-known that the surface magnetic fields observed in cool, lower-mass stars on the main sequence (MS) are generated by dynamos operating in their convective envelopes. However, higher-mass stars (above 1.5 Msun) pass their MS lives with a small convective core and a largely radiative envelope. Remarkably, notwithstanding the absence of energetically-important envelope convection, we observe very strong (from 300 G to 30 kG) and organised (mainly dipolar) magnetic fields in a few percent of the A and B-type stars on the MS, the origin of which is not well understood. In this poster we propose that these magnetic fields could be of fossil origin, and we present very strong observational results in favour of this proposal.

Alecian, E; Catala, C
2008-01-01

175

Stable magnetic equilibria and their evolution in the upper main sequence, white dwarfs, and neutron stars

  Astrophysics (arXiv)

Summary: [abbreviated] Long-lived, large-scale magnetic field configurations exist in upper main sequence, white dwarf, and neutron stars. Externally, these fields have a strong dipolar component, while their internal structure and evolution are uncertain, but highly relevant for several problems in stellar and high-energy astrophysics. We discuss the main properties expected for the stable magnetic configurations in these stars from physical arguments, and how these properties may determine the modes of decay of these configurations. Stable magneto-hydrostatic equilibria are likely to exist in stars whenever the matter in their interior is stably stratified (not barotropic). These equilibria are not force-free and not required to satisfy the Grad-Shafranov equation, but they do involve both toroidal and poloidal field components. We argue that the main mode of decay for these configurations are processes that lift the constraints set by stable stratification, such as heat diffusion in main-sequence envelopes and white dwarfs, and beta decays or particle diffusion in neutron stars. In the former, heat diffusion is not fast enough to make these equilibria evolve over the stellar lifetime. In neutron stars, a strong enough field might decay by overcoming the compositional stratification through beta decays (at the highest field strengths) or through ambipolar diffusion (for somewhat weaker fields). These processes convert magnetic energy to thermal energy, and they occur at significant rates only once the latter is smaller than the former, and therefore they substantially delay the cooling of the neutron star, while slowly decreasing its magnetic energy.

Andreas Reisenegger
2009-05-15

176

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

  Astrophysics (arXiv)

Summary: (Abridged) We present a detailed analysis of the main physical processes responsible for the transport of angular momentum and chemical species in the radiative regions of rotating stars. We focus on cases where meridional circulation and shear-induced turbulence only are included in the simulations. Our analysis is based on a 2-D representation of the secular hydrodynamics, which is treated using expansions in spherical harmonics. We present a full reconstruction of the meridional circulation and of the associated fluctuations of temperature and mean molecular weight along with diagnosis for the transport of angular momentum, heat and chemicals. In the present paper these tools are used to validate the analysis of two main sequence stellar models of 1.5 and 20 Msun for which the hydrodynamics has been previously extensively studied in the literature. We obtain a clear visualization and a precise estimation of the different terms entering the angular momentum and heat transport equations in radiative zones. This enables us to corroborate the main results obtained over the past decade by Zahn, Maeder, and collaborators concerning the secular hydrodynamics of such objects. We focus on the meridional circulation driven by angular momentum losses and structural readjustements. We confirm quantitatively for the first time through detailed computations and separation of the various components that the advection of entropy by this circulation is very well balanced by the barotropic effects and the thermal relaxation during most of the main sequence evolution. This enables us to derive simplifications for the thermal relaxation on this phase. The meridional currents in turn advect heat and generate temperature fluctuations that induce differential rotation through thermal wind thus closing the transport loop.

T. Decressin; S. Mathis; A. Palacios; L. Siess; S. Talon; C. Charbonnel; J. -P. Zahn
2008-12-01

177

Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities

  CERN Preprints

Summary: The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. In this paper, we evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic Giant Branch to distinguish stellar populations. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corre...

Lagarde, N; Charbonnel, C; Eggenberger, P; Ekström, S; Palacios, A
2012-01-01

178

Proper Motions of Pre-Main Sequence Stars in Southern Star-Forming Regions

  Astrophysics (arXiv)

Summary: We present proper motion measurements of pre-main sequence (PMS) stars associated with major star-forming regions of the southern hemisphere (Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis), situated in the galactic longitude range l = 290\\degr to l = 360\\degr. A list of PMS stars as complete as possible was established based on the Herbig and Bell catalogue and many new catalogues like the PDS survey, the catalogue of Herbig Ae/Be stars by Th\\'e et al. (1994), X-rays surveys, etc. The measurements made use of public material (mainly AC2000 and USNO--A2.0 catalogues) as well as scans of SERC-J Schmidt plates with the MAMA measuring machine (Paris) and Valinhos CCD meridian circle observations (Brazil). We derived proper motions for 214 stars, with an accuracy of 5 to 10 mas/yr depending mainly on the difference of epochs between the position sources. The main characteristics of the sample are discussed. We show that systematic motions of groups of stars exist, which are not explained by the reflex solar motion.

R. Teixeira; C. Ducourant; M. J. Sartori; J. I. B. Camargo; J. P. Perié; J. R. D. Lépine; P. Benevides-Soares
2000-08-24

179

Proper Motions of Pre-Main Sequence Stars in Southern Star-Forming Regions

  CERN Preprints

Summary: We present proper motion measurements of pre-main sequence (PMS) stars associated with major star-forming regions of the southern hemisphere (Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis), situated in the galactic longitude range l = 290\\degr to l = 360\\degr. A list of PMS stars as complete as possible was established based on the Herbig and Bell catalogue and many new catalogues like the PDS survey, the catalogue of Herbig Ae/Be stars by Th\\'e et al. (1994), X-rays surveys, etc. The measurements made use of public material (mainly AC2000 and USNO--A2.0 catalogues) as well as scans of SERC-J Schmidt plates with the MAMA measuring machine (Paris) and Valinhos CCD meridian circle observations (Brazil). We derived proper motions for 214 stars, with an accuracy of 5 to 10 mas/yr depending mainly on the difference of epochs between the position sources. The main characteristics of the sample are discussed. We show that systematic motions of groups of stars exist, which are not explained by the r...

Teixeira, R G; Sartori, M J; Camargo, J I B; Perié, J P; Lépine, J R D; Benevides-Soares, P
2000-01-01

180

Pre--Main-Sequence stellar populations across Shapley Constellation III. I. Photometric Analysis and Identification

  CERN Preprints

Summary: We present our investigation of pre--main-sequence (PMS) stellar populations in the Large Magellanic Cloud (LMC) from imaging with Hubble Space Telescope WFPC2 camera. Our targets of interest are four star-forming regions located at the periphery of the super-giant shell LMC 4 (Shapley Constellation III). The PMS stellar content of the regions is revealed through the differential Hess diagrams and the observed color-magnitude diagrams (CMDs). Further statistical analysis of stellar distributions along cross-sections of the faint part of the CMDs allowed the quantitative assessment of the PMS stars census, and the isolation of faint PMS stars as the true low-mass stellar members of the regions. These distributions are found to be well represented by a double Gaussian function, the first component of which represents the main-sequence field stars and the second the native PMS stars of each region. Based on this result, a cluster membership probability was assigned to each PMS star according to its CMD position....

Gouliermis, Dimitrios A; Robberto, Massimo; Gruendl, Robert A; Chu, You-Hua; Gennaro, Mario; Henning, Thomas; Rosa, Michael; Da Rio, Nicola; Brandner, Wolfgang; Romaniello, Martino; De Marchi, Guido; Panagia, Nino; Zinnecker, Hans
2011-01-01

181

Luminosity-metallicity relation for stars on the lower main sequence

  Astrophysics (arXiv)

Summary: We present a comparison of the predictions of stellar models with the luminosity of the lower main sequence ($5.5 < M_V < 7.3$) using K dwarfs in the Hipparcos catalog. The parallaxes of our comparison stars are known to better than 15% and metallicities have been recently determined from photometry. A major advantage of our comparison is that distances in our sample are known with good accuracy, while tests that involve open and globular clusters are constrained by potentially inaccurate distances. We show that the luminosity of the lower main sequence relative to a fiducial (solar metallicity) isochrone is a simple function of metallicity: $\\Delta M_V = 0.84375 \\times {\\rm [ Fe/H]} - 0.04577$. We compare the data with a range of isochrones from the literature. None of the models fit all the data, although some models do clearly better than others. In particular, metal rich isochrones seem to be difficult to construct. The relationship between luminosity, colour and metallicity for K dwarfs is found to be very tight. We are thus able to derive metallicities for K dwarfs based on their position in the Hipparcos colour-magnitude diagram with accuracies better than 0.1 dex. The metallicity-luminosity relation for K dwarfs leads to a new distance indicator with a wide range of possible applications.

Eira Kotoneva; Chris Flynn; Raul Jimenez
2002-03-07

182

Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars

  Astrophysics (arXiv)

Summary: The objective of this paper is to investigate whether a convective dynamo can account quantitatively for the observed lower limit of X-ray surface flux in solar-type main sequence stars. Our approach is to use 3D numerical simulations of a turbulent dynamo driven by convection to characterize the dynamic behavior, magnetic field strengths, and filling factors in a non-rotating stratified medium, and to predict these magnetic properties at the surface of cool stars. We use simple applications of stellar structure theory for the convective envelopes of main-sequence stars to scale our simulations to the outer layers of stars in the F0--M0 spectral range, which allows us to estimate the unsigned magnetic flux on the surface of non-rotating reference stars. With these estimates we use the recent results of \\citet{Pevtsov03} to predict the level of X-ray emission from such a turbulent dynamo, and find that our results compare well with observed lower limits of surface X-ray flux. If we scale our predicted X-ray fluxes to \\ion{Mg}{2} fluxes we also find good agreement with the observed lower limit of chromospheric emission in K dwarfs. This suggests that dynamo action from a convecting, non-rotating plasma is a viable alternative to acoustic heating models as an explanation for the basal emission level seen in chromospheric, transition region, and coronal diagnostics from late-type stars.

D. J. Bercik; G. H. FIsher; C. M. Johns-Krull; W. P. Abbett
2005-06-01

183

X-rays, pre-main sequence stars and planet formation

  Astrophysics (arXiv)

Summary: The study of magnetic activity of pre-main sequence (PMS) stars through their X-ray emission is entering a mature phase. We review here two recent developments. First, we present some early findings from the Chandra Orion Ultradeep Project (COUP) relating to the age-activity-rotation relations of PMS stars. COUP is a nearly continuous exposure of the Orion Nebula Cluster over 13 days and gives the most comprehensive view to date of X-ray emission from the PMS population. We find that the activity-rotation connection that dominates main sequence activity behavior is absent in PMS stars, although the activity-age relation is present as well as links to stellar mass and volume. Second, we discuss investigations of effects of PMS X-rays on their cold protoplanetary disks. It is possible that the the ionization, heating and spallogenic effects of high energy photons and particles produced in PMS stellar flares have significant effects on disk dynamics, heating and chemistry. X-ray ionization may induce MHD turbulence in the disk which, in turn, may regulate disk evolution and planet formation processes such as Jovian planet migration.

E. D. Feigelson
2005-01-12

184

The dynamical tide in a rotating 10 Msun main sequence star

  Astrophysics (arXiv)

Summary: We study the linear, but fully non-adiabatic tidal response of a uniformly rotating, somewhat evolved X_c=0.4, 10 Msun main sequence star to the dominant l=2 components of its binary companion's tidal potential. This is done numerically with a 2D implicit finite difference scheme. We assume the spin vector of the 10 Msun star to be aligned perpendicular to the orbital plane and calculate the frequency and width of the resonances with the prograde and retrograde gravity (g) modes as well as the resonances with quasi-toroidal rotational (r) modes for varying rotation rates of the main sequence star. For all applied forcing frequencies we determine the rate of tidal energy and angular momentum exchange with the companion. In a rotating star tidal energy is transferred from l=2 g-modes to g-modes of higher spherical degree (l=4,6,8,...) by the Coriolis force. These latter modes have shorter wavelength and are damped more heavily, so that the l=2 resonant tidal interaction tends to be reduced for large stellar rotation rates. On the other hand, the density of potential resonances (a broad l spectrum) increases. We find several inertially excited unstable l>4 g-modes, but not more than one (retrograde) unstable l=2 g-mode and that only for rapid rotation. Our numerical results can be applied to study the tidal evolution of eccentric binaries containing early type B-star components.

M. G. Witte; G. J. Savonije
1998-10-30

185

Winds of Main-Sequence Stars: Observational Limits and a Path to Theoretical Prediction

  Astrophysics (arXiv)

Summary: It is notoriously difficult to measure the winds of solar-type stars. Traditional spectroscopic and radio continuum techniques are sensitive to mass loss rates at least two to three orders of magnitude stronger than the Sun's relatively feeble wind. Much has been done with these methods to probe the more massive outflows of younger (T Tauri) and older (giant, AGB, supergiant) cool stars, but the main sequence remains terra incognita. This presentation reviews the limits on traditional diagnostics and outlines more recent ideas such as Lyman alpha astrospheres and charge-exchange X-ray emission. In addition, there are hybrid constraints on mass loss rates that combine existing observables and theoretical models. The Sackmann/Boothroyd conjecture of a more massive young Sun (and thus a much stronger ZAMS wind) is one such idea that needs to be tested further. Another set of ideas involves a strong proposed coupling between coronal heating and stellar mass loss rates, where the former is easier to measure in stars down to solar-like values. The combined modeling of stellar coronae and stellar winds is developing rapidly, and it seems to be approaching a level of development where the only remaining ``free parameters'' involve the sub-photospheric convection. This talk will also summarize these theoretical efforts to predict the properties of solar-type main-sequence winds.

Steven R. Cranmer
2007-01-19

186

Evidence of accretion triggered oscillations in the pre-main-sequence interacting binary AK Sco

  CERN Preprints

Summary: Pre-main sequence (PMS) binaries are surrounded by circumbinary disks from which matter falls onto both components. The material dragged from the circumbinary disk flows onto each star through independent streams channelled by the variable gravitational field. The action of the bar-like potential is most prominent in high eccentricity systems made of two equal mass stars. AK Sco is a unique PMS system composed of two F5 stars in an orbit with e=0.47. Henceforth, it is an ideal laboratory to study matter infall in binaries and its role in orbit circularization. In this letter, we report the detection of a 1.3mHz ultra low frequency oscillation in the ultraviolet light curve at periastron passage. This oscillation last 7 ks being most likely fed by the gravitational energy released when the streams tails spiralling onto each star get in contact at periastron passage enhancing the accretion flow; this unveils a new mechanism for angular momentum loss during pre-main sequence evolution and a new type of interacti...

de Castro, Ana I Gomez; Talavera, Antonio
2012-01-01

187

On the Nature of Collinder 121: Insights from the Low-Mass Pre-Main Sequence

  Astrophysics (arXiv)

Summary: We present a VI photometric catalogue towards the open cluster Cr 121. XMM-Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sight-line. de Zeeuw et al (1999) identified Cr 121 as a moving group, using HIPPARCOS data, at a distance of 592 pc. We reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sight-line, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050 pc. This is consistent with the original description of Cr 121 (Collinder, 1931), and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al (1999) resembles a foreground association and we agree with Eggen (1981) that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al (1999) census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.

Ben Burningham; Tim Naylor; R. D. Jeffries; C. R. Devey
2003-08-28

188

Can rotation explain the multiple main sequence turn-offs of Magellanic Cloud star clusters?

  CERN Preprints

Summary: Many intermediate age star clusters in the Magellanic Clouds present multiple main sequence turn-offs (MMSTO), which challenge the classical idea that star formation in such objects took place over short timescales. It has been recently suggested that the presence of fast rotators among main sequence stars could be the cause of such features (Bastian & de Mink 2009), hence relaxing the need for extended periods of star formation. In this letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour-magnitude diagrams. As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic colour-magnitude diagrams for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those ass...

Girardi, Leo; Miglio, Andrea
2011-01-01

189

Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones

  CERN Preprints

Summary: We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in the Galaxy. Each grid covers masses ranging from 5 to 60 Msun and initial rotation rates between 0 and about 600 km/s. To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities log g < 3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of t...

Brott, Ines; Cantiello, Matteo; Langer, Norbert; de Koter, Alex; Evans, Chris J; Hunter, Ian; Trundle, Carrie; Vink, Jorick S
2011-01-01

190

Rotation Periods of 34,030 Kepler Main-Sequence Stars: The Full Autocorrelation Sample

  CERN Preprints

Summary: We analyzed 3 years of data from the Kepler space mission to derive rotation periods of main-sequence stars below 6500 K. Our automated autocorrelation-based method detected rotation periods between 0.2 and 70 days for 34,030 (25.6%) of the 133,030 main-sequence Kepler targets (excluding known eclipsing binaries and Kepler Objects of Interest), making this the largest sample of stellar rotation periods to date. In this paper we consider the detailed features of the now well-populated period-temperature distribution and demonstrate that the period bimodality, first seen by McQuillan, Aigrain & Mazeh (2013) in the M-dwarf sample, persists to higher masses, becoming less visible above 0.6 M_sun. We show that these results are globally consistent with the existing ground-based rotation-period data and find that the upper envelope of the period distribution is broadly consistent with a gyrochronological age of 4.5 Gyrs, based on the isochrones of Barnes (2007), Mamajek & Hillenbrand (2008) and Meibom et al...

McQuillan, Amy; Aigrain, Suzanne
2014-01-01

191

An Unusual Eclipse of a Pre-Main Sequence Star in IC 348

  Astrophysics (arXiv)

Summary: A solar-like pre-main sequence star (TJ 108 = H 187 = LRLL 35 = HMW 15) in the extremely young cluster IC 348 has been found, which apparently experienced an eclipse lasting ~3.5 years, much longer than has ever been detected for any normal eclipsing binary. The light curve is flat-bottomed and rather symmetric, with a depth of 0.66 mag in Cousins I. During eclipse, the system reddened by \\~0.17 mag in R-I. We argue that the eclipsing body is not a star because of the small probability of detecting an eclipse in what would be a very widely separated binary. Instead, it appears that the eclipse was caused by a circumstellar or circumbinary cloud or disk feature which occulted the star, or one of its components, if it is a binary system. We emphasize the importance of more detailed study of this object, which appears to be a new member of a small class of pre-main sequence stars whose variability can be firmly linked to occultation by circumstellar (or circumbinary) matter.

R. E. Cohen; W. Herbst; E. C. Williams
2003-08-27

192

Anticorrelated CN and CH Variations on the 47 Tucanae Main-Sequence Turnoff

  Astrophysics (arXiv)

Summary: Observations of CN and CH band strengths among a random sample of main-sequence turn-off stars (+3.9 < MV < +4.6) in the globular cluster 47 Tuc (NGC 104, C0021-723) were made with the CTIO Argus fiber spectrograph for the purpose of determining the ratio of CN-strong to CN-weak stars and investigating the behavior of CH relative to CN. Of the 20 turn-off stars, 12 were found to be CN-strong while 8 appear to be CN-weak. This ratio of CN-strong to CN-weak stars is similar to the ratios found among the more luminous 47 Tuc stars and implies little change in the overall distribution of CN with evolutionary state, although the present sample size is small. A general anticorrelation between CN and CH is also observed in that the CN-weak main-sequence stars all (with one possible exception) exhibit strong CH bands - a trend similar to that found among the brighter stars. That these variations occur among such relatively un-evolved stars and that the overall CN distribution appears to be independent of evolutionary state presents serious challenges to internal or mixing theories of their origin. We therefore suggest that at least some component of the C and N abundance inhomogeneities observed among the brighter (more evolved) stars of this cluster appears to have been established prior to the commencement of evolution up the red giant branch.

M. M. Briley; J. E. Hesser; R. A. Bell; M. Bolte; G. H. Smith
1994-08-11

193

Impacts of WIMP dark matter upon stellar evolution: main-sequence stars

  HEP - Phenomenology (arXiv)

Summary: The presence of large amounts of WIMP dark matter in stellar cores has been shown to have significant effects upon models of stellar evolution. We present a series of detailed grids of WIMP-influenced stellar models for main sequence stars, computed using the DarkStars code. We describe the changes in stellar structure and main sequence evolution which occur for masses ranging from 0.3 to 2.0 solar masses and metallicities from Z = 0.0003-0.02, as a function of the rate of energy injection by WIMPs. We then go on to show what rates of energy injection can be obtained using realistic orbital parameters for stars near supermassive black holes, including detailed considerations of dark matter halo velocity and density profiles. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits, causing WIMP annihilation to provide up to 100 times the energy of hydrogen fusion in stars at the Galactic centre.

Pat Scott; Malcolm Fairbairn; Joakim Edsjö
2008-10-30

194

Luminosity-metallicity relation for stars on the lower main sequence

  CERN Preprints

Summary: We present a comparison of the predictions of stellar models with the luminosity of the lower main sequence ($5.5 < M_V < 7.3$) using K dwarfs in the Hipparcos catalog. The parallaxes of our comparison stars are known to better than 15% and metallicities have been recently determined from photometry. A major advantage of our comparison is that distances in our sample are known with good accuracy, while tests that involve open and globular clusters are constrained by potentially inaccurate distances. We show that the luminosity of the lower main sequence relative to a fiducial (solar metallicity) isochrone is a simple function of metallicity: $\\Delta M_V = 0.84375 \\times {\\rm [ Fe/H]} - 0.04577$. We compare the data with a range of isochrones from the literature. None of the models fit all the data, although some models do clearly better than others. In particular, metal rich isochrones seem to be difficult to construct. The relationship between luminosity, colour and metallicity for K dwarfs is found to...

Kotoneva, E; Jiménez, R; Kotoneva, Eira; Flynn, Chris; Jimenez, Raul
2002-01-01

195

Lithium Depletion Boundary in a Pre-Main Sequence Binary System

  Astrophysics (arXiv)

Summary: A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a \\~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature is detected from the primary (~M4). The physical companionship of the two stars is confirmed from common proper motions. Current theoretical pre-main sequence evolutionary models cannot simultaneously match the observed colors, brightnesses, and Li depletion patterns of this binary system. At the age upper limit of 20 Myr, contemporary theoretical evolutionary models predict too slow Li depletion. If true Li depletion is a faster process than predicted by theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391) estimated from the Li depletion boundary method are all overestimated. Because of the importance of the open cluster age scale, development of self-consistent theoretical models to match the HIP 112312 data is desirable.

Inseok Song; M. S. Bessell; B. Zuckerman
2002-11-06

196

The Rotation of Low-Mass Pre-Main-Sequence Stars

  Astrophysics (arXiv)

Summary: Major photometric monitoring campaigns of star-forming regions in the past decade have provided rich rotation period distributions of pre-main-sequence stars. The rotation periods span more than an order of magnitude in period, with most falling between 1 and 10 days. Thus the broad rotation period distributions found in 100 Myr clusters are already established by an age of 1 Myr. The most rapidly rotating stars are within a factor of 2-3 of their critical velocities; if angular momentum is conserved as they evolve to the ZAMS, these stars may come to exceed their critical velocities. Extensive efforts have been made to find connections between stellar rotation and the presence of protostellar disks; at best only a weak correlation has been found in the largest samples. Magnetic disk-locking is a theoretically attractive mechanism for angular momentum evolution of young stars, but the links between theoretical predictions and observational evidence remain ambiguous. Detailed observational and theoretical studies of the magnetospheric environments will provide better insight into the processes of pre-main-sequence stellar angular momentum evolution.

Robert D. Mathieu
2003-03-10

197

Binary Masses as a Test for Pre-Main-Sequence Models

  Astrophysics (arXiv)

Summary: Observations of binaries have traditionally provided the means for ascertaining stellar masses. Here, we use the published data on 8 pre-main-sequence pairs to gauge the accuracy of our own, recently calculated, evolutionary tracks (Palla & Stahler 1999). We consider both eclipsing, double-lined spectroscopic binaries, which provide the mass of each star separately, and non-eclipsing, double-lined systems, which yield only the ratio. We also analyze the visual, quadruple system GG Tau, for which the sum of the two component masses follows from observations of the circumbinary disk. In almost all cases, our theoretically derived masses or mass ratios are in good agreement with the empirical values. For two binaries (NTTS 162814-2427 and P1540), the observational results are still too uncertain for a proper comparison. We also find that the derived contraction ages within each pre-main-sequence pair are nearly equal. This result extends earlier findings regarding visual pairs, and indicates that the components of all binaries form in proximity, perhaps within the same dense cloud core. Finally, our study reveals that the Trapezium star BM Ori is very young, since both the star itself and its companion have contraction ages less than 10^5 years.

Francesco Palla; Steven W. Stahler
2001-02-08

198

Cool stars in NGC 2547 and pre main sequence lithium depletion

  Astrophysics (arXiv)

Summary: We present the results of a spectroscopic survey of X-ray selected, low-mass candidate members of the young open cluster NGC 2547. Using a combination of photometry, spectroscopic indices and radial velocities we refine our candidate list and then use our spectroscopy to study the progression of lithium depletion in low-mass pre main sequence stars. We derive lithium abundances or upper limits for all our candidate members, which have effective temperatures in the range 5000>Teff>3200K, and compare these with predictions for lithium burning and depletion provided by a number of models and also with the lithium depletion seen in younger and older stars. We find that some models can reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the Hertzsprung-Russell diagram. But the lack of significant further lithium depletion between NGC 2547 and older clusters argues for an age of at least 50Myr, more in keeping with the lack of lithium observed in even fainter NGC 2547 candidates. We show that reconciliation of these age estimates may require additions to the physics incorporated in current generations of pre main sequence models.

R. D Jeffries; J. M. Oliveira; D. Barrado y Navascues; J. R. Stauffer
2003-05-05

199

ROSAT and Hipparcos Observations of Isolated Pre-Main-Sequence Stars near HD 98800

  Astrophysics (arXiv)

Summary: We present new observations of the isolated young stars HD 98800 and CD-33_7795. Pointed ROSAT observations show that their X-ray properties, including X-ray luminosity and variability, are consistent with those of pre-main-sequence (PMS) stars. These observations do not reveal any additional PMS candidates in 40' fields centered on HD 98800 and CD-33_7795. Hipparcos observations of TW Hya (Wichmann et al. 1998) and HD 98800 (Soderblom et al. 1998) show that both stars are roughly 50 pc away and are PMS with ages of ~ 10^7 yr. We searched the Hipparcos catalog (complete down to ~ 2-3 L_sun at this distance) for other PMS stars in the same area. In a 10-pc radius volume of space centered on the previously known PMS stars, we find one additional candidate PMS star (CD-36_7429) with a low space velocity, X-ray emission comparable to that of HD 98800, and Li absorption. There are eight other stars in this area that have dwarf spectral types and lie above the main sequence, but based on their weak X-ray emission, high space velocities, and lack of Li in low-resolution spectra (i.e. EW(Li) < 0.1 Angstroms), these are probably mis-classified subgiants or giants. The current positions and proper motions of TW Hya, HD 98800, and CD-36_7429 are inconsistent with them having formed as a group.

Eric L. N. Jensen; David H. Cohen; Ralph Neuhauser
1998-06-01

200

SPIN-ORBIT INTERACTION IN NEUTRON STAR/MAIN SEQUENCE BINARIES AND IMPLICATIONS FOR PULSAR TIMING

  Astrophysics (arXiv)

Summary: The spin-induced quadrupole moment of a rapidly rotating star changes the orbital dynamics in a binary system, giving rise to advance (or regression) of periastron and precession of the orbital plane. We show that these effects are important in the recently discovered radio pulsar/main sequence star binary system PSR J0045$-$7319, and can reliably account for the observed peculiar timing residuals. Precise measurements of the apsidal motion and orbital plane precession can yield valuable information on the internal structure and rotation of the star. The detection of orbital precession implies that the spin of the companion star is not aligned with the orbital angular momentum, and suggests that the supernova gave the pulsar a kick out of the original orbital plane. Tidal excitation of g-mode oscillations in the PSR J0045$-$7319 system induces an orbital period change of order $|\\Delta P_{\\rm orb}/P_{\\rm orb}|\\sim 10^{-6}$ at each periastron passage, but the secular trend depends on the radiative damping time of the g-modes. We also discuss the spin-orbit coupling effects for the accreting X-ray pulsars and the other known radio pulsar/main sequence binary, PSR B1259$-$63.

Dong Lai; Lars Bildsten; Vicky Kaspi
1995-05-09

201

The effect of starspots on the radii of low-mass pre-main sequence stars

  CERN Preprints

Summary: A polytropic model is used to investigate the effects of dark photospheric spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun), pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to unspotted stars of the same luminosity, where beta is the equivalent covering fraction of dark starspots and N \\simeq 0.45+/-0.05. This is a much stronger inflation than predicted by the models of Spruit & Weiss (1986) for main sequence stars with the same beta, where N \\sim 0.2 to 0.3. These models have been compared to radii determined for very magnetically active K- and M-dwarfs in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked, low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs have radii inflated by \\sim 10 per cent compared to an empirical radius-luminosity relation that is defined by magnetically inactive field dwarfs with interferometrica...

Jackson, R J
2014-01-01

202

Spectrophotometric properties of pre-main sequence stars: the Epsilon Chamaeleontis cluster

  Astrophysics (arXiv)

Summary: We present a study of flux-calibrated low-resolution optical spectroscopy of ten stars belonging to eight systems in the ~ 5 Myr-old Epsilon Chamaeleontis (Eps Cha) pre-main-sequence (PMS) star cluster. Using synthetic broadband colours, narrow-band continuum, atomic and molecular lines derived from the spectra, we compare the Eps Cha stars to a slightly older PMS cluster, the ~ 8 Myr-old Eta Cha cluster, and to main-sequence dwarfs. Using synthetic VRI colours and other indices, we find that the relationship between broadband colours and spectroscopic temperature indicators for Eps Cha cluster members is indistinguishable from that of Gyr-old dwarfs. This result is identical to that found earlier in Eta Cha. Gravity-sensitive line indices place the cluster between the giant and dwarf sequences, and there is clear evidence that Eps Cha stars have lower surface gravity than Eta Cha stars. This result is consistent with Eps Cha being the slightly younger PMS association, a few Myr younger according to the Hertzsprung-Russell (HR) diagram placement of these two clusters and comparison with PMS evolutionary grids. Late M-type Eps Cha cluster members show a B-band flux excess of ~ 0.2 mag compared to observations of standard dwarfs, which might be related to enhanced magnetic activity. A similar level of excess B-band emission appears to be a ubiquitous feature of low mass members of young stellar populations with ages less than a few hundred Myr, a very similar timescale to the PMS phase of elevated relative X-ray luminosity.

A-Ran Lyo; Warrick A. Lawson; M. S. Bessell
2008-07-07

203

Generation of Longitudinal Flux Tube Waves in Theoretical Main-Sequence Stars: Effects of Model Parameters

  CERN Preprints

Summary: Context. Continued investigation of the linkage between magneto-acoustic energy generation in stellar convective zones and the energy dissipation and radiative emission in outer stellar atmospheres in stars of different activity levels. Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by R. L. Kurucz and D. F. Gray. Additionally, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the mixing-length concept is combined with the magneto-hydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave en...

Fawzy, Diaa E
2010-01-01

204

Post-main-sequence debris from rotation-induced YORP break-up of small bodies

  CERN Preprints

Summary: Although discs of dust and gas have been observed orbiting white dwarfs, the origin of this circumstellar matter is uncertain. We hypothesize that the in-situ breakup of small bodies such as asteroids spun to fission during the giant branch phases of stellar evolution provides an important contribution to this debris. The YORP effect, which arises from radiation pressure, accelerates the spin rate of asymmetric asteroids, which can eventually shear themselves apart. This pressure is maintained and enhanced around dying stars because the outward push of an asteroid due to stellar mass loss is insignificant compared to the resulting stellar luminosity increase. Consequently, giant star radiation will destroy nearly all bodies with radii in the range 100 m - 10 km that survive their parent star's main sequence lifetime within a distance of about 7 au; smaller bodies are spun apart to their strongest, competent components. This estimate is conservative, and would increase for highly asymmetric shapes or incorpora...

Veras, Dimitri; Gaensicke, Boris T
2014-01-01

205

White dwarf-main sequence binaries from LAMOST: the DR1 catalogue

  CERN Preprints

Summary: Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. Methods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to...

Ren, Juanjuan; Luo, Ali; Zhao, Yongheng; Xiang, Maosheng; Liu, Xiaowei; Zhao, Gang; Jin, Ge; Zhang, Yong
2014-01-01

206

KH 15D: Gradual Occultation of a Pre-Main-Sequence Binary

  Astrophysics (arXiv)

Summary: We propose that the extraordinary ``winking star'' KH 15D is an eccentric pre-main-sequence binary that is gradually being occulted by an opaque screen. This model accounts for the periodicity, depth, duration, and rate of growth of the modern eclipses; the historical light curve from photographic plates; and the existing radial velocity measurements. It also explains the re-brightening events that were previously observed during eclipses, and the subsequent disappearance of these events. We predict the future evolution of the system and its full radial velocity curve. Given the small velocity of the occulting screen relative to the center of mass of the binary, the screen is probably associated with the binary, and may be the edge of a precessing circumbinary disk.

Joshua N. Winn; Matthew J. Holman; John A. Johnson; Krzysztof Z. Stanek; Peter M. Garnavich
2004-01-21

207

ON BLUE STRAGGLER FORMATION BY DIRECT COLLISIONS OF MAIN-SEQUENCE STARS

  Astrophysics (arXiv)

Summary: We report the results of new SPH calculations of parabolic collisions between main-sequence (MS) stars. The stars are assumed to be close to the MS turn-off point in a globular cluster and are therefore modeled as $n=3$, $\\Gamma=5/3$ polytropes. We find that the high degree of central mass concentration in these stars has a profound effect on the hydrodynamics. In particular, very little hydrodynamic mixing occurs between the dense, helium-rich inner cores and the outer envelopes. As a result, and in contrast to what has been assumed in previous studies, blue stragglers formed by direct stellar collisions are not necessarily expected to have anomalously high helium abundances in their envelopes or to have their cores replenished with fresh hydrogen fuel.

James C. Lombardi, Jr.; Frederic A. Rasio; Stuart L. Shapiro
1995-02-28

208

Oscillation mode frequencies of 61 main-sequence and subgiant stars observed by Kepler

  CERN Preprints

Summary: Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology. We provide the mode frequencies required for performing comparison with those obtained from stellar modelling. Time series of 9 months of data have been used. The power spectra of 61 main-sequence and subgiant stars have been analysed using both Maximum Likelihood Estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We {{\\bf derived and describe}} a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individuals. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities. Here we give the mode frequencies of 61 stars (with their 1-sigma error bars), as well as their associated \\'echelle diagrams.

Appourchaux, T; Garcia, R A; Gruberbauer, M; Verner, G A; Antia, H M; Benomar, O; Campante, T L; Davies, G R; Deheuvels, S; Handberg, R; Hekker, S; Howe, R; Régulo, C; Salabert, D; Bedding, T R; White, T R; Ballot, J; Mathur, S; Aguirre, V Silva; Elsworth, Y P; Basu, S; Gilliland, R L; Christensen-Dalsgaard, J; Kjeldsen, H; Uddin, K; Stumpe, M C; Barclay, T
2012-01-01

209

Substellar Companions to Main Sequence Stars: No Brown Dwarf Desert at Wide Separations

  Astrophysics (arXiv)

Summary: We use three field L and T dwarfs which were discovered to be wide companions to known stars by the Two Micron All-Sky Survey (2MASS) to derive a preliminary brown dwarf companion frequency. Observed L and T dwarfs indicate that brown dwarfs are not unusually rare as wide (Delta >1000 A.U.) systems to F-M0 main-sequence stars (M>0.5M_sun, M_V<9.5), even though they are rare at close separation (Delta <3 A.U.), the ``brown dwarf desert.'' Stellar companions in these separation ranges are equally frequent, but brown dwarfs are >~ 10 times as frequent for wide than close separations. A brown dwarf wide-companion frequency as low as the 0.5% seen in the brown dwarf desert is ruled out by currently-available observations.

John E. Gizis; J. Davy Kirkpatrick; Adam Burgasser; I. Neill Reid; David G. Monet; James Liebert; John C. Wilson
2001-03-14

210

Element stratification in main sequence stars and its effect on stellar oscillations

  Astrophysics (arXiv)

Summary: The element settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars, which cannot be neglected in the computations of stellar structure. These processes where first introduced to account for abundance anomalies in "peculiar stars", but their importance in the so-called "normal" stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, like rotation-induced mixing or mass loss, which increase the settling time scales. We have recently obtained clear evidences that asteroseismology of main sequence solar type stars can give signatures of the chemical variations inside the stars and help for a better understanding of these processes.

Sylvie Vauclair
2003-09-30

211

Investigating the pre-main sequence magnetic chemically peculiar system HD 72106

  CERN Preprints

Summary: The origin of the strong magnetic fields observed in chemically peculiar Ap and Bp stars stars has long been debated. The recent discovery of magnetic fields in the intermediate mass pre-main sequence Herbig Ae and Be stars links them to Ap and Bp stars, providing vital clues about Ap and Bp stars and the origin and evolution of magnetic fields in intermediate and high mass stars. A detailed study of one young magnetic B star, HD 72106A, is presented. This star appears to be in a binary system with an apparently normal Herbig Ae star. A maximum longitudinal magnetic field strength of +391 +/- 65 G is found in HD 72106A, as are strong chemical peculiarities, with photospheric abundances of some elements ranging up to 100x above solar.

Folsom, C P; Hanes, D A; Catala, C; Alecian, E; Bagnulo, S; Böhm, T; Bouret, J C; Donati, J F; Landstreet, J D
2006-01-01

212

Circumstellar Disks Around Pre-Main-Sequence Stars: What ISO Can Tell Us

  Astrophysics (arXiv)

Summary: ISO observations will improve our understanding of disks around low-mass pre-main-sequence stars in various ways. In particular, ISO can measure simultaneously spectral energy distributions and low-resolution spectra from 2 to 12 \\um of T Tauri stars with greater sensitivity, greater spectral coverage and finer spectral resolution than available in ground-based observations or with IRAS. We illustrate the importance of such observations for our understanding of the disk structure and heating mechanisms, the disk evolution (in particular the formation of gaps caused by the presence of companion stars or large planets) and dissipation using preliminary results from PHOT and PHOT-S for T Tauri stars in the Chamaeleon cloud.

A. Natta; M. R. Meyer; S. V. W. Beckwith
1997-10-08

213

Mid-Infrared Spectra of Dust Debris Around Main-Sequence Stars

  Astrophysics (arXiv)

Summary: We report spectra obtained with the Spitzer Space Telescope in the wavelength range between 14 microns and 35 microns of 19 nearby main-sequence stars with infrared excesses. The six stars with strong dust emission show no recognizable spectral features, suggesting that the bulk of the emitting particles have diameters larger than 10 microns. If the observed dust results from collisional grinding of larger solids, we infer minimum masses of the parent body population between 0.004 of the Earth's mass and 0.06 of the Earth's mass. We estimate grain production rates of 10 Gg/s around lambda Boo and HR 1570; selective accretion of this matter may help explain their peculiar surface abundances. There appear to be inner truncations in the dust clouds at 48 AU, 11 AU, 52 AU and 54 AU around HR 333, HR 506, HR 1082 and HR 3927, respectively.

M. Jura; C. H. Chen; E. Furlan; J. Green; B. Sargent; W. J. Forrest; D. M. Watson; D. J. Barry; P. Hall; T. L. Herter; J. R. Houck; G. C. Sloan; K. Uchida; P. D. 'Alessio; B. R. Brandl; L. D. Keller; F. Kemper; P. Morris; J. Najita; N. Calvet; L. Hartmann; P. C. Myers
2004-05-31

214

Gaia-ESO Survey: The analysis of pre-main sequence stellar spectra

  CERN Preprints

Summary: This paper describes the analysis of UVES and GIRAFFE spectra acquired by the Gaia-ESO Public Spectroscopic Survey in the fields of young clusters whose population includes pre-main sequence (PMS) stars. Both methods that have been extensively used in the past and new ones developed in the contest of the Gaia-ESO survey enterprise are available and used. The internal precision of these quantities is estimated by inter-comparing the results obtained by such different methods, while the accuracy is estimated by comparison with independent external data, like effective temperature and surface gravity derived from angular diameter measurements, on a sample of benchmarks stars. Specific strategies are implemented to deal with fast rotation, accretion signatures, chromospheric activity, and veiling. The analysis carried out on spectra acquired in young clusters' fields during the first 18 months of observations, up to June 2013, is presented in preparation of the first release of advanced data products. Stellar par...

Lanzafame, A C; Damiani, F; Franciosini, E; Cottaar, M; Sousa, S G; Tabernero, H M; Klutsch, A; Spina, L; Biazzo, K; Prisinzano, L; Sacco, G G; Randich, S; Brugaletta, E; Mena, E Delgado; Adibekyan, V; Montes, D; Bonito, R; Gameiro, J F; Alcalá, J M; Hernández, J I González; Jeffries, R; Messina, S; Meyer, M; Gilmore, G; Asplund, M; Binney, J; Bonifacio, P; Drew, J E; Feltzing, S; Ferguson, A M N; Micela, G; Negueruela, I; Prusti, T; Rix, H-W; Vallenari, A; Alfaro, E J; Prieto, C Allende; Babusiaux, C; Bensby, T; Blomme, R; Bragaglia, A; Flaccomio, E; Francois, P; Hambly, N; Irwin, M; Koposov, S E; Korn, A J; Smiljanic, R; Van Eck, S; Walton, N; Bayo, A; Bergemann, M; Carraro, G; Costado, M T; Edvardsson, B; Heiter, U; Hill, V; Hourihane, A; Jackson, R J; Jofré, P; Lardo, C; Lewis, J; Lind, K; Magrini, L; Marconi, G; Martayan, C; Masseron, T; Monaco, L; Morbidelli, L; Sbordone, L; Worley, C C; Zaggia, S
2015-01-01

215

Testing evolutionary tracks of Pre-Main Sequence stars: the case of HD113449

  Astrophysics (arXiv)

Summary: Evolutionary tracks are of key importance for the understanding of star formation. Unfortunately, tracks published by various groups differ so that it is fundamental to have observational tests. In order to do this, we intend to measure the masses of the two components of the Pre-Main Sequence binary HD113449 by combining radial velocity measurements taken with HARPS, with infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit that has already been determined, combined with the first AMBER measurement, allows us to obtain a very first estimation of the inclination of the binary system and from this the masses of the two stars. More AMBER measurements of HD 113449 are needed to improve the precision on the masses: in the ESO period P82 two new measurements are scheduled.

F. Cusano; E. W. Guenther; M. Esposito; M. Mundt; E. Covino; J. M. Alcalá
2008-09-26

216

Testing evolutionary tracks of Pre-Main Sequence stars: the case of HD113449

  CERN Preprints

Summary: Evolutionary tracks are of key importance for the understanding of star formation. Unfortunately, tracks published by various groups differ so that it is fundamental to have observational tests. In order to do this, we intend to measure the masses of the two components of the Pre-Main Sequence binary HD113449 by combining radial velocity measurements taken with HARPS, with infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit that has already been determined, combined with the first AMBER measurement, allows us to obtain a very first estimation of the inclination of the binary system and from this the masses of the two stars. More AMBER measurements of HD 113449 are needed to improve the precision on the masses: in the ESO period P82 two new measurements are scheduled.

Cusano, F; Esposito, M; Mundt, M; Covino, E; Alcalá, J M
2008-01-01

217

Post Main Sequence Orbital Circularization of Binary Stars in the Large and Small Magellanic Clouds

  Astrophysics (arXiv)

Summary: We present results from a study of the orbits of eclipsing binary stars (EBs) in the Magellanic Clouds. The samples comprise 4510 EBs found in the Large Magellanic Cloud (LMC) by the MACHO project, 2474 LMC EBs found by the OGLE-II project (of which 1182 are also in the MACHO sample), 1380 in the Small Magellanic Cloud (SMC) found by the MACHO project, and 1317 SMC EBs found by the OGLE-II project (of which 677 are also in the MACHO sample); we also consider the EROS sample of 79 EBs in the bar of the LMC. Statistics of the phase differences between primary and secondary minima allow us to infer the statistics of orbital eccentricities within these samples. We confirm the well-known absence of eccentric orbit in close binary stars. We also find evidence for rapid circularization in longer period systems when one member evolves beyond the main sequence, as also found by previous studies.

Lorenzo Faccioli; Charles Alcock; Kem Cook
2007-11-17

218

Investigating the pre-main sequence magnetic chemically peculiar system HD 72106

  Astrophysics (arXiv)

Summary: The origin of the strong magnetic fields observed in chemically peculiar Ap and Bp stars stars has long been debated. The recent discovery of magnetic fields in the intermediate mass pre-main sequence Herbig Ae and Be stars links them to Ap and Bp stars, providing vital clues about Ap and Bp stars and the origin and evolution of magnetic fields in intermediate and high mass stars. A detailed study of one young magnetic B star, HD 72106A, is presented. This star appears to be in a binary system with an apparently normal Herbig Ae star. A maximum longitudinal magnetic field strength of +391 +/- 65 G is found in HD 72106A, as are strong chemical peculiarities, with photospheric abundances of some elements ranging up to 100x above solar.

C. P. Folsom; G. A. Wade; D. A. Hanes; C. Catala; E. Alecian; S. Bagnulo; T. Boehm; J. -C. Bouret; J. -F. Donati; J. D. Landstreet
2006-12-08

219

Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars

  CERN Preprints

Summary: We present an analysis of the intrinsic colors and temperatures of 5-30 Myr old pre-main sequence (pre-MS) stars using the F0 through M9 type members of nearby, negligibly reddened groups: Eta Cha cluster, TW Hydra Association, Beta Pic Moving Group, and Tucana-Horologium Association. To check the consistency of spectral types from the literature, we estimate new spectral types for 52 nearby pre-MS stars with spectral types F3 through M4 using optical spectra taken with the SMARTS 1.5-m telescope. Combining these new types with published spectral types, and photometry from the literature (Johnson-Cousins BVIc, 2MASS JHKs and WISE W1, W2, W3, and W4), we derive a new empirical spectral type-color sequence for 5-30 Myr old pre-MS stars. Colors for pre-MS stars match dwarf colors for some spectral types and colors, but for other spectral types and colors, deviations can exceed 0.3 mag. We estimate effective temperatures (Teff) and bolometric corrections (BCs) for our pre-MS star sample through comparing their ph...

Pecaut, Mark J
2013-01-01

220

Pre-main-sequence isochrones -- III. The Cluster Collaboration isochrone server

  CERN Preprints

Summary: We present an isochrone server for semi-empirical pre-main-sequence model isochrones in the following systems: Johnson-Cousins, Sloan Digital Sky Survey, Two-Micron All-Sky Survey, Isaac Newton Telescope (INT) Wide-Field Camera, and INT Photometric H$\\alpha$ Survey (IPHAS)/UV-Excess Survey (UVEX). The server can be accessed via the Cluster Collaboration webpage {http://www.astro.ex.ac.uk/people/timn/isochrones/}. To achieve this we have used the observed colours of member stars in young clusters with well-established age, distance and reddening to create fiducial loci in the colour-magnitude diagram. These empirical sequences have been used to quantify the discrepancy between the models and data arising from uncertainties in both the interior and atmospheric models, resulting in tables of semi-empirical bolometric corrections (BCs) in the various photometric systems. The model isochrones made available through the server are based on existing stellar interior models coupled with our newly derived semi-empiric...

Bell, Cameron P M; Naylor, Tim; Mayne, N J; Jeffries, R D; Mamajek, Eric E; Rowe, John
2014-01-01

221

HE 0437-5439 -- an unbound hyper-velocity main-sequence B-type star

  Astrophysics (arXiv)

Summary: We report the discovery of a 16th magnitude star, HE0437-5439, with a heliocentric radial velocity of +723+-3km/s. A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and the UVES spectrograph shows that HE0437-5439 is a main sequence B-type star with Teff=20350K, log g=3.77, solar within a factor of a few helium abundance and metal content, rotating at v sin i=54km/s. Using appropriate evolutionary tracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Its galactic rest frame velocity is at least 563km/s, almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole at the Galactic center (GC). HE0437-5439 needs about 100Myrs to travel from the GC to its presentposition, much longer than its main sequence lifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach its position from the center of the LMC. The proper motion predicted in this case is about 2mas/y (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis.

Heinz Edelmann; Ralf Napiwotzki; Uli Heber; Norbert Christlieb; Dieter Reimers
2005-11-10

222

Pre-main sequence stars in open clusters. I. The DAY-I catalogue

  Astrophysics (arXiv)

Summary: We present the project we are carrying out at present, the search for and characterisation of pre-main sequence (PMS) stars among the members of Galactic young clusters. The observations of 10 southern clusters, nine of them located in the Carina-Sagittarius spiral arm of the Milky Way are presented. We aim at listing candidate PMS member stars in young clusters. The catalogued stars will serve as a basis for future spectroscopic studies of individual objects to determine the properties of stellar formation in the last phases before the main sequence stage. Properties such as the presence of residual envelopes or disks, age spread among PMS members, and the possible presence of several episodes of star formation in the clusters, are to be addressed. Multicolour photometry in the UBVRcIc system has been obtained for 10 southern young clusters in the fourth Galactic quadrant, located between Galactic longitudes l=238 and l=310. For six clusters in the sample, the observations presented here provide the first published study based on CCD photometry. A quantitative comparison is performed with post-MS isochrones, and PMS isochrones from three different evolutionary models are used in the photometric membership analysis for possible PMS stars. The observations produce photometric indices in the Johnson-Cousins photometric systems for a total of 26962 stars. Matching with the 2MASS data base provides astrometric calibration for all stars, and JHK 2MASS indices for 60 per cent of them. Post-MS cluster ages range from 4 to 60 Myr, whereas the photometric membership analysis assigns PMS membership to a total of 842 stars, covering an age range between 1 and 10 Myr. A catalogue, named DAY-I, with the information on the PMS candidate members has been ellaborated.

A. J. Delgado; E. J. Alfaro; J. L. Yun
2007-03-14

223

The magnetic field of the pre-main sequence Herbig Ae star HD 190073

  Astrophysics (arXiv)

Summary: The general context of this paper is the study of magnetic fields in the pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are likely to play an important role in pre-main sequence evolution at these masses, in particular in controlling the gains and losses of stellar angular momentum. The particular aim of this paper is to announce the detection of a structured magnetic field in the Herbig Ae star HD 190073, and to discuss various scenarii for the geometry of the star, its environment and its magnetic field. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution and signal-to-noise circular polarization spectra which demonstrate unambiguously the presence of a magnetic field in the photosphere of this star. Nine circular polarization spectra were obtained, each one showing a clear Zeeman signature. This signature is suggestive of a magnetic field structured on large scales. The signature, which corresponds to a longitudinal magnetic field of 74+- 10 G, does not vary detectably on a one-year timeframe, indicating either an azimuthally symmetric field, a zero inclination angle between the rotation axis and the line of sight, or a very long rotation period. The optical spectrum of HD 190073 exhibits a large number of emission lines. We discuss the formation of these emission lines in the framework of a model involving a turbulent heated region at the base of the stellar wind, possibly powered by magnetic accretion. This magnetic detection brings an important element for our understanding of stellar magnetism at intermediate masses.

C. Catala; E. Alecian; J. -F. Donati; G. A. Wade; J. D. Landstreet; T. Boehm; J. -C. Bouret; S. Bagnulo; C. Folsom; J. Silvester
2006-10-17

224

Post Common Envelope Binaries from SDSS. V: Four eclipsing white dwarf main sequence binaries

  Astrophysics (arXiv)

Summary: We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main sequence (WDMS) binaries from the Sloan Digital Sky Survey, and report on follow-up observations of these systems. Orbital periods and ephemerides have been established from multi-season photometry. SDSS1435+3733, with Porb=3h has the shortest orbital period of all known eclipsing WDMS binaries. Time-resolved spectroscopic observations have been obtained and the radial velocities of the secondary stars in all four systems were measured. A spectral decomposition/fitting technique was then employed to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light curve modeling code to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of Mwd~0.4-0.6Msun, in line with the expectations from close binary evolution. The DC white dwarf in SDSS0303+0054 has a mass of Mwd>0.85Msun, making it unusually massive for a post-common envelope system. Our new additions raise the number of known eclipsing WDMS binaries to fourteen, and we find that the average white dwarf mass in this sample is =0.57+/-0.16Msun, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6Msun) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main sequence stars and of white dwarfs.

S. Pyrzas; B. T. Gänsicke; T. R. Marsh; A. Aungwerojwit; A. Rebassa-Mansergas; P. Rodríguez-Gil; J. Southworth; M. R. Schreiber; A. Nebot Gomez-Moran; D. Koester
2008-12-15

225

Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I

  Astrophysics (arXiv)

Summary: We report the discovery of quiescent emission from molecular hydrogen gas located in the circumstellar disks of six pre-main sequence stars, including two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon I star forming region. For two of these stars, we also place upper limits on the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence sources now known to be sources of quiescent near-infrared hydrogen emission, four possess transitional disks, which suggests that detectable levels of H$_2$ emission and the presence of inner disk holes are correlated. These H$_2$ detections demonstrate that these inner holes are not completely devoid of gas, in agreement with the presence of observable accretion signatures for all four of these stars and the recent detections of [Ne II] emission from three of them. The overlap in [Ne II] and H$_2$ detections hints at a possible correlation between these two features and suggests a shared excitation mechanism of high energy photons. Our models, combined with the kinematic information from the H$_2$ lines, locate the bulk of the emitting gas at a few tens of AU from the stars. We also find a correlation between H$_2$ detections and those targets which possess the largest H$\\alpha$ equivalent widths, suggesting a link between accretion activity and quiescent H$_2$ emission. We conclude that quiescent H$_2$ emission from relatively hot gas within the disks of TTS is most likely related to on-going accretion activity, the production of UV photons and/or X-rays, and the evolutionary status of the dust grain populations in the inner disks.

Jeffrey S. Bary; David A. Weintraub; Sonali J. Shukla; Jarron M. Leisenring; Joel H. Kastner
2008-01-17

226

The evolution of main sequence star + white dwarf binary systems towards Type~Ia supernovae

  Astrophysics (arXiv)

Summary: Close binaries consisting of a main sequence star and a white dwarf are considered as candidates for Type~Ia supernova progenitors. We present selfconsistent calculations of the time dependence of the structure of the main sequence star, the mass transfer rate, and the orbit by means of a binary stellar evolution program. In contrast to results based on simple estimates of the mass transfer rate in systems of the considered type, our results allow for the possibility that even systems with rather small initial white dwarf masses (~ 0.7 M_sun) may produce Type Ia supernovae. We present results for two different metallicities, Z=0.02 and Z=0.001. We find that for systems with the lower metallicity, the mass transfer rates are on average five times larger than in comparable system at solar metallicity. This leads to a systematic shift of the supernova Ia progenitor population. Firstly, while for Z=0.02 donor star initial masses in supernova progenitor systems are restricted to the range 1.6...2.3 M_sun, they are in the interval 1.4...1.8 M_sun at low Z. Secondly, the initial white dwarf masses need, on average, to be larger by 0.2 M_sun at low Z in order to obtain a Chandrasekhar mass white dwarf. This metallicity dependences may be responsible for a drop of the Type Ia supernova rate for low metallicity, and may introduce a Z-dependence in the properties of supernovae. We also estimate the X-ray luminosities of the computed systems, and investigate their donor star and orbital properties.

N. Langer; A. Deutschmann; S. Wellstein; P. Hoeflich
2000-08-29

227

Kholo Bay, Hawai`i, earthquake sequence of 2006: Relationship of the main shock slip with locations

  Geosciences Websites

Summary: Kholo Bay, Hawai`i, earthquake sequence of 2006: Relationship of the main shock slip with locations of the Island of Hawai`i on 15 October 2006, and static stress drops of small earthquakes that occurred in 2006, T., P. G. Okubo, and C. J. Wolfe (2010), Kholo Bay, Hawai`i, earthquake sequence of 2006

Wolfe, Cecily J.

228

An Objective Definition for the Main Sequence of Star-Forming Galaxies

  CERN Preprints

Summary: The Main Sequence (MS) of star-forming galaxies plays a fundamental role in driving galaxy evolution and in our efforts to understand it. However, different studies find significant differences in the normalization, slope and shape of the MS. These discrepancies arise mainly from the different selection criteria adopted to isolate star-forming galaxies, that may include or exclude galaxies with specific star formation rate (SFR) substantially below the MS value. To obviate this limitation of all current criteria, we propose an objective definition of the MS that does not rely at all on a pre-selection of star-forming galaxies. Constructing the 3D SFR-Mass-Number plot, the MS is then defined as the ridge line of the star-forming peak, as illustrated with various figures. The advantages of such definition are manifold. If generally adopted it will facilitate the inter-comparison of results from different groups using the same star formation rate (SFR) and stellar mass diagnostics, or to highlight the relative s...

Renzini, Alvio
2015-01-01

229

The Bottom of the Main Sequence and Beyond: Speculations, Calculations, Observations, and Discoveries (1958-2002)

  Astrophysics (arXiv)

Summary: In this paper, I briefly review the theoretical and observational work done since 1958 on the gaseous objects with mass below the H-burning limit. Special attention is paid to the theory that I developed during the period 1958-1962 for the hydrogen-rich gaseous objects with mass below the minimum main sequence mass of ~ 0.08 Msun. The three main predictions of this theory (Kumar 1962a; Kumar 1962b; Kumar 1963a; Kumar 1963b) are discussed. Fragmentation processes for interstellar clouds are discussed, and it is pointed out that the minimum mass of a gaseous fragment may be as low as 0.001 Msun. Observational results obtained since 1995 on the luminous and dark objects of very low mass are briefly reviewed. Comments are made on the basic nature of the stars and planets, and it is pointed out that processes of star formation are fundamentally different from those of planet formation. Arguments are presented to show that some of the very-low-mass dark companions, discovered since 1995, were most probably formed by the star formation processes and not by the planet formation processes.

Shiv S. Kumar
2002-08-05

230

The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4

  Astrophysics (arXiv)

Summary: The deepest optical image ever in a globular star cluster, a Hubble Space Telescope (HST) 123 orbit exposure in a single field of Messier 4, was obtained in 2 filters (F606W, F814W) over a 10 week period in early 2001. A somewhat shallower image obtained in 1995 allowed us to select out cluster and field objects via their proper motion displacement resulting in remarkably clean color-magnitude diagrams that reach to V = 30, I = 28. The cluster main sequence luminosity function contains very few stars fainter than MV = 15.0, MI = 11.8 which, in both filters, is more than 2 magnitudes brighter than our limit. This is about the faintest luminosity seen among field Population II subdwarfs of the same metallicity. However, there remains a sprinkling of potential cluster stars to lower luminosity all the way down to our limiting magnitudes. These latter objects are significantly redder than any known metal-poor field subdwarf. Comparison with the current generation of theoretical stellar models implies that the masses of the lowest luminosity cluster stars observed are near 0.09 Msun. We derive the mass function of the cluster in our field and find that it is very slowly rising towards the lowest masses with no convincing evidence of a turnover even below 0.1 Msun. The formal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter of 2.35) with a 99% confidence interval 0.55 - 1.05. A consistency check between these slopes and the number of observed cluster white dwarfs yields a range of possible conclusions, one of which is that we have indeed seen the termination of the white dwarf cooling sequence in M4.

Harvey B. Richer; James Brewer; Gregory G. Fahlman; Brad K. Gibson; Brad M. Hansen; Rodrigo Ibata; Jasonjot S. Kalirai; Marco Limongi; R. Michael Rich; Ivo Saviane; Michael M. Shara; Peter B. Stetson
2002-07-11

231

Convection in the atmospheres and envelopes of Pre-Main Sequence stars

  Astrophysics (arXiv)

Summary: The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. Since the convection penetrates into the stellar atmosphere, also the treatment of convection in the modeling of stellar atmospheres will affect the location of the Hayashi tracks. We present new non-grey PMS tracks for Teff>4000 K. We compute several grids of evolutionary tracks varying: 1. the treatment of convection: either the Mixing Length Theory (MLT) or Canuto et al. (1996, CGM) treatment; 2.the atmospheric boundary conditions: we use the new Vienna grids of ATLAS9 atmospheres (Heiter et al. 2002a), which were computed using either MLT (with alpha=Lambda/Hp=0.5) or CGM treatments. We compute as well grids of models with the NextGen (Allard & Hauschildt 1997) atmosphere models, and a 1 Msun grey MLT evolutionary track using the alpha calibration based on 2D-hydrodynamical models (Ludwig et al. 1999)}. These different grids of models allow us to analyze the effects of convection modeling on the non--grey PMS evolutionary tracks. We conclude that: 1. In spite of the solar calibration, if MLT convection is adopted a large uncertainty results in the shape and location of PMS tracks. 2. As long as the model of convection is not the same in the interior and in the atmosphere the optical depth at which we take the boundary conditions is an additional parameter of the models. 3. The comparison between NextGen and ATLAS9 based models shows that, in the Teff domain they have in common (4000--10000K), the PMS location is mainly determined by the treatment of the over-adiabatic convection. 4. Comparison with observations indicates that, for any convection model and for any of the atmosphere grids, convection in the PMS phase appears to be less efficient than what is necessary in order to fit the Sun.

J. Montalban; F. D'Antona; F. Kupka; U. Heiter
2003-10-23

232

The ISO-LWS far-infrared spectra of pre-Main Sequence objects

  Astrophysics (arXiv)

Summary: The Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO) satellite has allowed for the first time to investigate the whole far infrared (FIR) part of the spectrum (from 43 to 197 um), inaccessible from the ground. The FIR spectrum is crucial for the study of the early phases of protostellar evolution since the peak of the continuum emission of the protostars falls around 100 um and many molecular species emit in this range. In this paper we review the main results of the spectroscopic observations carried out with the LWS on a sample of pre-Main Sequence objects in different evolutionary stages. The FIR spectra are characterized by a strong continuum with superimposed atomic and molecular emission lines. The fine structure lines emitted by the OI and CII atoms are present in all the objects of our sample while emission from molecular species (CO, H_2O and OH) dominate the spectrum of the younger and more embedded objects; lines of highly ionized atoms (OIII, NIII, NII) are present only in high luminosity sources. We will show how both the FIR continuum and the emission lines are powerful indicators of the evolutionary status of young embedded objects. The analysis of the LWS spectra has allowed to study only the average properties of the young stellar objects, due to the large field of view (~ 80 arcsec), the limited sensitivity and the poor spectral resolution (~ 200) of the instrument in its grating mode. The higher spatial and spectral resolutions of the instruments on board the Herschel satellite will allow to address in deeper detail the scientific problems highlighted by the ISO results.

Milena Benedettini; Stefano Pezzuto; Luigi Spinoglio; Paolo Saraceno; Anna Maria di Giorgio
2003-10-03

233

A Spectroscopic Technique for Measuring Stellar Properties of Pre-Main Sequence Stars

  Astrophysics (arXiv)

Summary: We describe a technique for deriving effective temperatures, surface gravities, rotation velocities, and radial velocities from high resolution near-IR spectra. The technique matches the observed near-IR spectra to spectra synthesized from model atmospheres. For pre-main sequence stars, we use the same matching process to also measure the amount of excess near-IR emission. The information derived from high resolution spectra comes from line shapes and the relative line strengths of closely spaced lines. The values for the stellar parameters we derive are therefore independent of those derived from low resolution spectroscopy and photometry. The new method offers the promise of improved accuracy in placing young stellar objects on evolutionary model tracks. We discuss the possible systematic effects on our determination of the stellar parameters and evaluate the accuracy of the results derivable from high resolution spectra. The analysis of high resolution near-IR spectra of MK standards shows that the technique gives very accurate values for the effective temperature. The biggest uncertainty in comparing our results with optical spectral typing of MK standards is in the spectral type to effective temperature conversion for the standards themselves. Even including this uncertainty, the 1 sigma difference between the optical and IR temperatures for 3000-5800 K dwarfs is only 140 K. In a companion paper (Doppmann, Jaffe, & White 2003), we present an analysis of heavily extincted young stellar objects rho Oph.

G. W. Doppmann; D. T. Jaffe
2003-08-29

234

Submillimeter lines from circumstellar disks around pre-main sequence stars

  Astrophysics (arXiv)

Summary: Observations of submillimeter lines of CO (up to J=6-5), HCO+, HCN (up to J=4-3) and their isotopes from circumstellar disks around low mass pre-main sequence stars are presented. The different line ratios and optical depths indicate that most of the observed line emission arises from an intermediate disk layer with high densities of 1E6-1E8 cm-3 and moderately warm temperatures in the outer regions. The data are compared with three disk models from the literature using a full 2D Monte Carlo radiative transfer code. The abundances of the molecules are constrained from the more optically thin 13C species and indicate depletions of 1-30 for LkCa 15 and very high depletions of > 100 for TW Hya with respect to dark cloud abundances. Evidence for significant freeze-out (factors 10 or larger) of CO and HCO+ onto grain surfaces at temperatures below 22 K is found, but the abundances of these molecules must also be low in the warmer upper layer, most likely as a result of photodissociation. A warm upper layer near the surface of a flaring disk heated by stellar and interstellar radiation is an appropriate description of the observations of TW Hya. LkCa 15 seems to be cooler at the surface, perhaps due to dust settling.

Gerd-Jan van Zadelhoff; Ewine F. van Dishoeck; Wing-Fai Thi; Geoffrey A. Blake
2001-08-23

235

Oscillation mode linewidths of main-sequence and subgiant stars observed by Kepler

  CERN Preprints

Summary: Solar-like oscillations have been observed by {{\\it Kepler}} and CoRoT in several solar-type stars. We study the variations of stellar p-mode linewidth as a function of effective temperature. Time series of 9 months of Kepler data have been used. The power spectra of 42 cool main-sequence stars and subgiants have been analysed using both Maximum Likelihood Estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths and mode heights. Here we report on the mode linewidth at maximum power and at maximum mode height for these 42 stars as a function of effective temperature. We show that the mode linewidth at either maximum mode height or maximum amplitude follows a scaling relation with effective temperature, which is a combination of a power law plus a lower bound. The typical power law index is about 13 for the linewidth derived from the maximum mode height, and about 16 for the linewidth derived from the maximum amplitude while the lower bound is about 0.3 micr...

Appourchaux, T; Gruberbauer, M; Chaplin, W J; Garcia, R A; Handberg, R; Verner, G A; Antia, H M; Campante, T L; Davies, G R; Deheuvels, S; Hekker, S; Howe, R; Salabert, D; Bedding, T R; White, T R; Houdek, G; Aguirre, V Silva; Elsworth, Y P; Van Cleve, J; Clarke, B D; Hall, J R; Kjeldsen, H
2011-01-01

236

Updating the Dartmouth Stellar Evolution Model Grid: Pre-Main-Sequence Models & Magnetic Fields

  CERN Preprints

Summary: We present the current status of an effort to create an updated grid of low-mass stellar evolution mass tracks and isochrones computed using the Dartmouth stellar evolution code. Emphasis is placed on reliably extending the present grid to the pre-main-sequence, where modeling uncertainties have the greatest impact. Revisions to the original code release include: updated surface boundary conditions, the introduction of deuterium burning, and magnetic fields. The mass track grid contains models with a mass above 0.1 $M_{\\odot}$ and metallicities in the range of -0.5 to +0.5 dex. Magnetic mass tracks are calculated for surface magnetic field strengths between 0.1 kG and 4.0 kG using two different prescriptions for magneto-convection. Standard and magnetic model isochrones are available for ages older than 1 Myr. Tabulated quantities include the stellar fundamental properties, absolute photometric magnitudes, magnetic field properties, convective turnover times, apsidal motion constants, and lithium abundances. ...

Feiden, Gregory A; Chaboyer, Brian
2014-01-01

237

Winds of Main-Sequence Stars: Observational Limits and a Path to Theoretical Prediction

  CERN Preprints

Summary: It is notoriously difficult to measure the winds of solar-type stars. Traditional spectroscopic and radio continuum techniques are sensitive to mass loss rates at least two to three orders of magnitude stronger than the Sun's relatively feeble wind. Much has been done with these methods to probe the more massive outflows of younger (T Tauri) and older (giant, AGB, supergiant) cool stars, but the main sequence remains terra incognita. This presentation reviews the limits on traditional diagnostics and outlines more recent ideas such as Lyman alpha astrospheres and charge-exchange X-ray emission. In addition, there are hybrid constraints on mass loss rates that combine existing observables and theoretical models. The Sackmann/Boothroyd conjecture of a more massive young Sun (and thus a much stronger ZAMS wind) is one such idea that needs to be tested further. Another set of ideas involves a strong proposed coupling between coronal heating and stellar mass loss rates, where the former is easier to measure in sta...

Cranmer, S R
2007-01-01

238

Chemical peculiarities in magnetic and non-magnetic pre-main sequence A and B stars

  CERN Preprints

Summary: In A- and late B-type stars, strong magnetic fields are always associated with Ap and Bp chemical peculiarities. However, it is not clear at what point in a star's evolution those peculiarities develop. Strong magnetic fields have been observed in pre-main sequence A and B stars (Herbig Ae and Be stars), and these objects have been proposed to be the progenitors of Ap and Bp stars. However, the photospheric chemical abundances of these magnetic Herbig stars have not been studied carefully, and furthermore the chemical abundances of 'normal' non-magnetic Herbig stars remain poorly characterized. To investigate this issue, we have studied the photospheric compositions of 23 Herbig stars, four of which have confirmed magnetic fields. Surprisingly, we found that half the non-magnetic stars in our sample show lambda Bootis chemical peculiarities to varying degrees. For the stars with detected magnetic fields, we find one chemically normal star, one star with lambda Boo peculiarities, one star displaying weak Ap/Bp...

Folsom, C P; Wade, G A; Landstreet, J D; Alecian, E
2013-01-01

239

YSOVAR: Six pre-main-sequence eclipsing binaries in the Orion Nebula Cluster

  CERN Preprints

Summary: Eclipsing binaries (EBs) provide critical laboratories for empirically testing predictions of theoretical models of stellar structure and evolution. Pre-main-sequence (PMS) EBs are particularly valuable, both due to their rarity and the highly dynamic nature of PMS evolution, such that a dense grid of PMS EBs is required to properly calibrate theoretical PMS models. Analyzing multi-epoch, multi-color light curves for 2400 candidateOrion Nebula Cluster (ONC) members from our Warm Spitzer Exploration Science Program YSOVAR, we have identified 12 stars whose light curves show eclipse features. Four of these 12 EBs are previously known. Supplementing our light curves with follow-up optical and near-infrared spectroscopy, we establish two of the candidates as likely field EBs lying behind the ONC. We confirm the remaining six candidate systems, however, as newly identified ONC PMS EBs. These systems increase the number of known PMS EBs by over 50% and include the highest mass (Theta1 Ori E, for which we provide a ...

Morales-Calderón, M; Stassun, K G; Vrba, F J; Prato, L; Hillenbrand, L A; Terebey, S; Covey, K R; Rebull, L M; Terndrup, D M; Gutermuth, R; Song, I; Plavchan, P; Carpenter, J M; Marchis, F; García, E V; Margheim, S; Luhman, K L; Angione, J; Irwin, J M
2012-01-01

240

Surface pollution of main-sequence stars through encounters with AGB ejecta in omega Centauri

  Astrophysics (arXiv)

Summary: The origin of a double main-sequence (MS) in omega Centauri is explored. We have shown from theoretical calculations on the stellar evolution that the colors of MS stars are shifted to those of the observed blue MS if the surface layers are polluted by He-rich materials with the mass of ~ 0.1 solar mass. Stars are supposed to be polluted through numerous encounters with the ejecta descended from massive asymptotic giant-branch (AGB) stars. Two populations of stars with different kinematics exceptionally observed in omega Cen indicate that kinematically cooler stars are more polluted through encounters with AGB ejecta than kinematically hotter ones because the accretion rate is inversely proportional to the cube of the relative velocity. We propose that both of these factors split the MS in omega Cen. This theoretical scheme explains why only omega Cen exhibit a double MS and matches the amount of He necessary to produce the blue MS with that supplied from massive AGB stars. Furthermore, we predict that even if globular clusters (GCs) possess only one generation of stars, the velocity dispersion of stars broaden the MS in the color-magnitude diagram as long as the GCs are massive enough to keep the AGB ejecta after the burst of star formation. This view explains the broad MS recently found in the GC NGC 2808 which exhibits no scatter in [Fe/H] and thus is likely to consist of a single generation of stars unlike the case of omega Cen.

Takuji Tsujimoto; Toshikazu Shigeyama; Takuma Suda
2006-11-23

241

On the C and N Abundances of 47 Tucanae Main Sequence Stars

  Astrophysics (arXiv)

Summary: We report the results of an analysis of CN and CH band strengths among a large sample of 47 Tucanae main-sequence and turn-off stars presented earlier by Harbeck et al. The resulting C and N abundances derived from synthetic spectra demonstrate: 1) A strongly anti-correlated relationship between [C/Fe] and [N/Fe] with the CN-strong stars exhibiting depleted carbon and enhanced nitrogen. 2) The abundances of both elements agree remarkably well with those found among the evolved red giants of the cluster implying little change in surface abundances from at least Mv ~ +6.5 to the tip of the red giant branch. 3) The pattern of C-depletions and N-enhancements are quite similar to those seen among the turn-off stars of M71, a cluster of almost identical metallicity but lower central concentration and escape velocity. At the same time, similar if not smaller N-enhancements and larger C-depletions are evident among like stars in the more metal-poor cluster M5. We interpret these results, as did Harbeck et al., as evidence of the operation of some pollution/accretion event early in the cluster history - the most likely source being AGB ejecta. However, the present results rule out simple surface pollution and suggest that a substantial fraction of the present stars' masses must be involved.

Michael M. Briley; Daniel Harbeck; Graeme H. Smith; Eva K. Grebel
2003-12-15

242

New proper motions of pre-main sequence stars in Taurus-Auriga

  Astrophysics (arXiv)

Summary: We present proper motions of 72 T Tauri stars located in the central region of Taurus-Auriga. These proper motions are taken from a new proper motion catalogue called STARNET. Our sample comprises 17 classical T Tauri stars and 55 weak-line T Tauri stars, most of the latter discovered by ROSAT. 53 stars had no proper motion measurement before. Kinematically, 62 of these stars are members of the association. A velocity dispersion of less than 2-3 km/s is found which is dominated by the errors of the proper motions. This velocity dispersion correlates with a spread in distances. Furthermore we present proper motions of 58 stars located in a region just south of the Taurus molecular clouds and compare the kinematics of the youngest stars in this sample (younger than 3.5 10^7 yrs) with the kinematics of the pre-main sequence stars in the Taurus-Auriga association. From a comparison of the space velocities we find that the stars in the central region of Tau-Aur are kinematically different from the stars in the southern part. Among the stars with large proper motions far off the Taurus mean motion we find 2 Pleiades candidates and 7 possible Pleiades runaway stars.

S. Frink; S. Roeser; R. Neuhaeuser; M. F. Sterzik
1997-04-29

243

Stellar Diameters and Temperatures I. Main Sequence A, F, & G Stars

  CERN Preprints

Summary: We have executed a survey of nearby, main sequence A, F, and G-type stars with the CHARA Array, successfully measuring the angular diameters of fortyfour stars with an average precision of ~ 1.5%. We present new measures of the bolometric flux, which in turn leads to an empirical determination of the effective temperature for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale model isochrones to constrain the masses and ages of the stars. These results are compared to indirect estimates of these quantities obtained by collecting photometry of the stars and applying them to model atmospheres and evolutionary isochrones. We find that for most cases, the models overestimate the effective temperature by ~ 1.5-4%, when compared to our directly measured values. The overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses a...

Boyajian, Tabetha S; van Belle, Gerard; Gies, Douglas R; Brummelaar, Theo A ten; von Braun, Kaspar; Farrington, Chris; Goldfinger, P J; O'Brien, David; Parks, J Robert; Richardson, Noel D; Ridgway, Stephen; Schaefer, Gail; Sturmann, Laszlo; Sturmann, Judit; Touhami, Yamina; Turner, Nils H; White, Russel
2011-01-01

244

Searching for the signatures of terrestrial planets in F-, G-type main-sequence stars

  CERN Preprints

Summary: We have studied the volatile-to-refractory abundance ratios to investigate their possible relation with the low-mass planetary formation. We present a fully differential chemical abundance analysis using high-quality HARPS and UVES spectra of 61 late F- and early G-type main-sequence stars, 29 are planet hosts and 32 are stars without detected planets. As the previous sample of solar analogs, these stars slightly hotter than the Sun also provide very accurate Galactic chemical abundance trends in the metallicity range $-0.3<{\\rm [Fe/H]}<0.4$. Stars with and without planets show similar mean abundance ratios. Moreover, when removing the Galactic chemical evolution effects, these mean abundance ratios, $\\Delta {\\rm [X/Fe]_{SUN-STARS}}$, versus condensation temperature tend to exhibit less steep trends with nearly null or slightly negative slopes. We have also analyzed a sub-sample of 26 metal-rich stars, 13 with and 13 without known planets and find the similar, although not equal, abundance pattern with ...

Hernández, J I González; Sousa, S G; Israelian, G; Santos, N C; Adibekyan, V Zh; Udry, S
2013-01-01

245

Testing pre-main sequence models: the power of a Bayesian approach

  CERN Preprints

Summary: Pre-main sequence (PMS) models provide invaluable tools for the study of star forming regions as they allow to assign masses and ages to young stars. Thus it is of primary importance to test the models against observations of PMS stars with dynamically determined mass. We developed a Bayesian method for testing the present generation of PMS models which allows for a quantitative comparison with observations, largely superseding the widely used isochrones and tracks qualitative superposition. Using the available PMS data we tested the newest PISA PMS models establishing their good agreement with the observations. The data cover a mass range from ~0.3 to ~3.1 Msun, temperatures from ~3x10^3 to ~1.2x10^4 K and luminosities from ~3x10^-2 to ~60 Lsun. Masses are correctly predicted within 20% of the observed values in most of the cases and for some of them the difference is as small as 5%. Nevertheless some discrepancies are also observed and critically discussed. By means of simulations, using typical observation...

Gennaro, Mario; Tognelli, Emanuele
2011-01-01

246

Eclipses by a Circumstellar Dust Feature in the Pre-Main Sequence Star KH15D

  Astrophysics (arXiv)

Summary: Photometry and spectroscopy of the unique pre-main sequence eclipsing object KH15D in the young cluster NGC 2264 are presented. The orbital period is 48.34 days and both the length (~16 d) and depth (~3 mag) of the eclipse have increased with time. A brightening near the time of central eclipse is confirmed in the recent data but at a much smaller amplitude than was originally seen. Spectra taken when the star is bright show that the primary is a weak T Tauri star of spectral type K7. During eclipse there is no detectable change in spectral type or reddening, indicating that the obscuration is caused by rather large dust grains and/or macroscopic objects. Evidently the star is eclipsed by an extended feature in its circumstellar disk orbiting with a semi-major axis of ~0.2 AU. Continued photometric monitoring should allow us to probe the disk structure with a spatial resolution of ~3 x 10^6 km or better.

Catrina M. Hamilton; William Herbst; Candice Shih; Anthony J. Ferro
2001-06-08

247

The pre-main sequence binary HK Ori : Spectro-astrometry and EXPORT data

  Astrophysics (arXiv)

Summary: In this paper we present multi-epoch observations of the pre-main sequence binary HK Ori. These data have been drawn from the EXPORT database and are complemented by high quality spectro-astrometric data of the system. The spectroscopic data appear to be very well represented by a combination of an A dwarf star spectrum superposed on a (sub-)giant G-type spectrum. The radial velocity of the system is consistent with previous determinations, and does not reveal binary motion, as expected for a wide binary. The spectral, photometric and polarimetric properties and variability of the system indicate that the active object in the system is a T Tauri star with UX Ori characteristics. The spectro-astrometry of HK Ori is sensitive down to milli-arcsecond scales and confirms the speckle interferometric results from Leinert et al. The spectro-astrometry allows with fair certainty the identification of the active star within the binary, which we suggest to be a G-type T Tauri star based on its spectral characteristics.

D. Baines; R. D. Oudmaijer; A. Mora; C. Eiroa; the EXPORT collaboration
2004-06-24

248

Atmospheric mass loss by stellar wind from planets around main sequence M stars

  CERN Preprints

Summary: We present an analytic model for the interaction between planetary atmospheres and stellar winds from main sequence M stars, with the purpose of obtaining a quick test-model that estimates the timescale for total atmospheric mass loss due to this interaction. Planets in the habitable zone of M dwarfs may be tidally locked and may have weak magnetic fields, because of this we consider the extreme case of planets with no magnetic field. The model gives the planetary atmosphere mass loss rate as a function of the stellar wind and planetary properties (mass, atmospheric pressure and orbital distance) and an entrainment efficiency coefficient $\\alpha$. We use a mixing layer model to explore two different cases: a time-independent stellar mass loss and a stellar mass loss rate that decreases with time. For both cases we consider planetary masses within the range of $1\\to10$ M$_{\\oplus}$ and atmospheric pressures with values of 1, 5 and 10 atm. For the time dependent case, planets without magnetic field in the habit...

Zendejas, Jesus; Raga, Alejandro
2010-01-01

249

A Detailed Far-Ultraviolet Spectral Atlas of Main Sequence B Stars

  CERN Preprints

Summary: We have constructed a detailed spectral atlas covering the wavelength region 930A to 1225A for 10 sharp-lined B0-B9 stars near the main sequence. Most of the spectra we assembled are from the archives of the FUSE satellite, but for nine stars wavelength coverage above 1188A was taken from high-resolution IUE or echelle HST/STIS spectra. To represent the tenth star at type B0.2 V we used the Copernicus atlas of tau Sco. We made extensive line identifications in the region 949A to 1225A of all atomic features having published oscillator strengths at types B0, B2, and B8. These are provided as a supplementary data product - hence the term detailed atlas. Our list of found features totals 2288, 1612, and 2469 lines, respectively. We were able to identify 92%, 98%, and 98% of these features with known atomic transitions with varying degrees of certainty in these spectra. The remaining lines do not have published oscillator strengths. Photospheric lines account for 94%, 87%, and 91%, respectively, of all our iden- ...

Smith, Myron A
2009-01-01

250

A comprehensive set of simulations of high-velocity collisions between main sequence stars

  Astrophysics (arXiv)

Summary: We report on a very large set of simulations of collisions between two main sequence (MS) stars. These computations were done with the ``Smoothed Particle Hydrodynamics'' method. Realistic stellar structure models for evolved MS stars were used. In order to sample an extended domain of initial parameters space (masses of the stars, relative velocity and impact parameter), more than 15000 simulations were carried out. We considered stellar masses ranging between 0.1 and 75 Msun and relative velocities up to a few thousands km/s. To limit the computational burden, a resolution of 2000-30000 particles per star was used. The primary goal of this study was to build a complete database from which the result of any collision can be interpolated. This allows us to incorporate the effects of stellar collisions with an unprecedented level of realism into dynamical simulations of galactic nuclei and other dense stellar clusters. We make the data describing the initial condition and outcome (mass and energy loss, angle of deflection) of all our simulations freely available on the Internet. We find that the outcome of collisions depends sensitively on the stellar structure and that, in most cases, using polytropic models is inappropriate. Published fitting formulas for the collision outcomes, established from a limited set of collisions, prove of limited use because they do not allow robust extrapolation to other stellar structures or relative velocities.

Marc Freitag; Willy Benz
2005-05-05

251

A multisite photometric campaign on the Pre-Main-Sequence delta Scuti pulsator IP Per

  Astrophysics (arXiv)

Summary: We present the results of a photometric multisite campaign on the $\\delta$ Scuti Pre-Main-Sequence star IP Per. Nine telescopes have been involved in the observations, with a total of about 190 hours of observations over 38 nights. Present data confirms the multiperiodic nature of this star and leads to the identification of at least nine pulsational frequencies. Comparison with the predictions of linear non-adiabatic radial pulsation models allowed us to identify only five of the nine observed frequencies, and to constrain the position of IP Per in the HR diagram. The latter is in good agreement with the empirical determination of the stellar parameters obtained by Miroshnichenko et al. (2001). An initial interpretation of the observed frequencies using the Aarhus non-radial pulsation code suggests that three frequencies could be associated with non-radial ($l$=2) modes. Finally, we present new evolutionary and pulsation models at lower metallicity (Z=0.008) to take into account the possibility that IP Per is metal deficient, as indicated by Miroshnichenko et al. (2001).

V. Ripepi; S. Bernabei; M. Marconi; F. Palla; A. Arellano Ferro; A. Bonanno; P. Ferrara; A. Frasca; X. J. Jiang; S. -L. Kim; S. Marinoni; G. Mignemi; M. J. P. F. G. Monteiro; T. D. Oswalt; P. Reegen; R. Janulis; E. Rodriguez; A. Rolland; A. Ruoppo; L. Terranegra; K. Zwintz
2005-12-12

252

Pre-main sequence stars in the Lagoon Nebula (M8)

  Astrophysics (arXiv)

Summary: We report the discovery of new pre-main sequence (PMS) stars in the Lagoon Nebula (M8) at a distance of 1.25 kpc, based on intermediate resolution spectra obtained with the Boller & Chivens spectrograph at the 6.5-m Magellan I telescope (Las Campanas Observatory, Chile). According to the spectral types, the presence of emission lines and the lithium 6708A absorption line, we are able to identify 27 classical T Tauri stars, 7 weak-lined T Tauri stars and 3 PMS emission objects with spectral type G, which we include in a separated stellar class denominated "PMS Fe/Ge class". Using near-infrared photometry either from 2MASS or from our own previous work we derive effective temperatures and luminosities for these stars and locate them in the Hertzsprung-Russell diagram, in order to estimate their masses and ages. We find that almost all of our sample stars are younger than 3 10^6 years and span over a range of masses between 0.8 and 2.5 Msun. A cross-correlation between our spectroscopic data and the X-ray sources detected with the Chandra ACIS instrument is also presented.

Julia I. Arias; Rodolfo H. Barba; Nidia I. Morrell
2006-11-05

253

Fitting the young main-sequence; distances, ages and age spreads

  Astrophysics (arXiv)

Summary: We use several main-sequence models to derive distances (and extinctions), with statistically meaningful uncertainties for 11 star-forming-regions and young clusters. The model dependency is shown to be small, allowing us to adopt the distances derived using one model. Using these distances we have revised the age order for some of the clusters of Mayne et al (2007). The new (approximate) nominal ages are: 2 Myrs for NGC6530 and the ONC, 3 Myrs for Lambda Orionis, NGC2264 and Sigma Orionis, 4-5 Myrs for NGC2362, 13 Myrs for h and chi Per, 20 Myrs for NGC1960 and 40 Myrs for NGC2547. In cases of significantly variable extinction we have derived individual extinctions using a revised Q-method (Johnson and Morgan, 1953). These new data show that the largest remaining uncertainty in deriving an age ordering (and necessarily ages) is metallicity. We also discuss the use of a feature we term the R-C gap overlap to provide a diagnostic of isochronal age spreads or varying accretion histories within a given star-formation-region. Finally, recent derivations of the distance to the ONC lie in two groups. Our new more precise distance of 391(+12,-9) pc allows us to decisively reject the further distance, we adopt 400 pc as a convenient value.

N. J. Mayne; Tim Naylor
2008-01-26

254

The Distance to NGC 5904 (M 5) via the Subdwarfs Main Sequence Fitting Method

  Astrophysics (arXiv)

Summary: We present a determination of the distance modulus of the globular cluster NGC 5904 (M 5), obtained by means of the subdwarf main-sequence fitting on the (V,V-I) color-magnitude diagram. The subdwarf sample has been selected from the HIPPARCOS catalog in a metallicity range homogeneous with the cluster ([Fe/H] \\~= -1.1). Both the cluster and the subdwarfs have been observed with the same telescope+instrument+filters setup (namely, ESO-NTT equipped with the SUSI2 camera), in order to preserve homogeneity and reduce systematic uncertainties. A set of archival HST data has then been used to obtain a deep and precise ridge line. These have been accurately calibrated in the ground photometric system by using the NTT data and used to fit the cluster distance modulus. By adopting the most commonly accepted values for the reddening, E(B-V) = 0.035 and 0.03, we obtain respectively mu_0 = 14.44 +- 0.09 +- 0.07 and mu_0 = 14.41 +- 0.09 +- 0.07, in agreement with recent determinations.

Vincenzo Testa; Alessandro Chieffi; Marco Limongi; Gloria Andreuzzi; Gianni Marconi
2004-03-11

255

White dwarf-main sequence binaries from SDSS DR8: unveiling the cool white dwarf population

  CERN Preprints

Summary: The spectroscopic catalogue of white dwarf-main sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS) is the largest and most homogeneous sample of compact binary stars currently known. However, because of selection effects, the current sample is strongly biased against systems containing cool white dwarfs and/or early type companions, which are predicted to dominate the intrinsic population. In this study we present colour selection criteria that combines optical (ugriz DR8 SDSS) plus infrared (yjhk DR9 UKIRT Infrared Sky Survey (UKIDSS), JHK Two Micron All Sky Survey (2MASS) and/or w1w2 Wide-Field Infrared Survey Explorer (WISE)) magnitudes to select 3419 photometric candidates of harbouring cool white dwarfs and/or dominant (M dwarf) companions. We demonstrate that 84 per cent of our selected candidates are very likely genuine WDMS binaries, and that the white dwarf effective temperatures and secondary star spectral types of 71 per cent of our selected sources are expected to be below <~1000...

Rebassa-Mansergas, A; Schreiber, M R; Gaensicke, B T; Koester, D
2013-01-01

256

Ultraviolet-Selected Field and Pre-Main-Sequence Stars Towards Taurus and Upper Scorpius

  CERN Preprints

Summary: We have carried out a Galaxy Evolution Explorer (GALEX) Cycle 1 guest investigator program covering 56 square degrees near the Taurus T association and 12 square degrees along the northern edge of the Upper Scorpius OB association. We combined photometry in the GALEX FUV and NUV bands with data from the Two Micron All Sky Survey to identify candidate young (<100 Myr old) stars as those with an ultraviolet excess relative to older main sequence stars. Follow-up spectroscopy of a partial sample of these candidates suggest 5 new members of Taurus, with 8-20 expected from additional observations, and 5 new members of Upper Scorpius, with 3-6 expected from additional observations. These candidate new members appear to represent a distributed, non-clustered population in either region, although our sample statistics are as of yet too poor to constrain the nature or extent of this population. Rather, our study demonstrates the ability of GALEX observations to identify young stellar populations distributed over a ...

Findeisen, Krzysztof
2010-01-01

257

A probable pre-main sequence chemically peculiar star in the open cluster Stock 16

  CERN Preprints

Summary: We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large Telescope to obtain a high resolution and high signal-to-noise ratio spectrum of Stock 16-12, an early-type star which previous Delta-a photometric observations suggest being a chemically peculiar (CP) star. We used spectral synthesis to perform a detailed abundance analysis obtaining an effective temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence showing the star is most likely a member of the young Stock 16 open cluster (age 3-8 Myr). The probable cluster membership, the star's position in the Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun, depending upon the adopted spectroscopic o...

Netopil, M; Paunzen, E; Zwintz, K; Pintado, O I; Bagnulo, S
2014-01-01

258

The impact of type Ia supernovae on main sequence binary companions

  Astrophysics (arXiv)

Summary: The nature of Type Ia supernova progenitors is still unclear. The outstanding characteristic of the single-degenerate scenario is that it contains hydrogen in the binary companion of the exploding white dwarf star, which, if mixed into the ejecta of the supernova in large amounts may lead to conflicts with the observations thus ruling out the scenario. We investigate the effect of the impact of Type Ia supernova ejecta on a main sequence companion star of the progenitor system. With a series of simulations we investigate how different parameters of this system affect the amount of hydrogen stripped from the companion by the impact. The stellar evolution code GARSTEC is used to set up the structure of the companion stars mimicking the effect of a binary evolution phase. The impact itself is simulated with the smoothed particle hydrodynamics code GADGET2. We reproduce and confirm the results of earlier grid-based hydrodynamical simulation. Parameter studies of the progenitor system are extended to include the results of recent binary evolution studies. The more compact structure of the companion star found here significantly reduces the stripped hydrogen mass. The low hydrogen masses resulting from a more realistic companion structure are consistent with current observational constraints. Therefore, the single-degenerate scenario remains a valid possibility for Type Ia supernova progenitors. These new results are not a numerical effect, but the outcome of different initial conditions.

R. Pakmor; F. K. Roepke; A. Weiss; W. Hillebrandt
2008-07-21

259

Low-mass pre--main-sequence stars in the Magellanic Clouds

  CERN Preprints

Summary: [Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar stars form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature of these objects and the contamination of their samples by the evolved populations of the Galactic disk impose demanding observational techniques for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of special detection techniques unfeasible. Nonetheless, imaging with the Hubble Space Te...

Gouliermis, Dimitrios
2012-01-01

260

MHD simulations of near-surface convection in cool main-sequence stars

  CERN Preprints

Summary: The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected. We carried out 3D local-box MHD simulations of unipolar magnetized regions (average fields of 20, 100, and 500G) with parameters corresponding to six main-sequence stars (spectral types F3V to M2V). The influence of the magnetic field on the convection and the local thermodynamical structure were analyzed in detail. For three spectral lines, we determined the impact of the magnetic field on the disc-integrated Stokes-I profiles. Line weakening has i...

Beeck, Benjamin; Reiners, Ansgar
2014-01-01

261

The Mass Ratio Distribution in Main-Sequence Spectroscopic Binaries Measured by IR Spectroscopy

  Astrophysics (arXiv)

Summary: We report infrared spectroscopic observations of a large, well-defined sample of main-sequence, single-lined spectroscopic binaries in order to detect the secondaries and derive the mass ratio distribution of short-period binaries. The sample consists of 51 Galactic disk spectroscopic binaries found in the Carney and Latham high-proper-motion survey, with primary masses in the range of 0.6--0.85 msun. Our infrared observations detect the secondaries in 32 systems, two of which have mass ratios, q=M_2/M_1, as low as ~0.20. Together with 11 systems previously identified as double-lined binaries by visible light spectroscopy, we have a complete sample of 62 binaries, out of which 43 are double-lined. The mass ratio distribution is approximately constant over the range q=1.0 to 0.3. The distribution appears to rise at lower q values, but the uncertainties are sufficiently large that we cannot rule out a distribution that remains constant. The mass distribution derived for the secondaries in our sample, and that of the extra-solar planets, apparently represent two distinct populations.

T. Mazeh; M. Simon; L. Prato; B. Markus; S. Zucker
2003-09-03

262

Lithium abundances for 185 main-sequence stars - Galactic evolution and stellar depletion of lithium

  Astrophysics (arXiv)

Summary: We present a survey of lithium abundances in 185 main- sequence field stars with Teff between 5600 and 6600 K and [Fe/H] from -1.4 to +0.2 based on high-resolution spectra of 130 stars and a reanalysis of data from Lambert et al. (1991). The survey takes advantage of improved ways of determining effective temperature, metallicity, mass and age, offering an opportunity to investigate the behaviour of Li as a function of these parameters. An interesting result is the presence of a large gap in the Li-Teff plane, which distinguishes `Hyades-like, Li-dip' stars from other stars. These Li-dip stars have a well-defined mass, which decreases with metallicity. Stars above the gap, when divided into four metallicity groups, may show a correlation between Li abundance and stellar mass, but with a large dispersion that cannot be explained by observational errors or differences in metallicity and age, which ranges from 1.5 to 15 Gyr. This suggests that Li depletion occurs early in stellar life and that other parameters, e.g. initial rotation velocity and/or the rate of angular momentum loss, affect the degree of depletion. A comparison of the distribution of stars in the Li-[Fe/H] plane with evolutionary models of Romano et al. (1999) suggests that novae are a major source for the Li production in the Galactic disk.

Y. Q. Chen; P. E. Nissen; T. Benoni; G. Zhao
2001-04-03

263

Unstable quasi g-modes in rotating main-sequence stars

  Astrophysics (arXiv)

Summary: This paper studies the oscillatory stability of uniformly rotating main-sequence stars of mass 3-8 M_sun by solving the linearized non-adiabatic, non-radial oscillation equations with a forcing term and searching for resonant response to a complex forcing frequency. By using the traditional approximation the solution of the forced oscillation equations becomes separable, whereby the energy equation is made separable by approximation. It is found that the kappa-mechanism in rotating B-stars can destabilize not only gravity- or pressure modes, but also a branch of low frequency retrograde (in corotating frame) oscillations in between the retrograde g-modes and toroidal r-modes. These unstable quasi g-modes (or `q-modes') hardly exhibit rotational confinement to the equatorial regions of the star, while the oscillations are always prograde in the observer's frame, all in contrast to g-modes. The unstable q-modes occur in a few narrow period bands (defined by their azimuthal index m), and seem to fit the oscillation spectra observed in SPB stars rather well. The unstable q-mode oscillation spectrum of a very rapidly rotating 8 M_sun star appears similar to that of the well studied Be-star mu Cen. The unstable q-modes thus seem far better in explaining the observed oscillation spectra in SPB-stars and Be-stars than normal g-modes.

G. J. Savonije
2005-06-07

264

Timing the main-sequence-star binary pulsar J1740-3052

  CERN Preprints

Summary: PSR J1740-3052 is a young pulsar in orbit around a companion that is most likely a B-type main-sequence star. Since its discovery more than a decade ago, data have been taken at several frequencies with instruments at the Green Bank, Parkes, Lovell, and Westerbork telescopes. We measure scattering timescales in the pulse profiles and dispersion measure changes as a function of binary orbital phase and present evidence that both of these vary as would be expected due to a wind from the companion star. Using pulse arrival times that have been corrected for the observed periodic dispersion measure changes, we find a timing solution spanning 1997 November to 2011 March. This includes measurements of the advance of periastron and the change in the projected semimajor axis of the orbit and sets constraints on the orbital geometry. From these constraints, we estimate that the pulsar received a kick of at least ~50 km/s at birth. A quasi-periodic signal is present in the timing residuals with a period of 2.2 times th...

Madsen, E C; Kramer, M; Camilo, F; Hobbs, G B; Janssen, G H; Lyne, A G; Manchester, R N; Possenti, A; Stappers, B W
2012-01-01

265

The Lower Main Sequence and the Orbital Period Distribution of Cataclysmic Variable Stars

  Astrophysics (arXiv)

Summary: The color-magnitude diagram of the lower main sequence, as measured from a volume-limited sample of nearby stars, shows an abrupt downward jump between Mv = 12 and 13. This jump indicates that the observed mass-radius relationship steepens between 0.3 and 0.2 solar masses, but theoretical models show no such effect. It is difficult to isolate the source of this disagreement: the observational mass-radius relationship relies upon transformations that may not be sufficiently accurate, while the theoretical relationship relies upon stellar models that may not be sufficiently complete, particularly in their treatment of the complex physics governing the interior equation-of-state. If the features in the observationally derived mass-radius relationship are real, their existence provides a natural explanation for the well-known gap in the orbital period distribution of cataclysmic variables. This explanation relies only upon the observed mass-radius relationship of low-mass stars, and does not require ad hoc changes in magnetic braking or in the structure of cataclysmic variable secondaries. If correct, it will allow broader application of cataclysmic variable observations to problems of basic stellar physics.

J. Christopher Clemens; I. Neill Reid; John E. Gizis; M. Sean O'Brien
1997-10-27

266

Fundamental properties of pre-main sequence stars in young, southern star forming regions: metallicities

  CERN Preprints

Summary: Aims: The primary motivation for this project is to search for metal-rich star forming regions, in which, stars of super-solar metallicity will be created, as hopefully, will be extra-solar planets orbiting them ! Results: We find (pre-main sequence) model-dependent isochronal ages of the Lupus, Chamaeleon and CrA targets to be $9.1 \\pm 2.1$ Myr, $4.5 \\pm 1.6$ Myr and $9.0 \\pm 3.9$ Myr respectively. The majority of the stars have Li I 6707.8A equivalent widths similar to, or above those of, their similar mass Pleiades counterparts, confirming their youthfulness. Most stars are kinematic members, either single or binary, of their regions. We find a mean radial velocity for objects in the Lupus cloud to be ${\\bar {RV}}=+2.6 \\pm 1.8$ km s$^{-1}$, for the Chamaeleon I & II clouds, ${\\bar {RV}}=+12.8 \\pm 3.6$ km s$^{-1}$ whereas for the CrA cloud, we find ${\\bar {RV}}=-1.1 \\pm 0.5$ km s$^{-1}$. All stars are coronally and chromospherically active, exhibiting X-ray and H$\\alpha$ emission levels marginally less,...

James, D J; Santos, N C; Bouvier, J; James, David J.; Melo, Claudio; Santos, Nuno C.; Bouvier, Jerome
2005-01-01

267

Stellar and Circumstellar Properties of the Pre-Main Sequence Binary GV Tau from Infrared Spectroscopy

  Astrophysics (arXiv)

Summary: We report spatially resolved spectroscopy of both components of the low-mass pre-main-sequence binary GV Tau. High resolution spectroscopy in the K- and L-bands is used to characterize the stellar properties of the binary and to explore the nature of the circumstellar environment. We find that the southern component, GV Tau S, is a radial velocity variable, possibly as a result of an unseen low-mass companion. The strong warm gaseous HCN absorption reported previously toward GV Tau S (Gibb et al. 2007) was not present during the epoch of our observations. Instead, we detect warm (~500 K) molecular absorption with similar properties toward the northern infrared companion, GV Tau N. At the epoch of our observations, the absorbing gas toward GV Tau N was approximately at the radial velocity of the GV Tau molecular envelope, but it was redshifted with respect to the star by ~13 km/s. One interpretation of our results is that GV Tau N is also a binary and that most of the warm molecular absorption arises in a circumbinary disk viewed close to edge-on.

Greg W. Doppmann; Joan R. Najita; John S. Carr
2008-05-15

268

Post Common Envelope Binaries from SDSS. V: Four eclipsing white dwarf main sequence binaries

  CERN Preprints

Summary: We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main sequence (WDMS) binaries from the Sloan Digital Sky Survey, and report on follow-up observations of these systems. Orbital periods and ephemerides have been established from multi-season photometry. SDSS1435+3733, with Porb=3h has the shortest orbital period of all known eclipsing WDMS binaries. Time-resolved spectroscopic observations have been obtained and the radial velocities of the secondary stars in all four systems were measured. A spectral decomposition/fitting technique was then employed to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light curve modeling code to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of Mwd~0.4-0.6Msun, in line with the expectations from close binary evolution. The DC white d...

Pyrzas, S; Marsh, T R; Aungwerojwit, A; Rebassa-Mansergas, A; Rodríguez-Gil, P; Southworth, J; Schreiber, M R; Gomez-Moran, A Nebot; Köster, D
2008-01-01

269

Luminosity-Colours relations for thin disc main-sequence stars

  Astrophysics (arXiv)

Summary: In this study we present the absolute magnitude calibrations of thin disc main-sequence stars in the optical ($M_{V}$), and in the near-infrared ($M_{J}$). Thin disc stars are identified by means of Padova isochrones, and absolute magnitudes for the sample are evaluated via the newly reduced Hipparcos data. The obtained calibrations cover a large range of spectral types: from A0 to M4 in the optical and from A0 to M0 in the near-infrared. Also, we discuss the of effects binary stars and evolved stars on the absolute magnitude calibrations. The usage of these calibrations can be extended to the estimation of galactic model parameters for the thin disc individually, in order to compare these parameters with the corresponding ones estimated by $\\chi{^2}_{min}$ statistics (which provides galactic model parameters for thin and thick discs, and halo simultaneously) to test any degeneracy between them. The calibrations can also be used in other astrophysical researches where distance plays an important role in that study.

S. Bilir; S. Karaali; S. Ak; E. Yaz; A. Cabrera-Lavers; K. B. Coskunoglu
2008-08-13

270

The empirical upper limit for mass loss of cool main sequence stars

  Astrophysics (arXiv)

Summary: The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than M(lim) have lost an amount of mass equal to DeltaM. The actual value of DeltaM or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for DeltaM in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6x10^{-11} M(sun)/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs.

Anna Lednicka; K. Stepien
2008-02-26

271

Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler

  CERN Preprints

Summary: Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology. We provide the mode linewidths and mode heights of the oscillations of various stars as a function of frequency and of effective temperature. We used a time series of nearly two years of data for each star. The 23 stars observed belong to the simple or F-like category. The power spectra of the 23 main-sequence stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We study the source of systematic errors in the mode linewidths and mode heights, and we present a way to correct these errors with respect to a common reference fit. Using the correction, we could explain all sources of systematic errors, which could be reduced to less than $\\pm$15% for mode linewidths and heights, and less than $\\pm$5% for amplitude, when compared to the r...

Appourchaux, T; Benomar, O; Campante, T L; Davies, G R; Handberg, R; Howe, R; Régulo, C; Belkacem, K; Houdek, G; García, R A; Chaplin, W J
2014-01-01

272

The Pre-Main-Sequence Eclipsing Binary ASAS J052821+0338.5

  Astrophysics (arXiv)

Summary: In this paper we present the first results of a detailed spectroscopic and photometric analysis of the V = 11.7m eclipsing binary ASAS J052821+0338.5. With the FIES spectrograph at the Nordic Optical Telescope we have obtained a series of high-resolution spectra (R=47000) covering the entire orbit of the system. In addition we obtained simultaneous broadband photometry from three small aperture telescopes. From these spectroscopic and photometric data we have derived the system's orbital parameters and determined the fundamental stellar parameters of the two components. Our results indicate that ASAS J052821+0338.5 is a K1/K3 pre-main-sequence eclipsing binary, with component masses of 1.38 M_sun and 1.33 M_sun and a period of 3.87 days, located at a distance of 280 +/- 30 pc. The kinematics, physical location and the evolutionary status of the two stars suggest that ASAS J052821+0338.5 is a member of the approximately 11 Myr old Orion OB1a subassociation. The systems also exhibits smooth 0.15m out-of-eclipse variations that are similar to those found in RS CVn binaries. Furthermore the parameters we derived are consistent with the 10-13 Myr isochrones of the popular Baraffe stellar evolutionary models.

H. C. Stempels; L. Hebb; K. G. Stassun; J. Holtzman; N. Dunstone; L. Glowienka; S. Frandsen
2008-01-28

273

RX J0942.7-7726AB: an isolated pre-main sequence wide binary

  CERN Preprints

Summary: We report the discovery of two young M-dwarfs, RX J0942.7-7726 (M1) and 2MASS J09424157-7727130 (M4.5), that were found only 42 arcsec apart in a survey for pre-main sequence stars surrounding the open cluster eta Chamaeleontis. Both stars have congruent proper motions and near-infrared photometry. Medium-resolution spectroscopy reveals that they are coeval (age 8-12 Myr), codistant (100-150 pc) and thus almost certainly form a true wide binary with a projected separation of 4000-6000 AU. The system appears too old and dynamically fragile to have originated in eta Cha and a traceback analysis argues for its birth in or near the Scorpius-Centaurus OB Association. RX J0942.7-7726AB joins a growing group of wide binaries kinematically linked to Sco-Cen, suggesting that such fragile systems can survive the turbulent environment of their natal molecular clouds while still being dispersed with large velocities. Conversely, the small radial velocity difference between the stars (2.7 \\pm 1.0 km/s) could mean the syst...

Murphy, Simon J; Bessell, Michael S
2012-01-01

274

The star formation main sequence and stellar mass assembly of galaxies in the Illustris simulation

  CERN Preprints

Summary: Understanding the physical processes that drive star formation is a key challenge for galaxy formation models. In this article we study the tight correlation between the star formation rate (SFR) and stellar mass of galaxies at a given redshift, how halo growth influences star formation, and star formation histories of individual galaxies. We study these topics using Illustris, a state-of-the-art cosmological hydrodynamical simulation of galaxy formation. Illustris reproduces the observed relation (the star formation main sequence; SFMS) between SFR and stellar mass at redshifts z=0 and z=4, but at intermediate redshifts of z~2, the simulated SFMS has a significantly lower normalisation than reported by observations. The scatter in the relation is consistent with the observed scatter. However, the fraction of outliers above the SFR-stellar mass relation in Illustris is less than that observed. Galaxies with halo masses of ~10^{12} solar masses dominate the SFR density of the Universe, in agreement with the re...

Sparre, Martin; Springel, Volker; Vogelsberger, Mark; Genel, Shy; Torrey, Paul; Nelson, Dylan; Sijacki, Debora; Hernquist, Lars
2014-01-01

275

V4046 Sgr: Touchstone to Investigate Spectral Type Discrepancies for Pre-main Sequence Stars

  CERN Preprints

Summary: Determinations of the fundamental properties (e.g., masses and ages) of late-type, pre-main sequence (pre-MS) stars are complicated by the potential for significant discrepancies between the spectral types of such stars as ascertained via optical vs. near-infrared observations. To address this problem, we have obtained near-IR spectroscopy of the nearby, close binary T Tauri system V4046 Sgr AB with the NASA Infrared Telescope Facility (IRTF) SPEX spectrometer. The V4046 Sgr close binary (and circumbinary disk) system provides an important test case for spectral type determination thanks to the stringent observational constraints on its component stellar masses (i.e., ~0.9 Msun each) as well as on its age (12-21 Myr) and distance (73 pc). Analysis of the IRTF data indicates that the composite near-IR spectral type for V4046 Sgr AB lies in the range M0-M1, i.e., significantly later than the K5+K7 composite type previously determined from optical spectroscopy. However, the K5+K7 composite type is in better agre...

Kastner, Joel H; Sargent, Benjamin; Smith, C T; Rayner, John
2014-01-01

276

Surface pollution of main-sequence stars through encounters with AGB ejecta in omega Centauri

  CERN Preprints

Summary: The origin of a double main-sequence (MS) in omega Centauri is explored. We have shown from theoretical calculations on the stellar evolution that the colors of MS stars are shifted to those of the observed blue MS if the surface layers are polluted by He-rich materials with the mass of ~ 0.1 solar mass. Stars are supposed to be polluted through numerous encounters with the ejecta descended from massive asymptotic giant-branch (AGB) stars. Two populations of stars with different kinematics exceptionally observed in omega Cen indicate that kinematically cooler stars are more polluted through encounters with AGB ejecta than kinematically hotter ones because the accretion rate is inversely proportional to the cube of the relative velocity. We propose that both of these factors split the MS in omega Cen. This theoretical scheme explains why only omega Cen exhibit a double MS and matches the amount of He necessary to produce the blue MS with that supplied from massive AGB stars. Furthermore, we predict that even i...

Tsujimoto, T; Suda, T; Tsujimoto, Takuji; Shigeyama, Toshikazu; Suda, Takuma
2006-01-01

277

Deep near-IR variability survey of pre-main-sequence stars in Rho Ophiuchi

  Astrophysics (arXiv)

Summary: Variability is a common characteristic of pre-main-sequence stars (PMS). Near-IR variability surveys of young stellar objects (YSOs) can probe stellar and circumstellar environments and provide information about the dynamics of the on going magnetic and accretion processes. Furthermore, variability can be used as a tool to uncover new cluster members in star formation regions. We hope to achieve the deepest near-IR variability study of YSOs targeting the Rho Ophiuchi cluster. Fourteen epochs of observations were obtained with the Wide Field Camera (WFCAM) at the UKIRT telescope scheduled in a manner that allowed the study of variability on timescales of days, months, and years. Statistical tools, such as the multi-band cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability characteristics are compared to existing models of YSOs in order to relate them to physical processes, and then used to select new candidate members of this star-forming region. Variability in the near-IR is found to be present in 41% of the known population of Rho Ophiuchi recovered in our sample. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc. Using variability, a new population of objects has been uncovered that is believed to be part of the Rho Ophiuchi cluster.

Catarina Alves de Oliveira; Mark Casali
2008-04-09

278

Testing Rotational Mixing Predictions with New Boron Abundances in Main Sequence B-type Stars

  Astrophysics (arXiv)

Summary: (Abridged) New boron abundances for seven main-sequence B-type stars are determined from HST STIS spectroscopy around the BIII 2066A line. Boron abundances provide a unique and critical test of stellar evolution models that include rotational mixing since boron is destroyed in the surface layers of stars through shallow mixing long before other elements are mixed from the stellar interior through deep mixing. Boron abundances range from 12+log(B/H) = 1.0 to 2.2. The boron abundances are compared to the published values of their stellar nitrogen abundances (all have 12+log(N/H) < 7.8, i.e., they do not show significant CNO-mixing) and to their host cluster ages (4 to 16 Myr) to investigate the predictions from models of massive star evolution with rotational mixing effects (Heger & Langer 2000). Only three stars (out of 34) deviate from the model predictions, including HD36591, HD205021, and HD30836. These three stars suggest that rotational mixing could be more efficient than currently modelled at the highest rotation rates.

J. T. Mendel; K. A. Venn; C. R. Proffitt; A. M. Brooks; D. L. Lambert
2005-12-04

279

Hunting for millimeter flares from magnetic reconnection in pre-main sequence spectroscopic binaries

  CERN Preprints

Summary: Recent observations of the low-mass pre-main sequence, eccentric spectroscopic binaries DQ Tau and V773 Tau A reveal that their millimeter spectrum is occasionally dominated by flares from non-thermal emission processes. The transient activity is believed to be synchrotron in nature, resulting from powerful magnetic reconnection events when the separate magnetic structures of the binary components are capable of interacting and forced to reorganize, typically near periastron. We conducted the first systematic study of the millimeter variability toward a sample of 12 PMS spectroscopic binaries with the aim to characterize the proliferation of flares amongst sources likely to experience similar interbinary reconnection events. The source sample consists of short-period, close-separation binaries that possess either a high orbital eccentricity or a circular orbit. Using the MAMBO2 array on the IRAM 30m telescope, we carried out continuous monitoring at 1.25 mm over a 4-night period during which all of the high-e...

Kóspál, Á; Hogerheijde, M R; Moór, A; Blake, G A
2010-01-01

280

Evidence for Prolonged Main Sequence Stellar Evolution of F Stars in close binaries

  Astrophysics (arXiv)

Summary: Binary F stars exhibit large brightness anomaly, which is defined here as the difference between the absolute magnitude from the uvby photometry and the actual absolute magnitude of the star. We have found that the anomaly inversely correlates with the binary components separation. There is evidence that the correlation reflects actual population differences between close and wide binary pairs, in which case it indicates that the anomaly is somehow associated with the interaction of binary's components. The anomaly has also been found to correlate with both kinematics and metallicity. The sense of the correlations implies that the anomaly increases as the star evolves, suggesting a peculiar evolution of a primary F star in a tight binary pair. This conclusion has further been supported by the study of the age-velocity relation (AVR) of F stars that are cataloged in the HIPPARCOS as single. Among these stars, those with brightness anomaly were previously shown to be most likely unidentified close binaries. We have found that the AVR of these binary candidates is different from that of the ``truly single'' F stars. The discrepancy between the two AVRs indicates that the putative binaries are, on average, older than similar normal single F stars at the same effective temperature and luminosity, which is consistent with the inferred peculiar evolution in close binaries. It appears that this peculiarity is caused by the impact of the components interaction in a tight pair on stellar evolution, which results in the prolonged main sequence lifetime of the primary F star.

A. A. Suchkov
2001-03-01

281

Pre-main-sequence binaries with tidally disrupted discs: the Br gamma in HD 104237

  CERN Preprints

Summary: Active pre-main-sequence binaries with separations of around ten stellar radii present a wealth of phenomena unobserved in common systems. The study of these objects is extended from Classical T Tauri stars to the Herbig Ae star HD 104237. Spectro-interferometry with the VLTI/AMBER is presented. It is found that the K-band continuum squared visibilities are compatible with a circumbinary disc with a radius of ~0.5 AU. However, a significant fraction (~50 per cent) of the flux is unresolved and not fully accounted by the stellar photospheres. The stars probably don't hold circumstellar discs, in addition to the circumbinary disk, due to the combined effects of inner magnetospheric truncation and outer tidal truncation. This unresolved flux likely arises in compact structures inside the tidally disrupted circumbinary disc. Most ($\\gtrsim 90$ per cent) of the Br gamma line emission is unresolved. The line-to-continuum spectro-astrometry shifts in time, along the direction of the Ly alpha jet known to be driven b...

Garcia, P J V; Dougados, C; Bacciotti, F; Clausse, J -M; Massi, F; Mérand, A; Petrov, R; Weigelt, G
2013-01-01

282

New Pre-main-Sequence Stars in the Upper Scorpius Subgroup of Sco-Cen

  CERN Preprints

Summary: We present 237 new spectroscopically confirmed pre-main-sequence K and M-type stars in the young Upper Scorpius subgroup of the Sco-Cen association, the near- est region of recent massive star formation. Using the Wide-Field Spectrograph at the Australian National University 2.3 m telescope at Siding Spring, we observed 397 kinematically and photometrically selected candidate members of Upper Scorpius, and identified new members by the presence of Lithium absorption. The HR-diagram of the new members shows a spread of ages, ranging from ~3-20 Myr, which broadly agrees with the current age estimates of ~5-10 Myr. We find a significant range of Li 6708 equivalent widths among the members, and a minor dependence of HR-diagram position on the measured equivalent width of the Li 6708A line, with members that appear younger having more Lithium. This could indicate the presence of either popu- lations of different age, or a spread of ages in Upper Scorpius. We also use Wide-Field Infrared Survey Explorer data to inf...

Rizzuto, A C; Kraus, A L
2015-01-01

283

Age Constraints for an M31 Globular Cluster from Main Sequence Photometry

  Astrophysics (arXiv)

Summary: We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312 in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The cluster was included in deep observations taken to measure the star formation history of the M31 halo. Overcoming a very crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag below the main sequence turnoff. These are the first observations to allow a direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD and luminosity function using a finely-spaced grid of isochrones that have been calibrated using observations of Galactic clusters taken with the same camera and filters. The luminosity difference between the subgiant and horizontal branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison to isochrones yields an age of 10 +2.5/-1 Gyr.

Thomas M. Brown; Henry C. Ferguson; Ed Smith; Randy A. Kimble; Allen V. Sweigart; Alvio Renzini; R. Michael Rich; Don A. VandenBerg
2004-08-18

284

The Main Sequence and the Fundamental Metallicity Relation in MaGICC Galaxies: Evolution and Scatter

  CERN Preprints

Summary: Using cosmological galaxy simulations from the MaGICC project, we study the evolution of the stellar masses, star formation rates and gas phase abundances of star forming galaxies. We derive the stellar masses and star formation rates using observational relations based on spectral energy distributions by applying the new radiative transfer code GRASIL-3D to our simulated galaxies. The simulations match well the evolution of the stellar mass-halo mass relation, have a star forming main sequence that maintains a constant slope out to redshift z $\\sim$ 2, and populate projections of the stellar mass - star formation - metallicity plane, similar to observed star forming disc galaxies. We discuss small differences between these projections in observational data and in simulations, and the possible causes for the discrepancies. The light-weighted stellar masses are in good agreement with the simulation values, the differences between the two varying between 0.06 dex and 0.20 dex. We also find a good agreement betw...

Obreja, A; Stinson, G; Domínguez-Tenreiro, R; Gibson, B K; Silva, L; Granato, G L
2014-01-01

285

The Recurrent Eclipse of an Unusual Pre--Main-Sequence Star in IC 348

  Astrophysics (arXiv)

Summary: The recurrence of a previously documented eclipse of a solar-like pre--main-sequence star in the young cluster IC 348 has been observed. The recurrence interval is 4.7 $\\pm 0.1$ yr and portions of 4 cycles have now been seen. The duration of each eclipse is at least 3.5 years, or $\\sim 75$% of a cycle, verifying that this is not an eclipse by a stellar companion. The light curve is generally symmetric and approximately flat-bottomed. Brightness at maximum and minimum have been rather stable over the years but the light curve is not perfectly repetitive or smooth and small variations exist at all phases. We confirm that the star is redder when fainter. Models are discussed and it is proposed that this could be a system similar to KH 15D in NGC 2264. Specifically, it may be an eccentric binary in which a portion of the orbit of one member is currently occulted during some binary phases by a circumbinary disk. The star deserves sustained observational attention for what it may reveal about the circumstellar environment of low-mass stars of planet-forming age.

S. Nordhagen; W. Herbst; E. C. Williams; E. Semkov
2006-06-20

286

GOODS-Herschel: an infrared main sequence for star-forming galaxies

  CERN Preprints

Summary: We examine the infrared spectral energy distributions (SEDs) of galaxies at 03 x 10^10 Lsun kpc^-2) and high specific SFR. Locally, galaxies with high L[IR] are systematically in the SB mode, while this is not the case in the distant Universe where most (U)LIRGs form stars in the `normal' main sequence mode. This confusion between two modes of star formation is the cause of the so-called `mid-IR excess problem' found in galaxies at z>1.5 by previous studies. We find that the rest-frame UV-2700 A size of distant SB galaxies is typically half that of MS galaxies. MS galaxies present strong PAH emission line features, a broad far-IR bump resulting from a combination of Tdust~15 to 50 K and an effective Tdust~31 K, as derived from the peak wavelength of their IR SED. Galaxies in the starburst regime instead exhibit weak PAH EW and a sharper far-IR bump with an effective Tdust~40 K. Finally, we present evidence that power-law AGN systematically occur in compact, dusty starbursts, while in non power-law optical/X-r...

Elbaz, D; Hwang, H S; Diaz-Santos, T; Magdis, G; Magnelli, B; Borgne, D Le; Galliano, F; Pannella, M; Chanial, P; Armus, L; Charmandaris, V; Daddi, E; Aussel, H; Popesso, P; Kartaltepe, J; Altieri, B; Valtchanov, I; Coia, D; Dannerbauer, H; Dasyra, K; Leiton, R; Mazzarella, J; Buat, V; Burgarella, D; Chary, R -R; Gilli, R; Ivison, R J; Juneau, S; LeFloc'h, E; Lutz, D; Morrison, G E; Mullaney, J; Murphy, E; Pope, A; Scott, D; Alexander, D; Brodwin, M; Calzetti, D; Cesarsky, C; Charlot, S; Dole, H; Eisenhardt, P; Ferguson, H C; Foerster-Schreiber, N; Frayer, D; Giavalisco, M; Huynh, M; Koekemoer, A M; Papovich, C; Reddy, N; Surace, C; Teplitz, H; Yun, M S; Wilson, G
2011-01-01

287

Seismic diagnostics of mixing beyond the convective core in intermediate mass main-sequence stars

  CERN Preprints

Summary: We study prospects for seismic sounding the layer of a partial mixing above the convective core in main-sequence stars with masses in the 1.2 -- 1.9 solar mass range. There is an initial tendency to an increase of convective core mass in such stars and this leads to ambiguities in modeling. Solar-like oscillations are expected to be excited in such objects. Frequencies of such oscillations provide diagnostics, which are sensitive to the structure of the innermost part of the star and they are known as the small separations. We construct evolutionary models of stars in this mass range assuming various scenarios for element mixing, which includes formation of element abundance jumps, as well as semiconvective and overshooting layers. We find that the three point small separations employing frequencies of radial and dipole modes provide the best probe of the element distribution above the convective core. With expected accuracy of frequency measurement from the space experiments, a discrimination between various...

Popielski, B L
2005-01-01

288

Seismic diagnostics of mixing beyond the convective core in intermediate mass main-sequence stars

  Astrophysics (arXiv)

Summary: We study prospects for seismic sounding the layer of a partial mixing above the convective core in main-sequence stars with masses in the 1.2 -- 1.9 solar mass range. There is an initial tendency to an increase of convective core mass in such stars and this leads to ambiguities in modeling. Solar-like oscillations are expected to be excited in such objects. Frequencies of such oscillations provide diagnostics, which are sensitive to the structure of the innermost part of the star and they are known as the small separations. We construct evolutionary models of stars in this mass range assuming various scenarios for element mixing, which includes formation of element abundance jumps, as well as semiconvective and overshooting layers. We find that the three point small separations employing frequencies of radial and dipole modes provide the best probe of the element distribution above the convective core. With expected accuracy of frequency measurement from the space experiments, a discrimination between various scenarios should be possible.

B. L. Popielski; W. A. Dziembowski
2005-05-25

289

Lithium abundances of Main Sequence and Sub-Giant stars in the Globular Cluster NGC 6397

  CERN Preprints

Summary: We present FLAMES/GIRAFFE spectroscopy obtained at the Very Large Telescope (VLT). Using these observations we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a Globular Cluster. We also observed Li in a sample of Sub-Giant stars of the same B-V colour. Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a Li abundance higher by 0.1 dex than MS stars. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambigou...

Hernández, J I González; Caffau, E; Steffen, M; Ludwig, H -G; Behara, N; Sbordone, L; Cayrel, R; Zaggia, S
2009-01-01

290

New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets

  CERN Preprints

Summary: Using the MIPS instrument on the Spitzer telescope, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars - data that can be correlated with any planets that may eventually be found. We have found significant excess 70um emission toward 12 stars. Combined with an earlier study, we find an overall 70um excess detection rate of $13 \\pm 3$% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying 4 of these stars as having weak 24um excesses (fluxes $\\sim$10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24um exce...

Beichman, C; Stapelfeldt, K; al, et
2006-01-01

291

Constraints on the height of the inner disk rim in pre-main-sequence stars

  CERN Preprints

Summary: The structure of inner region of protoplanetary disks around young pre-main-sequence stars is still poorly understood. This part of the disk is shaped by various forces influencing dust and gas dynamics and by dust sublimation, which creates abrupt drops in the dust density. This region also emits a strong near-infrared excess that cannot be explained by classical accretion disk models, which suggests the existence of some unusual dust distribution or disk shape. The most prevalent explanation to date is the puffed-up inner disk rim model, where the disk exhibits an optically thin cavity around the star up to the distance of dust sublimation. The critical parameter in this model is the inner disk rim height $z_{\\rm max}$ relative to the rim's distance from the star $R_{\\rm in}$. Observations often require $z_{\\rm max}/R_{\\rm in}\\gtrsim0.2$ to reproduce the near-infrared excess in the spectra. In this paper we put together a comprehensive list of processes that can shape the inner disk rim and combined them to...

Vinkovi?, Dejan
2014-01-01

292

Pre-Main Sequence stars in the star forming complex Sh 2-284

  CERN Preprints

Summary: Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age ...

Cusano, F; Alcala, J M; Gandolfi, D; Marconi, M; Degl'Innocenti, S; Palla, F; Guenther, E W; Bernabei, S; Covino, E; Neiner, C; Puga, E; Hony, S
2010-01-01

293

The effect of heavy element opacity on pre-main sequence Li depletion

  Astrophysics (arXiv)

Summary: Recent 3-D analysis of the solar spectrum data suggests a significant change of the solar chemical composition. This may affect the temporal evolution of the surface abundance of light elements since the extension of the convective envelope is largely affected by the internal opacity value. We analyse the influence of the adopted solar mixture on the opacity in the convective envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion. The surface Li abundance depends on the relative efficiency of several processes, some of them still not known with the required precision; this paper thus analyses one of the aspects of this ``puzzle''. Focusing on PMS evolution, where the largest amount of Li burning occurs, we computed stellar models for three selected masses (0.8, 1.0 and 1.2 Msun, with Z=0.013, Y=0.27, alpha=1.9) by varying the chemical mixture, that is the internal element distribution in Z. We analysed the contribution of the single elements to the opacity at the temperatures and densities of interest for Li depletion. Several mixtures were obtained by varying the abundance of the most important elements one at a time; we then calculated the corresponding PMS Li abundance evolution. We found that a mixture variation does change the Li abundance: at fixed total metallicity, the Li depletion increases when increasing the fraction of elements heavier than O.

P. Sestito; S. Degl Innocenti; P. G. Prada Moroni; S. Randich
2006-04-07

294

The quadruple pre-main sequence system LkCa3: Implications for stellar evolution models

  CERN Preprints

Summary: We report the discovery that the pre-main sequence object LkCa3 in the Taurus-Auriga star-forming region is a hierarchical quadruple system of M stars. It was previously known to be a close (~0.5 arc sec) visual pair, with one component being a moderately eccentric 12.94-day single-lined spectroscopic binary. A re-analysis of archival optical spectra complemented with new near-infrared spectroscopy shows both visual components to be double-lined, the second one having a period of 4.06 days and a circular orbit. In addition to the orbital elements, we determine optical and near-infrared flux ratios, effective temperatures, and projected rotational velocities for all four stars. Using existing photometric monitoring observations of the system that had previously revealed the rotational period of the primary in the longer-period binary, we detect also the rotational signal of the primary in the 4.06-day binary, which is synchronized with the orbital motion. With only the assumption of coevality, a comparison of ...

Torres, Guillermo; Badenas, Mariona; Prato, L; Schaefer, G H; Wasserman, Lawrence H; Mathieu, Robert D; Latham, David W
2013-01-01

295

Asteroseismic measurement of slow, nearly-uniform surface-to-core rotation in the main sequence F star KIC 9244992

  CERN Preprints

Summary: We have found a rotationally split series of core g-mode triplets and surface p-mode multiplets in a main sequence F star, KIC 9244992. Comparison with models shows that the star has a mass of about 1.45 M$_\\odot$, and is at an advanced stage of main sequence evolution in which the central hydrogen abundance mass fraction is reduced to about 0.1. This is the second case, following KIC 11145123, of an asteroseismic determination of the rotation of the deep core and surface of an A-F main-sequence star. We have found, essentially model-independently, that the rotation near the surface, obtained from p-mode splittings, is 66 d, slightly slower than the rotation of 64 d in the core, measured by g-mode splittings. KIC 9244992 is similar to KIC 11145123 in that both are near the end of main-sequence stage with very slow and nearly uniform rotation. This indicates the angular momentum transport in the interior of an A-F star during the main sequence stage is much stronger than that expected from standard theoretical...

Saio, Hideyuki; Takata, Masao; Shibahashi, Hiromoto; Murphy, Simon J; Sekii, Takashi; Bedding, Timothy R
2014-01-01

296

The radio lighthouse CU Virginis: the spindown of a single main sequence star

  Astrophysics (arXiv)

Summary: The fast rotating star CU Virginis is a magnetic chemically peculiar star with an oblique dipolar magnetic field. The continuum radio emission has been interpreted as gyrosyncrotron emission arising from a thin magnetospheric layer. Previous radio observations at 1.4 GHz showed that a 100% circular polarized and highly directive emission component overlaps to the continuum emission two times per rotation, when the magnetic axis lies in the plane of the sky. This sort of radio lighthouse has been proposed to be due to cyclotron maser emission generated above the magnetic pole and propagating perpendicularly to the magnetic axis. Observations carried out with the Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this discovery show that this radio emission is still present, meaning that the phenomenon responsible for this process is steady on a timescale of years. The emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On the light of recent results on the physics of the magnetosphere of this star, the possibility of plasma radiation is ruled out. The characteristics of this radio lighthouse provides us a good marker of the rotation period, since the peaks are visible at particular rotational phases. After one year, they show a delay of about 15 minutes. This is interpreted as a new abrupt spinning down of the star. Among several possibilities, a quick emptying of the equatorial magnetic belt after reaching the maximum density can account for the magnitude of the breaking. The study of the coherent emission in stars like CU Vir, as well as in pre main sequence stars, can give important insight into the angular momentum evolution in young stars. This is a promising field of investigation that high sensitivity radio interferometers such as SKA can exploit.

C. Trigilio; P. Leto; G. Umana; C. S. Buemi; F. Leone
2007-11-21

297

The Helium abundance and Delta Y / Delta Z in Lower Main Sequence stars

  Astrophysics (arXiv)

Summary: We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio, a diagnostic of the chemical history of the Solar Neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content. Our theoretical isochrones have been checked against a congruous set of main sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations. Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity. We have investigated the effect of diffusion in stellar models and LTE assumption in deriving metallicities. Neither of these is able to resolve the low helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed. The helium-to-metal enrichment ratio is found to be Delta Y / Delta Z = 2.1 +/- 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars.

Luca Casagrande; Chris Flynn; Laura Portinari; Leo Girardi; Raul Jimenez
2007-12-10

298

The unusual pre-main-sequence star V718 Per (HMW 15)

  Astrophysics (arXiv)

Summary: The remarkable pre-main-sequence object V718 Per (HMW 15, H187) in the young cluster IC 348 periodically undergoes long-lasting eclipses caused by variable amounts of circumstellar dust in the line-of-sight to the star. It has been speculated that the star is a close binary and similar to another unusual eclipsing object, KH 15D. We have submitted V718 Per to a detailed photometric and spectroscopic study to find out more about the properties of the stellar object and the occulting circumstellar material, and to look for signatures of a possible binary component. Our photometric data show that the eclipses are very symmetric and persistent, and that the extinction deviates only little from what is expected for normal interstellar material. The spectroscopic data, obtained at minimum as well as at maximum brightness, indicate a primordial abundance of Li and a surface effective temperature of about 5200 K. Remarkably, the in-eclipse spectrum shows a significant broadening of the photospheric absorption lines, as well as a weak increase in emission components of H-alpha and the Ca II IR triplet. We did not detect any atomic or molecular features from to the occulting body in the in-eclipse spectrum. We also found no evidence of radial velocity changes in V718 Per to within about +/- 80 m/s, which for an edge-on system corresponds to a maximum companion mass of 6 Jupiter masses. Our observations suggest that V718 Per is a single star, and thus very different from KH 15D. We conclude that V718 Per is surrounded by an edge-on circumstellar disk with an irregular mass distribution orbiting at a distance of 3.3 AU from the star, presumably at the inner disk edge. We ascribe the broadening of photospheric absorption lines during the eclipse to forward scattering of stellar light in the circumstellar dust feature.

V. Grinin; H. C. Stempels; G. F. Gahm; S. Sergeev; A. Arkharov; O. Barsunova; L. Tambovtseva
2008-08-07

299

Pre-main sequence stars, emission stars and recent star formation in the Cygnus Region

  Astrophysics (arXiv)

Summary: The recent star formation history in the Cygnus region is studied using 5 clusters (IC 4996, NGC 6910, Berkeley 87, Biurakan 2 and Berkeley 86). The optical data from the literature are combined with the 2MASS data to identify the pre-main sequence (pre-MS) stars as stars with near IR excess. We identified 93 pre-MS stars and 9 stars with H$_\\alpha$ emission spectra. The identified pre-MS stars are used to estimate the turn-on age of the clusters. The duration of star formation was estimated as the difference between the turn-on and the turn-off age. We find that, NGC 6910 and IC 4996 have been forming stars continuously for the last 6 -- 7 Myr, Berkeley 86 and Biurakan 2 for 5 Myr and Berkeley 87 for the last 2 Myr. This indicates that the Cygnus region has been actively forming stars for the last 7 Myr, depending on the location. 9 emission line stars were identified in 4 clusters, using slit-less spectra (Be 87 - 4 stars; Be 86 - 2 stars, NGC 6910 - 2 stars and IC 4996 - 1 star). The individual spectra were obtained and analysed to estimate stellar as well as disk properties. All the emission stars are in the MS, well below the turn-off, in the core hydrogen burning phase. These stars are likely to be Classical Be (CBe) stars. Thus CBe phenomenon can be found in very young MS stars which are just a few (2 -- 7) Myrs old. This is an indication that CBe phenomenon need not be an evolutionary effect.

Bhavya B; Blesson Mathew; Annapurni Subramaniam
2008-04-09

300

Fundamental properties of pre-main sequence stars in young, southern star forming regions: metallicities

  Astrophysics (arXiv)

Summary: Aims: The primary motivation for this project is to search for metal-rich star forming regions, in which, stars of super-solar metallicity will be created, as hopefully, will be extra-solar planets orbiting them ! Results: We find (pre-main sequence) model-dependent isochronal ages of the Lupus, Chamaeleon and CrA targets to be $9.1 \\pm 2.1$ Myr, $4.5 \\pm 1.6$ Myr and $9.0 \\pm 3.9$ Myr respectively. The majority of the stars have Li I 6707.8A equivalent widths similar to, or above those of, their similar mass Pleiades counterparts, confirming their youthfulness. Most stars are kinematic members, either single or binary, of their regions. We find a mean radial velocity for objects in the Lupus cloud to be ${\\bar {RV}}=+2.6 \\pm 1.8$ km s$^{-1}$, for the Chamaeleon I & II clouds, ${\\bar {RV}}=+12.8 \\pm 3.6$ km s$^{-1}$ whereas for the CrA cloud, we find ${\\bar {RV}}=-1.1 \\pm 0.5$ km s$^{-1}$. All stars are coronally and chromospherically active, exhibiting X-ray and H$\\alpha$ emission levels marginally less, approximately equal or superior to that of their older IC 2602/2391 and/or Pleiades counterparts. All bar three of the targets show little or no signature of accretion from a circumstellar environment, according to their positions in a J$-$K/H$-$K$^{'}$ diagram. We have performed a metallicity analysis for 5 stars in Chamaeleon, 4 stars in Lupus and 3 stars in the CrA star forming regions. These results show that all three regions are slightly metal-poor, with marginally sub-solar metallicities, with $<$[Fe/H]$> = -0.11 \\pm 0.14$, $-0.10 \\pm 0.04$ & $-0.04 \\pm 0.05$ respectively.

David J. James; Claudio Melo; Nuno C. Santos; Jerome Bouvier
2005-10-20

301

An evolved disk surrounding the massive main sequence star MWC 297?

  Astrophysics (arXiv)

Summary: We present the results of the interferometric observations of the circumstellar disk surrounding MWC 297 in the continuum at 230 GHz (1.3 mm) and in the (J=2-1) rotational transitions of $^{12}$CO,$^{13}$CO and C$^{18}$O using the Submillimeter Array. At a distance of 250 pc, MWC 297 is one of the closest, young massive stars (M$_{\\star}$ $\\sim$10 M$_{\\odot}$) to us. Compact continuum emission is detected towards MWC 297 from which we estimate a disk mass (gas+dust) of 0.07 M$_{\\odot}$ and a disk radius of $\\le$ 80 AU. Our result demonstrates that circumstellar disks can survive around massive stars well into their main sequence phase even after they have become optically visible. Complementing our observations with the data compiled from the literature, we find the submm dust opacity index $\\beta$ to be between 0.1 and 0.3. If the emission is optically thin, the low value of $\\beta$ indicates the presence of relatively large grains in the disk, possibly because of grain growth. We do not detect any CO emission associated with the continuum source. We argue that the $^{13}$CO emission from the disk is likely optically thin, in which case, we derive an upper limit to the gas mass which implies significant depletion of molecular gas in the disk. The mass of this disk and the evolutionary trends observed are similar to those found for intermediate mass Herbig Ae stars and low mass T Tauri stars.

P. Manoj; Paul T. P. Ho; Nagayoshi Ohashi; Qizhou Zhang; T. Hasegawa; Huei-Ru Chen; H. C. Bhatt; N. M. Ashok
2007-08-15

302

O/Fe in metal-poor main sequence and subgiant stars

  Astrophysics (arXiv)

Summary: A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of weak FeII lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 < [Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging from -1.0 to -2.7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE effects on the triplet: the [OI] line and the triplet give consistent results with [O/Fe] increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs [Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] = -2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] < -1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to erase these differences.

P. E. Nissen; F. Primas; M. Asplund; D. L. Lambert
2002-05-22

303

The first magnetic maps of a pre-main sequence binary star system - HD 155555

  Astrophysics (arXiv)

Summary: We present the first maps of the surface magnetic fields of a pre-main sequence binary system. Spectropolarimetric observations of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) system were obtained at the Anglo-Australian Telescope in 2004 and 2007. Both datasets are analysed using a new binary Zeeman Doppler imaging (ZDI) code. This allows us to simultaneously model the contribution of each component to the observed circularly polarised spectra. Stellar brightness maps are also produced for HD 155555 and compared to previous Doppler images. Our radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes. We find rings of azimuthal field on both stars, most of which are found to be non-axisymmetric with the stellar rotational axis. We also examine the field strength and the relative fraction of magnetic energy stored in the radial and azimuthal field components at both epochs. A marked weakening of the field strength of the secondary star is observed between the 2004 and 2007 epochs. This is accompanied by an apparent shift in the location of magnetic energy from the azimuthal to radial field. We suggest that this could be indicative of a magnetic activity cycle. We use the radial magnetic maps to extrapolate the coronal field (by assuming a potential field) for each star individually - at present ignoring any possible interaction. The secondary star is found to exhibit an extreme tilt (~75 deg) of its large scale magnetic field to that of its rotation axis for both epochs. The field complexity that is apparent in the surface maps persists out to a significant fraction of the binary separation. Any interaction between the fields of the two stars is therefore likely to be complex also. Modelling this would require a full binary field extrapolation.

N. J. Dunstone; G. A. J. Hussain; A. Collier Cameron; S. C. Marsden; M. Jardine; H. C. Stempels; J. C. Ramirez Vlex; J. -F. Donati
2008-03-06

304

New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets

  Astrophysics (arXiv)

Summary: Using the MIPS instrument on the Spitzer telescope, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars - data that can be correlated with any planets that may eventually be found. We have found significant excess 70um emission toward 12 stars. Combined with an earlier study, we find an overall 70um excess detection rate of $13 \\pm 3$% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying 4 of these stars as having weak 24um excesses (fluxes $\\sim$10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24um excess is associated with a strong 70um excess. Interestingly, we find no evidence for debris disks around 23 stars cooler than K1, a result that is bolstered by a lack of excess around any of the 38 K1-M6 stars in 2 companion surveys. One motivation for this study is the fact that strong zodiacal emission can make it hard or impossible to detect planets directly with future observatories like the {\\it Terrestrial Planet Finder (TPF)}. The observations reported here exclude a few stars with very high levels of emission, $>$1,000 times the emission of our zodiacal cloud, from direct planet searches. For the remainder of the sample, we set relatively high limits on dust emission from asteroid belt counterparts.

C. Beichman; G. Bryden; K. Stapelfeldt
2006-11-21

305

Evolution of Magnetic Fields in Stars Across the Upper Main Sequence: II. Observed distribution of the magnetic field geometry

  Astrophysics (arXiv)

Summary: We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M_sun are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M_sun seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities beta is detected in both higher and lower mass stars. The obliquity angle distribution as inferred from the distribution of r-values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle beta tends to reach values close to either 90 deg or 0 deg for M < 3 M_sun. The evolution of the obliquity angle beta seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of beta with the elapsed time on the main sequence for stars with M > 3 M_sun, no similar trend is found for stars with M < 3 M_sun. However, the predominance of high values of beta at advanced ages in these stars is notable.

S. Hubrig; P. North; M. Schoeller
2007-04-11

306

Preliminary Abundance Analysis of Galactic Bulge Main Sequence, Subgiant, and Giant Branch Stars Observed During Microlensing with Keck/HIRES

  Astrophysics (arXiv)

Summary: We present an abundance analysis of six main sequence turnoff, subgiant, and giant branch stars toward the Galactic bulge that were observed with Keck/HIRES during microlensing events. This is an early look at the first detailed chemical analysis of main sequence stars in the Galactic bulge. Lensing events allow the effective aperture of Keck to be increased beyond its current dimensions; although, some events still stretched its spectroscopic capabilities. Future large telescopes with high resolution and high throughput spectrometers will allow the study of abundances in distant stellar populations and in less evolved stars with greater ease.

R. M. Cavallo; K. H. Cook; D. Minniti; T. Vandehei
2002-09-10

307

The Spectra of Main Sequence Stars in Galactic Globular Clusters II. CH and CN Bands in M71

  Astrophysics (arXiv)

Summary: Spectra with a high signal-to-noise ratio of 79 stars which are just below the main sequence turnoff of M71 are presented. They yield indices for the strength of the G band of CH and the ultraviolet CN band at 3885 \\AA. These indices are each to first order bimodal and they are anti-correlated. There are approximately equal numbers of CN weak/CH strong and CN strong/CH weak main sequence stars in M71. It is not yet clear whether these star-to-star variations arise from primordial variations or from mixing within a fraction of individual stars as they evolve.

J. G. Cohen
1999-01-26

308

WIYN Open Cluster Study. XIX. Main Sequence Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams

  Astrophysics (arXiv)

Summary: We have combined existing optical magnitudes for stars in seven open clusters and 54 field stars with the corresponding JHKs photometry from the Two Micron All Sky Survey (2MASS). Combining optical with near-IR photometry broadens the color baseline minimizing the influence of photometric errors and allows better discrimination between cluster stars and contaminating foreground and background populations. The open clusters in this study include NGC 2516, M35, M34, NGC 3532, M37, M67, and NGC 188. The field stars we are using possess high quality Hipparcos parallaxes and well-determined metal abundances allowing us to empirically determine the dependence of (V-K) color on metal abundance in the range -0.45<=[Fe/H]<=+0.35. Using this relation along with the parallaxes of the field stars, we are able to construct an unevolved main sequence in the [Mv,(V-K)o] diagram for a specific abundance. These diagrams are then used to fit to the cluster main sequences in the [V,V-K] color-magnitude diagram in order to estimate a distance for each open cluster. We find that the resultant distances are within the range of distances found in the literature via the main sequence fitting technique. It is hoped that this will spur an expansion of the current (limited) database of star clusters with high quality (V-K) photometry down to the unevolved main sequence.

Ata Sarajedini; Ken Brandt; Aaron Grocholski; Glenn Tiede
2003-10-30

309

The Young Open Clusters King 12, NGC 7788, and NGC 7790: Pre-Main Sequence Stars and Extended Stellar Halos

  CERN Preprints

Summary: The stellar contents of the open clusters King 12, NGC 7788, and NGC 7790 are investigated using MegaCam images. Comparisons with isochrones yield an age < 20 Myr for King 12, 20--40 Myr for NGC 7788, and 60 -- 80 Myr for NGC 7790 based on the properties of stars near the main sequence turn-off (MSTO) in each cluster. The reddening of NGC 7788 is much larger than previously estimated. The luminosity functions (LFs) of King 12 and NGC 7788 show breaks that are attributed to the onset of pre-main sequence (PMS) objects, and comparisons with models of PMS evolution yield ages that are consistent with those measured from stars near the MSTO. In contrast, the r' LF of main sequence stars in NGC 7790 is matched to r' = 20 by a model that is based on the solar neighborhood mass function. The structural properties of all three clusters are investigated by examining the two-point angular correlation function of blue main sequence stars. King 12 and NGC 7788 are each surrounded by a stellar halo that extends out to ...

Davidge, T J
2012-01-01

310

CONVECTIVE DYNAMOS AND THE MINIMUM X-RAY FLUX IN MAIN-SEQUENCE STARS D. J. Bercik,1

  Physics Websites

Summary: CONVECTIVE DYNAMOS AND THE MINIMUM X-RAY FLUX IN MAIN-SEQUENCE STARS D. J. Bercik,1 G. H. Fisher,1 ABSTRACT The objective of this paper is to investigate whether a convective dynamo can account approach is to use three-dimensional numerical simulations of a turbulent dynamo driven by convection

Abbett, Bill

311

8. The evolution of stars a more detailed picture 8.1Pre Main-Sequence Evolution

  Physics Websites

Summary: the centre, and the star moves to higher effective temperatures. Slowly nuclear burning starts in the core8. The evolution of stars ­ a more detailed picture 8.1Pre Main-Sequence Evolution Evolution onto is transformed into kinetic energy, which gets thermalised, so the temperature goes up. This phase lasts

Peletier, Reynier

312

A split exact sequence of Mackey functors 1. The main theorems

  Mathematics Websites

Summary: (G) - 0 where if is a face of g then = (|g) · cg-1 · resG Gg = (|g) · indG Gg · cg 1 #12;and otherwise at a G-set is (|g)M (1 � G Gg · cg) : M( � G/G) M( � G/G ) in the case of the first sequence, or (|g)M(1 � G Gg · cg) : M( � G/G ) M( � G/G) 2 #12;in the case of the second sequence, where cg : G/G G/Gg

Webb, Peter

313

The pre-main sequence star HD 34282: A very short period delta Scuti-type pulsator

  Astrophysics (arXiv)

Summary: HD 34282 has been found to pulsate during a systematic search for short-term photometric variability in Herbig Ae/Be stars with the goal of determining the position and size of the pre-main sequence instability strip. Simultaneous Stromgren photometry is used in the frequency analysis, yielding two frequencies with values of nu1=79.5 and nu2=71.3 c/d. The light curve with the largest amplitude is that of the u band. This behaviour, which is not common for delta Scuti stars, is explained as pulsation in a high radial order in stars near the blue edge of the instability strip. The main period, with a value of 18.12 min, represents the shortest period observed so far for a delta Scuti-type pulsator. A seismic modelling, including instability predictions and rotation effects, has been attempted. It is found that both main sequence and pre-main sequence models predict modes in the range of 56 to 82 c/d (between 648 and 949 microHz), corresponding to oscillations of radial order n from 6 to 8. The highest of the observed frequencies only becomes unstable for models of low metallicity, in agreement with results from spectroscopic measurements.

P. J. Amado; A. Moya; J. C. Suarez; S. Martin-Ruiz; R. Garrido; E. Rodriguez; C. Catala; M. J. Goupil
2004-06-04

314

Mg II h + k emission lines as stellar activity indicators of main sequence F-K stars

  Astrophysics (arXiv)

Summary: The main purpose of this study is to use the IUE spectra in the analysis of magnetic activity of main sequence F-K stars. Combining IUE observations of MgII and optical spectroscopy of Ca II, the registry of ctivity of stars can be extended in time. We retrieved all the high-resolution spectra of F, G, and K main sequence stars observed by IUE (i.e. 1623 spectra of 259 F to K dwarf stars). We obtained the continuum surface flux near the Mg II h+k lines near 2800 \\AA and the MgII line-core surface flux from the IUE spectra. We obtained a relation between the mean continuum flux near the MgII lines with the colour $B-V$ of the star. For a set of 117 nearly simultaneous observations of Mg II and Ca II fluxes of 21 F5 to K3 main sequence stars, we obtained a colour dependent relation between the Mount Wilson CaII S-index and the MgII emission line-core flux. As an application of this calibration, we computed the Mount Wilson index for all the dF to dK stars which have high resolution IUE spectra. For some of the most frequently observed main sequence stars, we analysed the Mount Wilson index S from the IUE spectra, together with the ones derived from visible spectra. We confirm the cyclic chromospheric activity of epsilon Eri (HD 22049) and beta Hydri (HD 2151), and we find a magnetic cycle in alpha Cen B (HD 128621). Complete abstract in the paper.

Andrea P. Buccino; Pablo J. D. Mauas
2008-04-07

315

A MULTIRESOLUTION APPROACH FOR MAIN MOBILE OBJECT LOCALIZATION IN VIDEO SEQUENCES

  Biology and Medicine Websites

Summary: with Grant #99164. and intrusion detection. However, the rapidly growing ideas of object-based video coding adequate results, they provide non- satisfactory execution times. The main consideration in mobile object


316

A split exact sequence of Mackey functors 1. The main theorems

  Mathematics Websites

Summary: is projective relative to X . (iv) For every Y # Y, M(Y ) = 0. Then there are split exact sequences 0 -# M(G) -# # ##G\\#0(#) M(G # ) (### ) -# # ##G\\#1(#) M(G # ) -# · · · -# # ##G\\#d(#) M(G # ) -# 0 and 0 #- M(G) #- # ##G\\#0(#) M(G # ) (### ) #- # ##G\\#1(#) M(G # ) #- · · · #- # ##G\\#d(#) M(G # ) #- 0 where

Webb, Peter

317

The effect of the electron donor H3+ on the pre-main and Main Sequence evolution of low mass zero metallicity stars

  Astrophysics (arXiv)

Summary: H3+ has been shown (Lenzuni et al. 1991, ApJS, 76, 759) to be the dominant positive ion, in a zero metallicity gas at low temperature and intermediate to high density. It therefore affects both the number of free electrons and the opacity of the gas. The most recent H3+ partition function (Neale and Tennyson, 1995, ApJ, 454, L169) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H3+ is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000 < T(K) < 9000 and -14 < log10(density [g/cc]) < -2. In the calculation of these opacities we have made use of the latest collision induced absorption data as well as the most recent H3+ partition function and line opacity data. It is shown that these updated and new sources of opacity give rise to a Rosseland mean opacity for a hydrogen-helium gas which is in general greater than that calculated in earlier works. The new opacity data are then used to model the evolution of low mass 0.15-0.8 Mo zero metallicity stars, from pre-Main Sequence collapse to Main Sequence turn-off. To investigate the effect of H3+ on the evolution of low mass zero metallicity stars, we repeat our calculations neglecting H3+ as a source of electrons and line opacity. We find that H3+ can have an effect on the structure and evolution of stars of mass ~0.5 Mo or less. A gray atmosphere is used for the calculation, which is sufficient to demonstrate that H3+ affects the evolution of very low mass stars to a greater degree than previously believed.

G. J. Harris; A. E. Lynas-Gray; S. Miller; J. Tennyson
2003-09-17

318

Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. II. Fully Convective Main Sequence Stars

  CERN Preprints

Summary: We examine the hypothesis that magnetic fields are inflating the radii of fully convective main sequence stars in detached eclipsing binaries (DEBs). The magnetic Dartmouth stellar evolution code is used to analyze two systems in particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming stabilization of convection and also by assuming reductions in convective efficiency due to a turbulent dynamo. We find that magnetic stellar models are unable to reproduce the properties of inflated fully convective main sequence stars, unless strong interior magnetic fields in excess of 10 MG are present. Validation of the magnetic field hypothesis given the current generation of magnetic stellar evolution models therefore depends critically on whether the generation and maintenance of strong interior magnetic fields is physically possible. An examination of this requirement is provided. Additionally, an analysis of previous studies invoking the influence of star spots is presented to assess the suggestio...

Feiden, Gregory A
2014-01-01

319

Vibrational instability of Population III very massive main-sequence stars due to the $\\varepsilon$-mechanism

  CERN Preprints

Summary: Very massive stars are thought to be formed in the early Universe because of a lack of cooling process by heavy elements, and might have been responsible for the later evolution of the Universe. We had an interest in vibrational stability of their evolution and carried out the linear nonadiabatic analysis of radial and nonradial oscillations for population III very massive main-sequence stars with $500-3000M_{\\sun}$. We found that only the radial fundamental mode becomes unstable due to the $\\varepsilon$-mechanism for these stars. The instability appears just after the CNO cycle is activated and the nuclear energy generation rate becomes large enough to stop the pre--main-sequence contraction, and continues during the early stage of the core hydrogen burning. Besides, we roughly estimated amount of mass loss due to the instability to know its significance.

Sonoi, Takafumi
2012-01-01

320

A Dynamical Mass Constraint for Pre-Main-Sequence Evolutionary Tracks: The Binary NTT 045251+3016

  Astrophysics (arXiv)

Summary: We present an astrometric/spectroscopic orbital solution for the pre-main-sequence binary NTT 045251+3016. Our measurements for the primary and secondary masses are 1.45 +/- 0.19 M_sun and 0.81 +/- 0.09 M_sun, respectively, and 145 +/- 8 pc for the distance of the system, consistent with prior estimates for the Taurus-Auriga star-forming region. The evolutionary tracks of D'Antona & Mazzitelli (1997), Baraffe et al. (1998), and Palla & Stahler (1999) are tested against these dynamical mass measurements. Due to the intrinsic color/T_eff variation within the K5 spectral class, each pre-main-sequence model provides a mass range for the primary. The theoretical mass range derived from the Baraffe et al. (1998) tracks that use a mixing length parameter alpha=1.0 is closest to our measured primary mass, deviating between 1.3 and 1.6 sigma. The set of Baraffe et al. (1998) tracks that use alpha=1.9 deviate between 1.6 and 2.1 sigma from our measured primary mass. The mass range given by the Palla & Stahler (1999) tracks for the primary star deviate between 1.6 and 2.9 sigma. The D'Antona & Mazzitelli (1997) tracks give a mass range that deviates by at least 3.0 sigma from our derived primary mass, strongly suggesting that these tracks are inconsistent with our observation. Observations of the secondary are less constraining than those of the primary, but the deviations between the dynamical mass of the secondary and the mass inferred for the secondary from the various pre-main-sequence tracks mirror the deviations of the primary star. All of the pre-main-sequence tracks are consistent with coevality of the components of NTT 045251+3016.

A. T. Steffen; R. D. Mathieu; M. G. Lattanzi; D. W. Latham; T. Mazeh; L. Prato; M. Simon; H. Zinnecker; D. Loreggia
2001-05-01

321

Effects of "extra-mixing" processes on the periods of high-order gravity modes in main-sequence stars

  Astrophysics (arXiv)

Summary: In main-sequence stars, the chemical composition gradient that develops at the edge of the convective core is responsible for a non-uniform period spacing of high-order gravity modes. In this work we investigate, in the case of a 1.6 Msun star, the effects on the period-spacing of extra mixing processes in the core (such as diffusion and overshooting).

A. Miglio; J. Montalban; A. Noels
2005-10-21

322

Improved $u'g'r'i'z'$ to $UBVR_CI_C$ Transformation Equations for Main Sequence Stars

  Astrophysics (arXiv)

Summary: We report improved transformation equations between the $u'g'r'i'z'$ and $UBVR_CI_C$ photometric systems. Although the details of the transformations depend on luminosity class, we find a typical rms scatter on the order of 0.001 magnitude if the sample is limited to main sequence stars. Furthermore, we find an accurate transformation requires complex, multi-color dependencies for the bluer bandpasses. Results for giant stars will be reported in a subsequent paper.

Christopher T. Rodgers; Ron Canterna; J. Allyn Smith; Michael J. Pierce; Douglas L. Tucker
2006-09-27

323

High-resolution simulations of stellar collisions between equal-mass main-sequence stars in globular clusters

  Astrophysics (arXiv)

Summary: We performed high-resolution simulations of two stellar collisions relevant for stars in globular clusters. We considered one head-on collision and one off-axis collision between two 0.6 M_sun main sequence stars. We show that a resolution of about 100 000 particles is sufficient for most studies of the structure and evolution of blue stragglers. We demonstrate conclusively that collision products between main-sequence stars in globular clusters do not have surface convection zones larger than 0.004 M_sun after the collision, nor do they develop convection zones during the `pre-main-sequence' thermal relaxation phase of their post-collision evolution. Therefore, any mechanism which requires a surface convection zone (i.e. chemical mixing or angular momentum loss via a magnetic wind) cannot operate in these stars. We show that no disk of material surrounding the collision product is produced in off-axis collisions. The lack of both a convection zone and a disk proves a continuing problem for the angular momentum evolution of blue stragglers in globular clusters.

Alison Sills; Tim Adams; Melvyn Davies; Matthew Bate
2001-12-12

324

Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center

  Astrophysics (arXiv)

Summary: We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf < -275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 - 4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is also allowed.

Warren R. Brown; Margaret J. Geller; Scott J. Kenyon; Michael J. Kurtz; Benjamin C. Bromley
2007-09-13

325

Evolution of Magnetic Fields in Stars Across the Upper Main Sequence: II. Observed distribution of the magnetic field geometry

  CERN Preprints

Summary: We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M 3 M_sun seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities beta is detected in both higher and lower mass stars. The obliquity angle distribution as inferred fro...

Hubrig, S; Schöller, M
2007-01-01

326

At the Bottom of the Main Sequence: Activity and magnetic fields beyond the threshold to complete convection

  Astrophysics (arXiv)

Summary: The bottom of the main sequence hosts objects with fundamentally different properties. At masses of about 0.3 M$_{\\odot}$, stars become fully convective and at about 0.08 M$_{\\odot}$ the hydrogen-burning main sequence ends; less massive objects are brown dwarfs. While stars and brown dwarfs experience very different evolutions, their inner structure has relatively little impact on the atmospheres. The generation of magnetic fields and activity is obviously connected to the threshold between partial and complete convection, because dynamo mechanisms involving a layer of shear like the solar $\\alpha\\Omega$-dynamo must cease. Hence a change in stellar activity can be expected there. Observations of stellar activity do not confirm a rapid break in activity at the convection boundary, but the fraction of active stars and rapid rotators is higher on the fully convective side. I summarize the current picture of stellar activity and magnetic field measurements at the bottom of the main sequence and present recent results on rotational braking beyond.

Ansgar Reiners
2007-12-18

327

The Blue Straggler and Main-sequence Binary Population of the Low-Mass Globular Cluster Palomar 13

  Astrophysics (arXiv)

Summary: We present high-precision VI photometry of stars from the middle of the giant branch to about 5 magnitudes below the main-sequence turnoff in the globular cluster Palomar 13 based on images obtained with the Keck II 10m telescope. We tabulate a complete sample of blue stragglers in the cluster out to about 18 core radii. The blue straggler population is significantly more centrally concentrated than the giant star sample, which is in turn significantly more centrally concentrated than the main-sequence star sample. Palomar 13 has one of the highest specific frequencies of blue stragglers of any known globular cluster, but the specific frequency of blue stragglers in the outskirts of the cluster does not increase as has been seen in denser clusters. We also identify a group of faint blue stragglers (bluer than the turnoff, but having about the same magnitude) that outnumbers the brighter stragglers by more than a factor of 2. The cluster's color-magnitude diagram shows a large excess of stars to the red of the main sequence, indicating that the cluster's binary fraction is at least 30% +/- 4%, which appears to be similar to that of the low-mass cluster E3 but significantly higher than that of the more massive clusters Pal 5 and NGC 288.

L. Lee Clark; Eric L. Sandquist; Michael Bolte
2004-09-10

328

Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13

  Astrophysics (arXiv)

Summary: An analysis of the CN and CH band strengths measured in a large sample of M71 and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic spectra to quantify the underlying C and N abundances. In the case of M71 it is found that the observed CN and CH band strengths are best matched by the {\\it{identical}} C/N/O abundances which fit the bright giants, implying: 1) little if any mixing is taking place during red giant branch ascent in M71, and 2) a substantial component of the C and N abundance inhomogeneities is in place before the main sequence turn-off. The unlikelihood of mixing while on the main sequence requires an explanation for the abundance variations which lies outside the present stars (primordial inhomogeneities or intra-cluster self enrichment). For M13 it is shown that the 3883\\AA CN bands are too weak to be measured in the spectra for any reasonable set of expected compositions. A similar situation exists for CH as well. However, two of the more luminous program stars do appear to have C abundances considerably greater than those found among the bright giants thereby suggesting deep mixing has taken place on the M13 red giant branch.

Michael M. Briley; Judith G. Cohen
2001-04-05

329

Constraints on the pairing properties of main sequence stars from observations of white dwarfs in binary systems

  CERN Preprints

Summary: Observations of main sequence stars conducted over the last several decades have clearly shown that something like 50 per cent of stars of spectral types G and F occur in multiple systems. For earlier spectral types, the incidence of multiplicity is even higher. Thus, a volume limited sample of white dwarfs should reflect the percentage of binarity observed in stars of F to late B spectral types, which are their Main Sequence progenitors. However, a study of the local volume limited sample of white dwarfs (20 pc from the Sun) conducted by Holberg has shown that a white dwarf has a probability of only $\\sim 32$ per cent of occurring in a binary system, in stark contrast to the observations of multiplicity of Main Sequence stars. Others studies have also led to the same conclusion. In this paper, we argue that the "hidden" white dwarfs are either in double white dwarf systems or in Sirius-like systems. We also show that the white dwarf progenitors of the SDSS white dwarf - M dwarf wide binaries are distributed ...

Ferrario, Lilia
2012-01-01

330

GOODS–Herschel: an infrared main sequence for star-forming galaxies

  University of Kansas - KU ScholarWorks

Summary: with the PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010) in- struments. Thanks to the unique power of Herschel to determine the bolometric output of star-forming galaxies, we demonstrate that incorrect extrapolations of LtotIR from 24 ?m..., stellar masses and photometric redshifts. The IR main se- quence is presented in Sect. 4 where the so-called “mid-IR ex- cess problem” is addressed and a solution proposed using the IR8 bolometric correction factor. This parameter, which relies on the same...

Wilson, Graham Wallace; Elbaz, D.; Dickinson, M.; Hwang, H. S.; Dí az-Santos, T.; Magdis, G.; Magnelli, B.; Le Borgne, D.; Galliano, F.; Pannella, M.; Chanial, P.; Armus, L.; Charmandaris, V.; Daddi, E.
2011-09-01

331

The M4 Core Project with HST -- II. Multiple Stellar Populations at the Bottom of the Main Sequence

  CERN Preprints

Summary: The M4 Core Project with HST is designed to exploit the Hubble Space Telescope to investigate the central regions of M4, the Globular Cluster closest to the Sun. In this paper we combine optical and near-infrared photometry to study multiple stellar populations in M4. We detected two sequences of M-dwarfs containing ~38% (MS_I) and ~62% (MS_II) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC2808, which is the only other GCs where multiple MSs of very low-mass stars have been studied to date. We calculate synthetic spectra for M-dwarfs, assuming the chemical composition mixture inferred from spectroscopic studies of stellar populations along the red giant branch, and different Helium abundances, and we compare predicted and observed colors. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.

Milone, A P; Bedin, L R; Piotto, G; Cassisi, S; Dieball, A; Anderson, J; Jerjen, H; Asplund, M; Bellini, A; Brogaard, K; Dotter, A; Giersz, M; Heggie, D C; Knigge, C; Rich, R M; Berg, M van den; Buonanno, R
2014-01-01

332

Three-dimensional simulations of near-surface convection in main-sequence stars - I. Overall structure

  CERN Preprints

Summary: The near-surface layers of cool main-sequence stars are structured by convective flows, which are overshooting into the atmosphere. The flows and the associated spatio-temporal variations of density and temperature affect spectral line profiles and thus have an impact on estimates of stellar properties such as effective temperature, gravitational acceleration, and abundances. We aim at identifying distinctive properties of the thermodynamic structure of the atmospheres of different stars and understand their causes. We ran comprehensive 3D radiation hydrodynamics simulations of the near-surface layers of six simulated stars of spectral type F3V to M2V with the MURaM code. We carry out a systematic parameter study of the mean stratifications, flow structures, and the energy flux in these stars.\\par Results: We find monotonic trends along the lower main sequence in granule size, flow velocity, and intensity contrast. The convection in the M-star models differs substantially from that of the hotter stars, mainly...

Beeck, Benjamin; Reiners, Ansgar; Schüssler, Manfred
2013-01-01

333

Parameters of a Stromgren Sphere Let's assume that we have a 40,000 K B-main sequence star sur-

  Physics Websites

Summary: Parameters of a Str¨omgren Sphere Let's assume that we have a 40,000 K B-main sequence star sur- rounded by an interstellar medium composed entirely of hydrogen (density N(H) = 1 particles cm-3 ). How long will a neutral hydrogen atom stay neutral? The photon luminosity of a B main sequence star is 0 L

Satyapal, Shobita

334

Linear line polarimetry modelling of pre-main sequence Jorick S. Vink 1 , Janet E. Drew 1 , Tim J. Harries 2 , Rene D. Oudmaijer 3

  Physics Websites

Summary: Linear line polarimetry modelling of pre-main sequence stars Jorick S. Vink 1 , Janet E. Drew 1 around pre-main sequence stars. For a sample of both Herbig Ae/Be stars and T Tauri stars, we #12;nd line pro#12;les scattered in rotating accretion disks. We consider both the case of a central star

Vink, Jorick

335

Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M5

  Astrophysics (arXiv)

Summary: We present the iron abundance and abundance ratios for 18 elements with respect to Fe in a sample of stars with a wide range in luminosity from luminous giants to stars near the turnoff in the globular cluster M5. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R=35,000). We find that the neutron capture, the iron peak and the alpha-element abundance ratios show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff To within the precision of the measurements (~0.1 dex), gravitationally induced heavy element diffusion does not appear to be present among the stars near the main sequence turnoff studied here. Our work and other recent studies suggest that heavy element diffusion is inhibited in the surface layers of metal poor stars. Differences in the Na abundance from star to star which extend to the main sequence turnoff are detected in our sample in M5. The anti-correlation between O and Na abundances, observed in other metal poor globular clusters, is not detected in our sample, but it may be hidden among stars with only upper limits for their O abundances. Overall the abundance ratios of M5 appear very similar to those of M71, with the possible exception of the neutron capture element Ba, where we argue that the apparent difference may be due to difficulties in the analysis. As in M71, the alpha-elements Mg, Ca, Si and Ti are overabundant relative to Fe. The results of our abundance analysis of 25 stars in M5 provide further evidence of abundance variations among specific light elements at unexpectedly low luminosities, which cannot be explained by our current understanding of stellar evolution.

Solange V. Ramirez; Judith G. Cohen
2002-10-10

336

On the nature of a secondary main sequence turn-off in the rich LMC cluster NGC 1868

  Astrophysics (arXiv)

Summary: Evidence for a second main-sequence turn-off in a deep colour-magnitude diagram of NGC 1868 is presented. The data were obtained with HST/WFPC2 and reach down to m_{555} ~ 25. Besides the usual t ~ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m_{555} ~ 21 (M_V ~ 2.5), which is consistent with an age of t ~ 3 Gyrs. This CMD feature stands out clearly especially when contaminating field LMC stars are statistically removed. The background subtracted CMD also visibly displays a red subgiant branch extending about 1.5 mag below the younger turn-off and the clump of red giants. The significance of the secondary turn-off in NGC 1868 was confirmed with Monte-Carlo simulations and bootstrapping techniques. Star-counts in selected regions in the cluster CMD indicate a mass ratio of old population/young population in the range 5% <= M_{old} / M_{young} <= 12%, depending on the mass function slope. The existence of such a subpopulation in NGC 1868 is significant even in the presence of uncertainties in background subtraction. The possibility that the secondary turn-off is associated with the field star population was examined by searching for similar features in CMDs of field stars. Statistically significant excesses of stars redwards of the main-sequence were found in all such fields in the range 20 <= m_{555} <= 22. These however are much broader features that do not resemble the main-sequence termination of a single population. We also discuss other alternative explanations for the feature at m_{555} ~ 21, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap.

B. Santiago; L. Kerber; R. Castro; R. de Grijs
2002-07-08

337

A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC

  Astrophysics (arXiv)

Summary: An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE TLUSTY model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of H II regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars.

I. Hunter; P. L. Dufton; R. S. I. Ryans; D. J. Lennon; W. R. J. Rolleston; I. Hubeny; T. Lanz
2005-03-30

338

Gomez's Hamburger (IRAS 18059-3211): A pre main-sequence A-type star

  Astrophysics (arXiv)

Summary: We study the nature of Gomez's Hamburger (IRAS 18059-3211), a nebula that has been proposed to be a post-AGB object. Such a classification is not confirmed; instead, we argue that it will be a key object in the study of disks rotating around young stars. We present high resolution SMA maps of CO J=2--1 in Gomez's Hamburger. The data are analyzed by means of a code that simulates the emission of a nebula showing a variety of physical conditions and kinematics. Our observations clearly show that the CO emitting gas in Gomez's Hamburger forms a spectacular disk in keplerian rotation. Model calculations undoubtly confirm this result. The central (mainly stellar) mass is found to be high, ~ 4 Mo for a distance of 500 pc. The mass and (relatively low) luminosity of the source are, independent of the assumed distance, very different from those possible in evolved stars. Gomez's Hamburger is probably a transitional object between the pre-MS and MS phases, still showing interstellar material around the central star or stellar system.

V. Bujarrabal; K. Young; D. Fong
2008-04-28

339

REM near-IR and optical photometric monitoring of Pre-Main Sequence Stars in Orion

  CERN Preprints

Summary: We performed an intensive photometric monitoring of the PMS stars falling in a field of about 10x10 arc-minutes in the vicinity of the Orion Nebula Cluster (ONC). Photometric data were collected between November 2006 and January 2007 with the REM telescope in the VRIJHK' bands. The largest number of observations is in the I band (about 2700 images) and in J and H bands (about 500 images in each filter). From the observed rotational modulation, induced by the presence of surface inhomogeneities, we derived the rotation periods for 16 stars and improved previous determinations for the other 13. The analysis of the spectral energy distributions and, for some stars, of high-resolution spectra provided us with the main stellar parameters (luminosity, effective temperature, mass, age, and vsini). We also report the serendipitous detection of two strong flares in two of these objects. In most cases, the light-curve amplitudes decrease progressively from the R to H band as expected for cool starspots, while in a few ...

Frasca, A; Spezzi, L; Alcalá, J M; Marilli, E; Furesz, G; Gandolfi, D
2009-01-01

340

Polarimetric variations of binary stars. VI. Orbit-induced variations in the pre-main-sequence binary AK Sco

  Astrophysics (arXiv)

Summary: We present simultaneous UBV polarimetric and photometric observations of the pre-main-sequence binary AK Sco, obtained over 12 nights, slightly less than the orbital period of 13.6 days. The polarization is a sum of interstellar and intrinsic polarization, with a significant intrinsic polarization of 1% at 5250A, indicating the presence of circumstellar matter distributed in an asymmetric geometry. The polarization and its position angle are clearly variable on time scales of hours and nights, in all 3 wavelengths, with a behavior related to the orbital motion. The variations have the highest amplitudes seen so far for pre-main-sequence binaries (~1%, ~30deg) and are sinusoidal with periods similar to the orbital period and half of it. The polarization variations are generally correlated with the photometric ones: when the star gets fainter, it also gets redder and its polarization increases. The color-magnitude diagram B-V, V exhibits a ratio of total to selective absorption R=4.3 higher than in normal interstellar clouds (R=3.1). The interpretation of the simultaneous photometric and polarimetric observations is that a cloud of circumstellar matter passes in front of the star, decreasing the amount of direct, unpolarized light, and hence increasing the contribution of scattered (blue) light. We show that the large amplitude of the polarization variations can not be reproduced with a single scattering model and axially symmetric circumbinary or circumstellar disks.

N. Manset; P. Bastien; C. Bertout
2004-10-06

341

K Sco, first detection of a highly disturbed atmosphere in a pre-main sequence close binary

  CERN Preprints

Summary: AK Sco is a unique source: a ~10 Myrs old pre-main sequence spectroscopic binary composed of two nearly equal F5 stars that at periastron are separated by barely eleven stellar radii so, the stellar magnetospheres fill the Roche lobe at periastron. The orbit is not yet circularized (e=0.47) and very strong tides are expected. This makes of AK Sco, the ideal laboratory to study the effect of gravitational tides in the stellar magnetic field building up during pre-main sequence (PMS) evolution. In this letter, the detection of a highly disturbed (sigma ~ 100 km/s) and very dense atmosphere (ne = 1.6e10cm-3) is reported. Significant line broadening blurs any signs of ion belts or bow shocks in the spectrum of the atmospheric plasma. The radiative loses cannot be accounted solely by the dissipation of energy from the tidal wave propagating in the stellar atmosphere; neither by the accreting material. The release of internal energy from the star seems to be the most likely source of the plasma heating. This is the...

de Castro, Ana I Gomez
2009-01-01

342

Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark

  CERN Preprints

Summary: We present a novel approach to derive the age of very young star clusters, by using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram (CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS luminosity function (LF) of the cluster, the TOn is identified as a peak followed by a dip. We propose that by combining the CMD analysis with the monitoring of the spatial distribution of MS stars it is possible to reliably identify the TOn in extragalactic star forming regions. Compared to alternative methods, this technique is complementary to the turn-off dating and avoids the systematic biases affecting the PMS phase. We describe the method and its uncertainties, and apply it to the star forming region NGC346, which has been extensively imaged with the Hubble Space Telescope (HST). This study extends the LF approach in crowded extragalactic regions and opens the way for future studies with HST/WFC3, JWST and from the ground with adaptive optics.

Cignoni, M; Sabbi, E; Nota, A; Degl'Innocenti, S; Moroni, P G Prada; Gallagher, J S
2010-01-01

343

Dusty OB Stars in the Small Magellanic Cloud - I: Optical Spectroscopy Reveals Predominantly Main-Sequence OB Stars

  CERN Preprints

Summary: We present the results of optical spectroscopic follow-up of 125 candidate main sequence OB stars in the Small Magellanic Cloud (SMC) that were originally identified in the S3MC infrared imaging survey as showing an excess of emission at 24 um indicative of warm dust, such as that associated with a transitional or debris disks. We use these long-slit spectra to investigate the origin of the 24 um emission and the nature of these stars. A possible explanation for the observed 24 um excess, that these are emission line stars with dusty excretion disks, is disproven for the majority of our sources. We find that 88 of these objects are normal stars without line emission, with spectral types mostly ranging from late-O to early-B; luminosity classes a sub-set of our sample indicate that most are main-sequence stars. We further identify 17 emission-line stars, 7 possible emission-line stars, and 5 other stars with forbidden-line emission. We discover a new O6 Iaf star; it exhibits strong HeII 4686 A emission but rel...

Sheets, Holly A; van Loon, Jacco Th; Sandstrom, Karin; Simon, Joshua D; Oliveira, Joana M; Barbá, Rodolfo H
2013-01-01

344

High Resolution X-ray Spectroscopy of Pre-Main-Sequence Stars: TWA 5 and PZ Tel

  Astrophysics (arXiv)

Summary: We report on the analysis of high resolution X-ray spectra of two pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri star in the TW Hydrae association while PZ Tel is a rapidly rotating weak-lined T Tauri star in the beta-Pictoris moving group. For both stars we have reconstructed the emission measure distribution and derived the coronal abundances to check for possible patterns of the abundances related to the first ionization potential of the various elements. We have also derived estimates of the plasma density from the analysis of the He-like triplets. We compare the characteristics of our targets with those of other pre-main sequence stars previously analyzed by other authors: TW Hya, HD 98800 and HD 283572. Our findings suggest that X-ray emission from classical T Tauri and weak-lined T Tauri stars is produced in all cases by magnetically-heated coronae, except for TW Hya which has unique plasma temperatures and densities. Moreover we derive that TWA 5 has the same peculiar Ne/Fe abundance ratio as TW Hya.

C. Argiroffi; J. J. Drake; F. R. Harnden; A. Maggio; G. Peres; S. Sciortino; B. Stelzer; R. Neuhaeuser
2004-10-11

345

Deep Wide-Field Imaging down to the oldest Main Sequence Turnoffs in the Sculptor dwarf spheroidal galaxy

  CERN Preprints

Summary: We present wide-field photometry of resolved stars in the nearby Sculptor dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude Diagrams can be used to constrain the ages of different stellar populations, and also their spatial distribution. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old) which can be easily resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. By using spectroscopic metallicities of RGB stars in combination with our deep Main Sequence Turn-Off photometry we can constrain the ages of different stellar populations with particular accuracy. We find that the known metallicity gradient in Sculptor is well matched to an age gradient. This is the first time that this link with age has been directly quantified. This gradient has been previously o...

de Boer, T J L; Saha, A; Olsen, K; Irwin, M J; Battaglia, G; Hill, V; Shetrone, M D; Fiorentino, G; Cole, A
2011-01-01

346

Dynamical Masses for Low-Mass Pre-Main Sequence Stars: A Preliminary Physical Orbit for HD 98800 B

  Astrophysics (arXiv)

Summary: We report on Keck Interferometer observations of the double-lined binary (B) component of the quadruple pre-main sequence (PMS) system HD 98800. With these interferometric observations combined with astrometric measurements made by the Hubble Space Telescope Fine Guidance Sensors (FGS), and published radial velocity observations we have estimated preliminary visual and physical orbits of the HD 98800 B subsystem. Our orbit model calls for an inclination of 66.8 $\\pm$ 3.2 deg, and allows us to infer the masses and luminosities of the individual components. In particular we find component masses of 0.699 $\\pm$ 0.064 and 0.582 $\\pm$ 0.051 M$_{\\sun}$ for the Ba (primary) and Bb (secondary) components respectively. Modeling of the component SEDs finds temperatures and luminosities in agreement with previous studies, and coupled with the component mass estimates allows for comparison with PMS models in the low-mass regime with few empirical constraints. Solar abundance models seem to under-predict the inferred component temperatures and luminosities, while assuming slightly sub-solar abundances bring the models and observations into better agreement. The present preliminary orbit does not yet place significant constraints on existing pre-main sequence stellar models, but prospects for additional observations improving the orbit model and component parameters are very good.

A. F. Boden; A. I. Sargent; R. L. Akeson; J. M. Carpenter; G. Torres; D. W. Latham; D. R. Soderblom; E. Nelan; O. G. Franz; L. H. Wasserman
2005-08-15

347

A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus Molecular Cloud

  Astrophysics (arXiv)

Summary: This work is part of a systematic X-ray survey of the Taurus star forming complex with XMM-Newton. We study the time series of all X-ray sources associated with Taurus members, to statistically characterize their X-ray variability, and compare the results to those for pre-main sequence stars in the Orion Nebula Cluster and to expectations arising from a model where all the X-ray emission is the result of a large number of stochastically occurring flares. We find that roughly half of the detected X-ray sources show variability above our sensitivity limit, and in ~ 26 % of the cases this variability is recognized as flares. Variability is more frequently detected at hard than at soft energies. The variability statistics of cTTS and wTTS are undistinguishable, suggesting a common (coronal) origin for their X-ray emission. We have for the first time applied a rigorous maximum likelihood method in the analysis of the number distribution of flare energies on pre-main sequence stars. In its differential form this distribution follows a power-law with index alpha = 2.4 +- 0.5, in the range typically observed on late-type stars and the Sun. The flare energy distribution is probably steep enough to explain the heating of stellar coronae by nano-flares (alpha > 2), albeit associated with a rather large uncertainty that leaves some doubt on this conclusion.

B. Stelzer; E. Flaccomio; K. Briggs; G. Micela; L. Scelsi; M. Audard; I. Pillitteri; M. Guedel
2006-08-30

348

Rotational mixing in early-type stars: the main-sequence evolution of a 9 Mo star

  Astrophysics (arXiv)

Summary: We describe the main-sequence evolution of a rotating 9 $M_\\odot$ star. Its interior rotation profile is determined by the redistribution of angular momentum through the meridian circulation and through the shear turbulence generated by the differential rotation; the possible effect of internal waves is neglected. We examine the mixing of chemicals produced by the same internal motions. Our modelization is based on the set of equations established by Zahn (1992) and completed in Matias, Talon & Zahn (1996). Our calculations show that the amount of mixing associated with a typical rotation velocity of $\\sim 100 km/s$ yields stellar models whose global parameters are very similar to those obtained with the moderate overshooting ($d/H_P \\simeq 0.2$) which has been invoked until now to fit the observations. Fast rotation ($\\sim 300 km/s$) leads to significant changes of the C/N and O/N surface ratios, but the abundance of He is barely increased. The modifications of the internal composition profile due to such rotational mixing will certainly affect the post--main-sequence evolution.

Suzanne Talon; Jean-Paul Zahn; Andre Maeder; Georges Meynet
1996-11-18

349

A Low-Mass Main-Sequence Star and Accretion Disk in the Very Faint Transient M15 X-3

  CERN Preprints

Summary: We present near-simultaneous Chandra/HST observations of the very faint ($L_{x} < 10^{36}$ erg s$^{-1}$) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be $< 10^{34}$ erg s$^{-1}$ in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power-law with break energy $E_{\\rm break} = 2.7^{+0.4}_{-0.6}$ keV, and power law indices of $\\Gamma_{1} = 1.3^{+0.1}_{-0.2}$ and $\\Gamma_{2} = 1.9^{+0.2}_{-0.2}$ over a single power law. We fit our new F438W ($B$), F606W (broad $V$), and F814W ($I$) HST data on the blue optical counterpart with a model for an accretion disk and a metal-poor main sequence star. From this fit, we determine the companion to be consistent with a main sequence star of mass $0.440^{+0.035}_{-0.060}$ $M_{\\odot}$ in a $\\sim$4-hour orbit. X-ray irradiation of the companion is likely to be a factor in the optical emission from the system, ...

Arnason, Robin; Heinke, Craig; Cohn, Haldan; Lugger, Phyllis
2015-01-01

350

The Mdot - M* relation of pre-main sequence stars: a consequence of X-ray driven disc evolution

  CERN Preprints

Summary: We analyse current measurements of accretion rates onto pre-main sequence stars as a function of stellar mass, and conclude that the steep dependance of accretion rates on stellar mass is real and not driven by selection/detection threshold, as has been previously feared. These conclusions are reached by means of statistical tests including a survival analysis which can account for upper limits. The power-law slope of the Mdot-M* relation is found to be in the range of 1.6-1.9 for young stars with masses lower than 1 Msun. The measured slopes and distributions can be easily reproduced by means of a simple disc model which includes viscous accretion and X-ray photoevaporation. We conclude that the Mdot-M* relation in pre-main sequence stars bears the signature of disc dispersal by X-ray photoevaporation, suggesting that the relation is a straight- forward consequence of disc physics rather than an imprint of initial conditions.

Ercolano, B; Owen, J E; Rosotti, G; Manara, C F
2013-01-01

351

WIYN Open Cluster Study. XIX. Main Sequence Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams

  CERN Preprints

Summary: We have combined existing optical magnitudes for stars in seven open clusters and 54 field stars with the corresponding JHKs photometry from the Two Micron All Sky Survey (2MASS). Combining optical with near-IR photometry broadens the color baseline minimizing the influence of photometric errors and allows better discrimination between cluster stars and contaminating foreground and background populations. The open clusters in this study include NGC 2516, M35, M34, NGC 3532, M37, M67, and NGC 188. The field stars we are using possess high quality Hipparcos parallaxes and well-determined metal abundances allowing us to empirically determine the dependence of (V-K) color on metal abundance in the range -0.45<=[Fe/H]<=+0.35. Using this relation along with the parallaxes of the field stars, we are able to construct an unevolved main sequence in the [Mv,(V-K)o] diagram for a specific abundance. These diagrams are then used to fit to the cluster main sequences in the [V,V-K] color-magnitude diagram in order to...

Sarajedini, A; Grocholski, A; Tiede, G P; Sarajedini, Ata; Brandt, Ken; Grocholski, Aaron; Tiede, Glenn
2004-01-01

352

Biological Damage due to Photospheric, Chromospheric and Flare Radiation in the Environments of Main-Sequence Stars

  CERN Preprints

Summary: We explore the biological damage initiated in the environments of F, G, K, and M-type main-sequence stars due to photospheric, chromospheric and flare radiation. The amount of chromospheric radiation is, in a statistical sense, directly coupled to the stellar age as well as the presence of significant stellar magnetic fields and dynamo activity. With respect to photospheric radiation, we also consider detailed synthetic models, taking into account millions or hundred of millions of lines for atoms and molecules. Chromospheric UV radiation is increased in young stars in regard to all stellar spectral types. Flare activity is most pronounced in K and M-type stars, which also has the potential of stripping the planetary atmospheres of close-in planets, including planets located in the stellar habitable zone. For our studies, we take DNA as a proxy for carbon-based macromolecules, guided by the paradigm that carbon might constitute the biochemical centerpiece of extraterrestrial life forms. Planetary atmospheric ...

Cuntz, M; Kurucz, R L
2009-01-01

353

Detection of Lithium in a Main Sequence Bulge Star Using Keck I as a 15m Diameter Telescope

  Astrophysics (arXiv)

Summary: The bulge contains the most chemically evolved old stellar population in the Milky Way. Thanks to microlensing, it is now possible to obtain high resolution echelle spectra of bulge stars near the main sequence turn-off, and study the abundance of elements that are affected by stellar evolution, such as lithium. We have observed with the HIRES spectrograph on the Keck I 10m telescope the source star of the MACHO microlensing event 97BLG45 while it was magnified by about 1 magnitude. Here we report the detection of the Li I line at \\lambda 6707.8 \\AA in the echelle spectrum of this star, and measure the bulge lithium abundance for the first time: A(Li) = 2.25 \\pm 0.25.

D. Minniti; T. Vandehei; K. H. Cook; K. Griest; C. Alcock
1997-12-03

354

A new Bohm-Vitense gap in the temperature range 5560 to 5610K in the Main Sequence

  Astrophysics (arXiv)

Summary: Highly precise temperatures (sigma = 10-15 K) have been determined from line depth ratios for a set of 248 F-K field dwarfs of about solar metallicity (-0.5 < [Fe/H] < +0.4), based on high resolution (R=42000), high S/N echelle spectra. A new gap has been discovered in the distribution of stars on the Main Sequence in the temperature range 5560 to 5610 K. This gap coincides with a jump in the microturbulent velocity Vt and the well-known Li depression near 5600 K in field dwarfs and open clusters. As the principal cause of the observed discontinuities in stellar properties we propose the penetration of the convective zone into the inner layers of stars slightly less massive than the Sun and related to it, a change in the temperature gradient.

V. V. Kovtyukh; C. Soubiran; S. I. Belik
2004-09-30

355

The Great Escape III: Placing post-main-sequence evolution of planetary and binary systems in a Galactic context

  CERN Preprints

Summary: Our improving understanding of the life cycle of planetary systems prompts investigations of the role of the Galactic environment before, during and after Asymptotic Giant Branch (AGB) stellar evolution. Here, we investigate the interplay between stellar mass loss, Galactic tidal perturbations, and stellar flybys for evolving stars which host one planet, smaller body or stellar binary companion and reside in the Milky Way's bulge or disc. We find that the potential evolutionary pathways from a main sequence (MS) to a white dwarf (WD) planetary system are a strong function of Galactocentric distance only with respect to the prevalence of stellar flybys. Planetary ejection and collision with the parent star should be more common towards the bulge. At a given location anywhere in the Galaxy, if the mass loss is adiabatic, then the secondary is likely to avoid close flybys during AGB evolution, and cannot eventually escape the resulting WD because of Galactic tides alone. Partly because AGB mass loss will shrink ...

Veras, Dimitri; Wyatt, Mark C; Tout, Christopher A
2013-01-01

356

The Spectra of Main Sequence Stars in Galactic Globular Clusters I. CH and CN Bands in M13

  Astrophysics (arXiv)

Summary: Spectra with a high signal-to-noise ratio of 50 stars which are just below the main sequence turnoff and are members of M13 are presented. They yield indices for the strength of the CH and the ultraviolet CN band. There is no evidence for a variation in the strength of either feature from star to star in this intermediate-metallicity galactic globular cluster, and thus no evidence for primordial variations in the abundance of C and N in M13. This supports the hypothesis that abundance variations found among the light elements in the evolved stars of M13 by Suntzeff (1981), and commonly seen on the giant and subgiant branches of globular clusters of comparable metallicity, are due primarily or entirely to mixing within a fraction of individual stars as they evolve.

J. G. Cohen
1999-01-26

357

Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars

  CERN Preprints

Summary: Magnetic fields are at the heart of the observed stellar activity in late-type stars, and they are presumably generated by a dynamo mechanism at the interface layer between the radiative and the convective stellar regions. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar code allows for explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, even though there are formidable mathematical and numerical challenges involved. As examples, we present theoretical estimates for both the local (tau_c) and global (tau_g) convective turnover times for rotating pre-main sequence solar-type stars, based on up-to-date input physics for stellar models. Our theoretical predictions are compared with the previous ones available in the literature. In addition, we investigate the dependence of the convective turnover time on convection regimes, the presence of rotatio...

Landin, N R; Vaz, L P R
2010-01-01

358

Combined stellar structure and atmosphere models for massive stars; 1, interior evolution and wind properties on the main sequence

  CERN Preprints

Summary: We present the first "combined stellar structure and atmosphere models" (CoStar) for massive stars, which consistently treat the entire mass loosing star from the center out to the asymptotic wind velocity. The models use up-to-date input physics and state-of-the-art techniques to model both the stellar interior and the spherically expanding non--LTE atmosphere including line blanketing. Our models thus yield consistent predictions regarding not only the basic stellar parameters, including abundances, but also theoretical spectra along evolutionary tracks. On the same ground they allow us to study the influence of stellar winds on evolutionary models. In this first paper, we present our method and investigate the wind properties and the interior evolution on the main sequence (MS) at solar metallicity. The wind momentum and energy deposition associated with the MS evolution is given and the adopted wind properties are discussed. From our atmosphere calculations, we also derive theoretical estimates of mass lo...

Schärer, D; Schmutz, W; Maeder, A
1995-01-01

359

Using seismic inversions to obtain an internal mixing processes indicator for main-sequence solar-like stars

  CERN Preprints

Summary: Determining accurate and precise stellar ages is a major problem in astrophysics. These determinations are either obtained through empirical relations or model-dependent approaches. Currently, seismic modelling is one of the best ways of providing accurate ages. However, current methods are affected by simplifying assumptions concerning mixing processes. In this context, providing new structural indicators which are less model-dependent and more sensitive to such processes is crucial. We build a new indicator for core conditions on the main sequence, which should be more sensitive to structural differences and applicable to older stars than the indicator t presented in a previous paper. We also wish to analyse the importance of the number and type of modes for the inversion, as well as the impact of various constraints and levels of accuracy in the forward modelling process that is used to obtain reference models for the inversion. First, we present a method to obtain new structural kernels and use them to bu...

Buldgen, G; Dupret, M A
2015-01-01

360

Non-LTE modelling of the HeI 10830A line in early-type main sequence stars

  Astrophysics (arXiv)

Summary: The near-IR HeI 10830A transition is a highly sensitive diagnostic for non-LTE effects in the helium atom. So far, non-LTE line-formation computations have failed to quantitatively reproduce observations of this line in the entire range of early-A to late-O main sequence stars. It is shown that the non-LTE modelling was insufficient, for the most part either because of inaccurate photoionization cross-sections for the 2s 3S state or the neglect of line blocking. New calculations based on state-of-the-art atomic data give excellent agreement with observation for the HeI 10830 A feature, while profiles of the HeI lines in the visual are retained.

N. Przybilla
2005-08-02

361

Evolution of very low mass pre-main sequence stars and young brown dwarfs under accretion: A phenomenological approach

  Astrophysics (arXiv)

Summary: In the poster presented in Cool Star 15, we analyzed the effect of disk accretion on the evolution of very low mass pre-main sequence stars and young brown dwarfs and the resulting uncertainties on the determination of masses and ages. We use the Lyon evolutionary 1-D code assuming a magnetospheric accretion process, i.e., the material falls covering a small area of the radiative surface, and we take into account the internal energy added from the accreted material as a free parameter $\\epsilon$. Even if the approach to this problem is phenomenological, our formalism provides important hints about characteristics of disk accretion, which are useful for improved stellar interior calculations. Using the accretion rates derived from observations our results show that accretion does not affect considerably the position of theoretical isochrones as well as the luminosity compared with standard non-accreting models. See more discussions in a forthcoming paper by Gallardo, Baraffe and Chabrier (2008).

Jose Gallardo; Isabelle Baraffe; Gilles Chabrier
2008-10-16

362

NGC 146: A young open cluster with a Herbig Be star and intermediate mass pre-main sequence stars

  Astrophysics (arXiv)

Summary: We present UBV CCD photometry and low-resolution spectra of stars in the field of the young open cluster NGC 146. UBV photometry of 434 stars were used to estimate the E(B$-$V) reddening of 0.55 $\\pm$ 0.04 mag and BV photometry of 976 stars were used to estimate a distance modulus of (m$-$M)$_0$ = 12.7 $\\pm$0.2 mag, corresponding to a distance of 3470$^{+335}_{-305}$ pc. We estimated 10 -- 16 Myr as the turn-off age for the upper main sequence of the cluster using isochrones and synthetic colour magnitude diagrams. We identified two B type stars with H$_\\alpha$ in emission and located on the MS using slit-less spectra. A higher resolution spectrum of the brighter Be star indicated the presence of a number of emission lines, with some lines showing the signature of gas infall. This star was found to be located in the region of Herbig Ae/Be stars in the (J$-$H) vs (H$-$K) colour-colour diagram. Thus, we identify this star as a Herbig Be star. On the other hand, 54 stars were found to show near infrared excess, of which 17 were found to be located in the region of Herbig Ae/Be stars and 18 stars were found to be located in the region of Be stars in the NIR colour-colour diagram. Thus NGC 146 is a young cluster with a large number of intermediate mass pre-main sequence stars. The turn-on age of the cluster is found to be $\\sim$ 3 Myr. Though NGC 146 shows an older turn off, the bulk of stars in this cluster seems to belong to the younger population of 3 Myr.

A. Subramaniam; D. K. Sahu; R. Sagar; P. Vijitha; ;
2005-05-16

363

A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud

  Astrophysics (arXiv)

Summary: We report on HST/ACS photometry of the rich intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud, which clearly reveals the presence of a double main sequence turn-off in this object. Despite this, the main sequence, sub-giant branch, and red giant branch are all narrow and well-defined, and the red clump is compact. We examine the spatial distribution of turn-off stars and demonstrate that all belong to NGC 1846 rather than to any field star population. In addition, the spatial distributions of the two sets of turn-off stars may exhibit different central concentrations and some asymmetries. By fitting isochrones, we show that the properties of the colour-magnitude diagram can be explained if there are two stellar populations of equivalent metal abundance in NGC 1846, differing in age by approximately 300 Myr. The absolute ages of the two populations are ~1.9 and ~2.2 Gyr, although there may be a systematic error of up to +/-0.4 Gyr in these values. The metal abundance inferred from isochrone fitting is [M/H] ~ -0.40, consistent with spectroscopic measurements of [Fe/H]. We propose that the observed properties of NGC 1846 can be explained if this object originated via the tidal capture of two star clusters formed separately in a star cluster group in a single giant molecular cloud. This scenario accounts naturally for the age difference and uniform metallicity of the two member populations, as well as the differences in their spatial distributions.

A. D. Mackey; P. Broby Nielsen
2007-04-25

364

Post Common Envelope Binaries from SDSS. I: 101 white dwarf main sequence binaries with multiple SDSS spectroscopy

  Astrophysics (arXiv)

Summary: We present a detailed analysis of 101 white dwarf-main sequence binaries (WDMS) from the Sloan Digital Sky Survey (SDSS) for which multiple SDSS spectra are available. We detect significant radial velocity variations in 18 WDMS, identifying them as post common envelope binaries (PCEBs) or strong PCEB candidates. Strict upper limits to the orbital periods are calculated, ranging from 0.43 to 7880 d. Given the sparse temporal sampling and relatively low spectral resolution of the SDSS spectra, our results imply a PCEB fraction of >=15% among the WDMS in the SDSS data base. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS in our sample. Two independent distance estimates are obtained from the flux scaling factors between the WDMS spectra, and the white dwarf models and main sequence star templates, respectively. Approximately one third of the systems in our sample show a significant discrepancy between the two distance estimates. In the majority of discrepant cases, the distance estimate based on the secondary star is too large. A possible explanation for this behaviour is that the secondary star spectral types that we determined from the SDSS spectra are systematically too early by 1-2 spectral classes. This behaviour could be explained by stellar activity, if covering a significant fraction of the star by cool dark spots will raise the temperature of the inter-spot regions. Finally, we discuss the selection effects of the WDMS sample provided by the SDSS project.

A. Rebassa-Mansergas; B. T. Gaensicke; P. Rodriguez-Gil; M. R. Schreiber; D. Koester
2007-07-27

365

A VLT/NACO Survey for Triple and Quadruple Systems among Visual Pre-Main Sequence Binaries

  Astrophysics (arXiv)

Summary: This paper describes a systematic search for high-order multiplicity among wide visual Pre-Main Sequence (PMS) binaries. We conducted an Adaptive Optics survey of a sample of 58 PMS wide binaries from various star-forming regions, which include 52 T Tauri systems with mostly K- and M-type primaries, with the NIR instrument NACO at the VLT. Of these 52 systems, 7 are found to be triple (2 new) and 7 quadruple (1 new). The new close companions are most likely physically bound based on their probability of chance projection and, for some of them, on their position on a color-color diagram. The corresponding degree of multiplicity among wide binaries (number of triples and quadruples divided by the number of systems) is 26.9 +/- 7.2% in the projected separation range 0.07-12 arcsec, with the largest contribution from the Taurus-Auriga cloud. We also found that this degree of multiplicity is twice in Taurus compared to Ophiuchus and Chamaeleon for which the same number of sources are present in our sample. Considering a restricted sample composed of systems at distance 140-190pc, the degree of multiplicity is 26.8 +/- 8.1%, in the separation range 10/14 AU - 1700/2300 AU (30 binaries, 5 triples, 6 quadruples). The observed frequency agrees with results from previous multiplicity surveys within the uncertainties, although a significant overabundance of quadruple systems compared to triple systems is apparent. Tentatively including the spectroscopic pairs in our restricted sample and comparing the multiplicity fractions to those measured for solar-type main-sequence stars in the solar neighborhood leads to the conclusion that both the ratio of triples to binaries and the ratio of quadruples to triples seems to be in excess among young stars. [...

S. Correia; H. Zinnecker; Th. Ratzka; M. F. Sterzik
2006-08-30

366

A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods

  Astrophysics (arXiv)

Summary: We use mid-IR (primarily 10 $\\mu$m) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre-main sequence stars in Orion and Taurus-Auriga for which rotation periods are known and which do not show evidence for inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence for excess mid-IR emission. While these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk-locking, and there exist very young stars already rotating near breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr, which may allow the models to explain the rotational evolution of low-mass pre-main sequence stars in a way that does not depend upon braking by disks.

Keivan G. Stassun; Robert D. Mathieu; Frederick J. Vrba; Tsevi Mazeh; Arne Henden
2000-10-31

367

Spectroscopic Binaries in Globular Clusters. I. A Search for Ultra-Hard Binaries on the Main Sequence in M4

  Astrophysics (arXiv)

Summary: A search for spectroscopic binaries on the main sequence of the nearby globular cluster M4 has been undertaken with Argus, the multi-object spectrograph on the CTIO 4.0m telescope. A pair of radial velocities (median precision $\\simeq$ 2 km/s) separated by 11 months have been obtained for 33 turnoff dwarfs in the magnitude range 16.9 $\\le$ V $\\le$ 17.4. Monte-Carlo simulations have been used to derive a binary fraction, X, for systems with periods in the range 2 days $\\lae {\\rm P} \\lae$ 3 years and mass ratios between 0.2 and 1.0. This short-period cutoff is more than an order of magnitude smaller than those of existing radial velocity surveys and is comparable to the shortest periods possible for main-sequence turnoff stars. Our survey therefore provides a first glimpse into the abundance of ``ultra-hard" spectroscopic binaries in globular clusters. Although no star shows a velocity variation larger than 14 km/s, two objects are observed to have chi-square probabilities below 0.1\\%. No such stars are expected in a sample of 33. We find a best-fit binary fraction of X $\\simeq$ 0.15, a value which is consistent with recent estimates based on deep HST color-magnitude diagrams, as well as with the binary fraction of X $\\simeq$ 0.1 for nearby solar-type stars having similar periods and mass ratios. Our derived binary fraction suggests that exchange interactions with pre-existing binaries are a plausible means of explaining the origin of the hierarchical triple system containing the pulsar PSR 1620-26.

Patrick Cote; Phil Fischer
1996-05-02

368

Evolution of Magnetic Fields in Stars Across the Upper Main Sequence: Results from Recent Measurements with FORS 1 at the VLT

  Astrophysics (arXiv)

Summary: We rediscuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. FORS1 at the VLT in spectropolarimetric mode has been used to carry out a systematic search for magnetic fields in chemically peculiar stars whose magnetic field has never been studied before. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that the distribution of the magnetic stars of mass below 3 M_o differs significantly from that of normal stars in the same temperature range. Normal A stars occupy the whole width of the main sequence, without a gap, whereas magnetic stars are concentrated towards the centre of the main-sequence band. We show that, in contrast, higher mass magnetic Bp stars may well occupy the whole main-sequence width.

S. Hubrig; P. North; T. Szeifert
2005-01-23

369

From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters

  Astrophysics (arXiv)

Summary: I explore the current ability of both white dwarf cooling theory and main sequence stellar evolution theory to accurately determine stellar population ages by comparing ages derived using both techniques for open clusters ranging from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence evolutionary ages over the entire age range currently available for study. I also find that directly comparing main sequence turn-off ages to white dwarf ages is only weakly sensitive to realistic levels of errors in cluster distance, metallicity, and reddening. Additional detailed comparisons between white dwarf and main sequence ages have tremendous potential to refine and calibrate both of these important clocks, and I present new simulations of promising open cluster targets. The most demanding requirement for these white dwarf studies are very deep (V > 25-28) cluster observations made necessary by the faintness of the oldest white dwarfs.

Ted von Hippel
2005-02-14

370

EINSTEIN Observations of T Tauri Stars in TaurusAuriga. II: Relationships between Xray Emission and PreMainSequence Activity

  Physics Websites

Summary: EINSTEIN Observations of T Tauri Stars in Taurus­Auriga. II: Relationships between X­ray Emission Einstein IPC X­ray data on Tau­Aur pre­main­sequence stars, presented in a companion paper, we have studied the relationships between the X­ray emis­ sion and typical diagnostics of pre­main­sequence activity, to obtain


371

Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: A Correlation Between Turnoff Width and Early Escape Velocity

  CERN Preprints

Summary: We present color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1 - 2 Gyr old) star clusters in the Magellanic Clouds, including 8 clusters for which new data was obtained. We find that ${\\it all}$ star clusters in our sample feature extended main sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate dynamical evolution of clusters with and without initial mass segregation. Our main results are: (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages $\\leq 1.35$ Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity $v_{\\rm esc}$, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects ...

Goudfrooij, Paul; Kozhurina-Platais, Vera; Kalirai, Jason S; Platais, Imants; Puzia, Thomas H; Correnti, Matteo; Bressan, Alessandro; Chandar, Rupali; Kerber, Leandro; Marigo, Paola; Rubele, Stefano
2014-01-01

372

Photometric determination of the mass accretion rates of pre-main sequence stars. III. Results in the Large Magellanic Cloud

  CERN Preprints

Summary: We present a multi-wavelength study of three star forming regions, spanning the age range 1-14 Myr, located between the 30 Doradus complex and supernova SN1987A in the Large Magellanic Cloud (LMC). We reliably identify about 1000 pre-main sequence (PMS) star candidates actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (Macc). Our measurements represent the largest Macc dataset of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical \\dot{M} of PMS stars in the LMC is higher than for galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and \\dot{M} estimates, the difference in Macc between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of dis...

Spezzi, L; Panagia, N; Sicilia-Aguilar, A; Ercolano, B
2011-01-01

373

HST WFPC2 Observations of the Peculiar Main Sequence of the Double Star Cluster NGC 2011 in the Large Magellanic Cloud

  Astrophysics (arXiv)

Summary: We report the serendipitous discovery of a peculiar main sequence in archived Hubble Space Telescope WFPC2 observations of the young star cluster NGC 2011 in the Large Magellanic Cloud. The bright part of this main sequence exhibits a prominent double, fork-like feature, as if it consists of twin main sequences, one of them being redder. The color-magnitude diagram, constructed from the stars found in the only available WFPC2 field of the cluster, is used to distinguish the stars according to their membership to each of these sequences and to study their spatial distribution. We find that there are two well distinguished populations in the sense that the redder main sequence is dominated by stars that belong to the main body of the cluster, while the stars of the bluer main sequence belong to the surrounding region. Providing that NGC 2011 is a verified binary cluster, with the second companion unfortunately not observed, and taking into account the general region where this cluster is located, we discuss the possible scenarios from both star formation, and early dynamical evolution point-of-view that might explain this unique discovery.

D. A. Gouliermis; S. Lianou; M. Kontizas; E. Kontizas; A. Dapergolas
2006-11-22

374

ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster

  Astrophysics (arXiv)

Summary: We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this lumino- sity function, using available pre-main sequence tracks and plausible star for- mation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the IMF of the embedded cluster. We estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low- mass index of -0.35+/-0.25, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no evidence for a low-mass cutoff down to at least 0.06 Msun.

S. Bontemps; P. Andre; A. A. Kaas; L. Nordh; G. Olofsson; M. Huldtgren; A. Abergel; J. Blommaert; F. Boulanger; M. Burgdorf; C. J. Cesarsky; D. Cesarsky; E. Copet; J. Davies; E. Falgarone; G. Lagache; T. Montmerle; M. Perault; P. Persi; T. Prusti; J. L. Puget; F. Sibille
2001-05-17

375

Measuring the Mass of a Pre-Main Sequence Binary Star Through the Orbit of TWA 5A

  Astrophysics (arXiv)

Summary: We present the results of a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the W.M. Keck 10 m telescopes. These measurements were taken as part of our ongoing monitoring of pre-main sequence (PMS) binaries in an effort to increase the number of dynamically determined PMS masses and thereby calibrate the theoretical PMS evolutionary tracks. Our observations have allowed us to obtain the first determination of this system's astrometric orbit. We find an orbital period of 5.94 +- 0.09 years and a semi-major axis of 0.066" +- 0.005". Combining these results with a kinematic distance, we calculate a total mass of 0.71 +- 0.14 M_sun (D/44 pc)^3. for this system. This mass measurement, as well as the estimated age of this system, are consistent to within 2$\\sigma$ of all theoretical models considered. In this analysis, we properly account for correlated uncertainties, and show that while these correlations are generally ignored, they increase the formal uncertainties by up to a factor of five and therefore are important to incorporate. With only a few more years of observation, this type of measurement will allow the theoretical models to be distinguished.

Q. M. Konopacky; A. M. Ghez; G. Duchene; C. McCabe; B. A. Macintosh
2007-01-19

376

On the Interpretation of Sub-Giant Branch Morphologies of Intermediate-Age Star Clusters with Extended Main Sequence Turnoffs

  CERN Preprints

Summary: Recent high-quality photometry of many star clusters in the Magellanic Clouds with ages of 1$\\,-\\,$2 Gyr revealed main sequence turnoffs (MSTOs) that are significantly wider than can be accounted for by a simple stellar population (SSP). Such extended MSTOs (eMSTOs) are often interpreted in terms of an age spread of several $10^8$ yr, challenging the traditional view of star clusters as being formed in a single star formation episode. Li et al. and Bastian & Niederhofer recently investigated the sub-giant branches (SGBs) of NGC 1651, NGC 1806, and NGC 1846, three star clusters in the Large Magellanic Cloud (LMC) that exhibit an eMSTO. They argued that the SGB of these star clusters can be explained only by a SSP. We study these and two other similar star clusters in the LMC, using extensive simulations of SSPs including unresolved binaries. We find that the shapes of the cross-SGB profiles of all star clusters in our sample are in fact consistent with their cross-MSTO profiles when the latter are interpre...

Goudfrooij, Paul; Rosenfield, Philip; Bressan, Alessandro; Marigo, Paola; Correnti, Matteo; Puzia, Thomas H
2015-01-01

377

Resolved Near-Infrared Spectroscopy of the Mysterious Pre-Main Sequence Binary Systems T Tau S

  Astrophysics (arXiv)

Summary: We obtained new near-infrared images of the prototypical pre-main sequence triple system T Tau, as well as the first resolved medium-resolution spectra of the close pair T Tau S. At the time of our observations, the tight binary had a 13 AU projected separation and showed significant motion since its discovery, three years before. The orbit cannot be strongly constrained yet, but the observed motion of T Tau Sb with respect to T Tau Sa suggests that the system is at least as massive as T Tau N itself. This may indicate that T Tau N is not the most massive star in the system. The spectrum of T Tau Sa, which is totally featureless except for a strong Br gamma emission line, identifies this component with the ``infrared companion'', whose exact nature remains obscure but may be the consequence of it being the most massive component of the system. Contrasting sharply with T Tau Sa, the spectrum of T Tau Sb shows numerous photospheric features consistent with an early-M spectral type. The presence of a strong Br gamma emission line and of a significant veiling continuum classifies this object as a deeply embedded T Tauri star. From these observations, we conclude that both components of T Tau S are embedded in their own dense circumstellar cocoon of material, which are probably fed by a much more extended structure.

G. Duchene; A. M. Ghez; C. McCabe
2001-12-05

378

Comparison of different exoplanet mass detection limit methods using a sample of main-sequence intermediate-type stars

  CERN Preprints

Summary: The radial velocity (RV) technique is a powerful tool for detecting extrasolar planets and deriving mass detection limits that are useful for constraining planet pulsations and formation models. Detection limit methods must take into account the temporal distribution of power of various origins in the stellar signal. These methods must also be able to be applied to large samples of stellar RV time series We describe new methods for providing detection limits. We compute the detection limits for a sample of ten main sequence stars, which are of G-F-A type, in general active, and/or with detected planets, and various properties. We use them to compare the performances of these methods with those of two other methods used in the litterature. We obtained detection limits in the 2-1000 day period range for ten stars. Two of the proposed methods, based on the correlation between periodograms and the power in the periodogram of the RV time series in specific period ranges, are robust and represent a significant impr...

Meunier, Nadège; De Bondt, Katrien
2012-01-01

379

Jupiter will become a hot Jupiter: Consequences of Post-Main-Sequence Stellar Evolution on Gas Giant Planets

  CERN Preprints

Summary: When the Sun ascends the red giant branch (RGB), its luminosity will increase and all the planets will receive much greater irradiation than they do now. Jupiter, in particular, might end up more highly irradiated than the hot Neptune GJ 436b and, hence, could appropriately be termed a "hot Jupiter." When their stars go through the RGB or asymptotic giant branch (AGB) stages, many of the currently known Jupiter-mass planets in several-AU orbits will receive levels of irradiation comparable to the hot Jupiters, which will transiently increase their atmospheric temperatures to ~1000 K or more. Furthermore, massive planets around post-main-sequence stars could accrete a non-negligible amount of material from the enhanced stellar winds, thereby significantly altering their atmospheric chemistry as well as causing a significant accretion luminosity during the epochs of most intense stellar mass loss. Future generations of infrared observatories might be able to probe the thermal and chemical structure of such hot ...

Spiegel, David S
2012-01-01

380

Post-common envelope binaries from SDSS - XIV. The DR7 white dwarf-main sequence binary catalogue

  CERN Preprints

Summary: We present an updated version of the spectroscopic white dwarf-main sequence (WDMS) binary catalogue from the Sloan Digital Sky Survey (SDSS). 395 new systems are serendipitous discoveries from the spectroscopic SDSSI/II Legacy targets. As part of SEGUE, we have carried out a dedicated and efficient (64 per cent success rate) search for WDMS binaries with a strong contribution of the companion star, which were underrepresented by all previous surveys, identifying 251 additional systems. In total, our catalogue contains 2248 WDMS binaries, and includes, where available, magnitudes from the GALEX All Sky Survey in the ultraviolet and from the UKIRT Infrared Sky Survey (UKIDSS) in the near-infrared. We also provide radial velocities of the companion stars, measured from the SDSS spectroscopy using the NaI8183.27,8194.81 absorption doublet and/or the Halpha emission. Using an updated version of our spectral decomposition/fitting technique we determine/update the white dwarf effective temperatures, surface graviti...

Rebassa-Mansergas, A; Schreiber, M R; Gansicke, B T; Schwope, A; Gallardo, J; Koester, D
2011-01-01

381

The Keele-Exeter young cluster survey: I. Low mass pre-main sequence stars in NGC 2169

  Astrophysics (arXiv)

Summary: We have used R_C I_C CCD photometry from the Isaac Newton telescope and intermediate resolution spectroscopy from the Gemini North telescope to identify and characterise low-mass (0.15main sequence stars in the young open cluster NGC 2169. Isochrone fitting to the high- and low-mass populations yields an intrinsic distance modulus of 10.13{+0.06}{-0.09} mag and a model-dependent age of 9+/-2 Myr. Compared with the nearby, kinematically defined groups of a similar age, NGC 2169 has a large low-mass population which potentially offers a more precise statistical investigation of several aspects of star formation and early stellar evolution. By modelling the distribution of low-mass stars in the I_C versus R_C-I_C diagram we find that any age spread among cluster members has a Gaussian full width at half maximum <=2.5 Myr. A young age and small age spread (<10 Myr) are supported by the lack of significant lithium depletion in the vast majority of cluster members. There is no clear evidence for accretion or warm circumstellar dust in the low-mass members of NGC 2169, bolstering the idea that strong accretion has ceased and inner discs have dispersed in almost all low-mass stars by ages of 10 Myr.

R. D Jeffries; J. M. Oliveira; Tim Naylor; N. J. Mayne; S. P. Littlefair
2006-11-20

382

PS1-10jh: The Disruption of a Main-Sequence Star of Near-Solar Composition

  CERN Preprints

Summary: When a star comes within a critical distance to a supermassive black hole (SMBH), immense tidal forces can remove a significant fraction of the star's mass, resulting in a stream of debris that falls back onto the black hole and powers a luminous flare. In this paper, we perform two hydrodynamical simulations of the disruption of a main-sequence star by a SMBH to characterize the evolution of the debris stream after a tidal disruption. We demonstrate that this debris stream is confined by self-gravity in the two directions perpendicular to the original direction of the star's travel, restricting its width and height to be only a factor of a few larger than its original size. As a consequence, the stream has a negligible surface area and makes almost no contribution to either the continuum or line emission. We propose that any observed emission lines are not the result of photoionization in the unbound debris, but are produced in the region above and below the forming elliptical accretion disk, the same region...

Guillochon, James; Ramirez-Ruiz, Enrico
2013-01-01

383

An internet server for update pre-main sequence tracks of low- and intermediate-mass stars

  Astrophysics (arXiv)

Summary: We present new grids of pre-main sequence (PMS) tracks for stars in the mass range 0.1 to 7.0 Msun. The computations were performed for four different metallicities (Z=0.01, 0.02, 0.03 and 0.04). A fifth table has been computed for the solar composition (Z=0.02), including a moderate overshooting. We describe the update in the physics of the Grenoble stellar evolution code which concerns mostly changes in the equation of state (EOS) adopting the formalism proposed by Pols et al. (1995) and in the treatment of the boundary condition. Comparisons of our models with other grids demonstrate the validity of this EOS in the domain of very low-mass stars. Finally, we present a new server dedicated to PMS stellar evolution which allows the determination of stellar parameters from observational data, the calculation of isochrones, the retrieval of evolutionary files and the possibility to generate graphic outputs. WWW site : http://www-laog.obs.ujf-grenoble.fr/activites/starevol/evol.html

L. Siess; E. Dufour; M. Forestini
2000-03-31

384

Variations of the ISM Conditions Across the Main Sequence of Star-Forming Galaxies: Observations and Simulations

  CERN Preprints

Summary: (abridged) Significant evidence has been gathered suggesting the existence of a main sequence (MS) of star-forming galaxies that relates their star formation rate and their stellar mass: $SFR \\propto M_*^{\\alpha}$. Several ideas have been suggested to explain fundamental properties of the MS, such as its slope, its dispersion, and its evolution with redshift. However, no consensus has been reached regarding its true nature, or whether the membership of particular galaxies to this MS implies the existence of two different modes of star formation. In order to advance our understanding of the MS, here we use a statistically robust Bayesian Spectral Energy Distribution (SED) analysis (CHIBURST) to consistently analyze the star-forming properties of a set of hydro-dynamical simulations of mergers, as well as observations of real mergers and luminous galaxies, both local and at intermediate redshift. We find a very tight correlation between the specific star formation rate (sSFR) of our fitted galaxies, and the typ...

Martínez-Galarza, Juan R; Lanz, Lauranne; Hayward, Christopher C; Zezas, Andreas; Rosenthal, Lee; Weiner, Aaron; Hung, Chao-Ling; Ashby, Matthew L N; Groves, Brent
2014-01-01

385

On the magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters

  CERN Preprints

Summary: We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [$\\alpha$/Fe], employing photometric data collected with the Advanced Camera for Survey on board the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observations, the theoretical red giant branch bump magnitudes being too bright by on average $\\sim0.2$~mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated to the determination of the horizontal branch level from colour magnitude diagrams, and to uncertainties in the luminosity of horizontal branch stellar m...

Cassisi, Santi; Salaris, Maurizio; Aparicio, Antonio; Monelli, Matteo; Pietrinferni, Adriano
2010-01-01

386

The Star-Formation Main Sequence: The Dependence of Specific Star Formation Rate and Its Dispersion on Galaxy Stellar Mass

  CERN Preprints

Summary: The dispersion of the star-formation main sequence (SFMS) reflects the diversity of star formation histories and variation in star formation rates (SFRs) in star-forming galaxies (SFGs) with similar stellar masses ($M^\\ast$). We examine the dispersion of local SFMS using a complete sample of Sloan Digital Sky Survey galaxies at 0.01$$8.8. The SFRs are estimated from H$\\alpha$ in combination with 22$\\mu m$ observation from WISE. The catalog of bulge+disk decomposition from Simard et al. (2011) is available for the sample galaxies. We measure the dispersion of specific SFR (SSFR) as a function of $M^*$. We confirm that the dispersion increases with $M^*$ from 0.37$\\pm0.01$dex at $\\log(M^\\ast/M_\\odot)$10.2. Despite star formation is mostly associated with disks, the dispersion of disk SSFR still increases with $M^*$. We conclude that the presence of bulges/bars is likely responsible for the large dispersion of SSFR in massive SFGs while low-mass SFGs are mostly disk-dominated and thus with small dispersion. Our ...

Guo, Kexin; Wang, Tao; Fu, Hai
2015-01-01

387

On the origin of the correlations between the accretion luminosity and emission line luminosities in pre-main sequence stars

  CERN Preprints

Summary: Correlations between the accretion luminosity and emission line luminosities (L_acc and L_line) of pre-main sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) L_acc-L_line correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L_star) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the L_acc-L_line correlations depend on the L_acc-L_star relationship. We conclude that because PMS stars show the L_acc-L_star correlation immediately implies that L_acc also correlates with the luminosity of all emission lines, for which the L_acc-L_line correlations alone do not prove any phy...

Mendigutía, I; Rigliaco, E; Fairlamb, J R; Calvet, N; Muzerolle, J; Cunningham, N; Lumsden, S L
2015-01-01

388

ALMA and Herschel reveal that AGN and main-sequence galaxies have different star formation rate distributions

  CERN Preprints

Summary: Using deep Herschel and ALMA observations, we investigate the star formation rate (SFR) distributions of X-ray AGN host galaxies at 0.5main-sequence", or MS) galaxies. We find 34-55 per cent of AGNs have SFRs at least a factor of two below that of the average MS galaxy, compared to ~15 per cent of all MS galaxies, suggesting significantly different SFR distributions. Indeed, when both are modelled as log-normal distributions, the mass and redshift-normalised SFR distributions of AGNs are roughly twice as broad, and peak ~0.4 dex lower, than that of MS galaxies. However, like MS galaxies, the normalised SFR distribution of AGNs appears not to evolve with redshift. Despite AGNs and MS galaxies having different SFR distributions, the linear-mean SFR of AGNs derived from our distributions is remarkably consistent with that of MS galaxies, and thus with previous results derived from stacked Herschel data. This apparent contradic...

Mullaney, J R; Aird, J; Bernhard, E; Daddi, E; Del Moro, A; Dickinson, M; Elbaz, D; Harrison, C M; Juneau, S; Liu, D; Pannella, M; Rosario, D; Santini, P; Sargent, M; Schreiber, C; Simpson, J; Stanley, F
2015-01-01

389

White-dwarf+main-sequence binaries identified from the tenth data release of Solan Digital Sky Survey

  CERN Preprints

Summary: We have presented 309 new white-dwarf (WD) + main-sequence (MS) star binaries identified from the Sloan Digital Sky Survey (SDSS) Tenth Data Release (DR10). The majority of them consist of a white dwarf and a low-mass secondary (typically M dwarf) companion. The SDSS spectra of the newly found WDMS binaries with a DA/DB white dwarf and an M/late-K dwarf companion are analyzed based on a spectral decomposition/fitting method. White dwarf effective temperatures, surface gravities and masses together with the secondary star spectral types are obtained, and the stellar parameters of DA WDs with $T_{\\rm eff}\\la 14,000$ K are revised to the results in the case of 3D model atmosphere. Two independent distance estimates are derived from the flux-scaling factors between the WDMS SDSS spectra and the white dwarf and M-dwarf model spectra. It is found that about more than 20 per cent of the newly found WDMS binaries show a significant discrepancy between the two distance estimates. This might be caused by the effects of...

Li, Lifang; Zhang, Fenghui; Han, Quanwang; Kong, Xiaoyang
2014-01-01

390

Pre-main sequence accretion in the low metallicity Galactic star-forming region Sh 2-284

  CERN Preprints

Summary: We present optical spectra of pre-main sequence (PMS) candidates around the H$\\alpha$ region taken with the Southern African Large Telescope, SALT, in the low metallicity ($Z$) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of $Z$ $\\sim$ 1/5 $Z_{\\odot}$. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, $\\dot M_{\\rm{acc}}$, are a function of $Z$. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-$Z$ star-forming region. Our data-set was enlarged with literature data of H$\\alpha$ emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 $M_{\\odot}$ and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find $\\dot M_{\\rm{acc}}$ in the 1 - 2 $M_{\\odot}$ interval to d...

Kalari, V M
2014-01-01

391

Long-term evolution of three-planet systems to the post-Main Sequence and beyond

  CERN Preprints

Summary: We study the stability of systems of three giant planets orbiting 3 - 8 M_Sol stars at orbital distances of >10 au as the host star ages through the Main Sequence (MS) and well into the White Dwarf (WD) stage. Systems are stable on the MS if the planets are separated by more than ~9 Hill radii. Most systems surviving the MS will remain stable until the WD phase, although planets scattered onto small pericentres in unstable systems can be swallowed by the expanding stellar envelope when the star ascends the giant branches. Mass loss at the end of the asymptotic giant branch triggers delayed instability in many systems, leading to instabilities typically occurring at WD cooling ages of a few 100 Myr. This instability occurs both in systems that survived the star's previous evolution unscathed, and in systems that previously underwent scattering instabilities. The outcome of such instability around WDs is overwhelmingly the ejection of one of the planets from the system, with several times more ejections occurri...

Mustill, Alexander; Villaver, Eva
2013-01-01

392

The Solar Neighborhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star

  CERN Preprints

Summary: We present the results of a multi-technique investigation of the M4.5Ve flare star AP Col, which we discover to be the nearest pre-main-sequence star. These include astrometric data from the CTIO 0.9m, from which we derive a proper motion of 342.0+/-0.5 mas yr^-1, a trigonometric parallax of 119.21+/-0.98 mas (8.39+/-0.07 pc), and photometry and photometric variability at optical wavelengths. We also provide spectroscopic data, including radial velocity (22.4+/-0.3 km s^-1), lithium Equivalent Width (EW) (0.28+/-0.02 A), H-alpha EW (-6.0 to -35 A), {\\it vsini} (11+/-1 km s^-1), and gravity indicators from the Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle spectrographs. The combined observations demonstrate that AP Col is the closer of only two known systems within 10 pc of the Sun younger than 100 Myr. Given its space motion and apparent age of 12-50 Myr, AP Col is likely a member of the recently proposed ~40 Myr old Argus/IC 2391 association.

Riedel, Adric R; Henry, Todd J; Melis, Carl; Jao, Wei-Chun; Subasavage, John P; 10.1088/0004-6256/142/4/104
2011-01-01

393

The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence

  Astrophysics (arXiv)

Summary: (Abridged) Projected rotational velocities (vsini) have been measured for a sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun) located in the Orion star-forming complex. These measurements have been supplemented with data from the literature for Orion stars with masses as low as 0.1 Msun. The primary finding from analysis of these data is that the upper envelope of the observed values of angular momentum per unit mass (J/M) varies as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3 Msun. This power law extends smoothly into the domain of more massive stars (3 to 10 Msun), which in Orion are already on the ZAMS. This result stands in sharp contrast to the properties of main sequence stars, which show a break in the power law and a sharp decline in J/M with decreasing mass for stars with M <2 Msun. A second result of our study is that this break is seen already among the PMS stars in our Orion sample that are on radiative tracks, even though these stars are only a few million years old. A comparison of rotation rates seen for stars on either side of the convective-radiative boundary shows that stars do not rotate as solid bodies during the transition from convective to radiative tracks.

S. C. Wolff; S. E. Strom; L. A. Hillenbrand
2003-10-09

394

Photometric determination of the mass accretion rate of pre-main sequence stars. IV. Recent star formation in NGC 602

  CERN Preprints

Summary: We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Halpha bands. We have identified about 300 pre-main sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognise at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100 arcsec north of the centre of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of th...

De Marchi, Guido; Panagia, Nino
2013-01-01

395

Simulator of Galaxy Millimeter/Submillimeter Emission (SIGAME): The [CII]-SFR Relationship of Massive z=2 Main Sequence Galaxies

  CERN Preprints

Summary: We present SIGAME simulations of the [CII] 157.7 {\\mu}m fine structure line emission from cosmological smoothed particle hydrodynamics (SPH) simulations of main sequence galaxies at z = 2. Using sub-grid physics prescriptions the gas in our galaxy simulations is modelled as a multi-phased interstellar medium (ISM) comprised of molecular gas residing in the inner regions of giant molecular clouds, an atomic gas phase associated with photodissociation regions at the surface of the clouds, and a diffuse, fully ionized gas phase. Adopting a density profile of the clouds and taking into account heating by the local FUV radiation field and cosmic rays - both scaled by the local star formation rate density - we calculate the [CII] emission from each of the aforementioned ISM phases using a large velocity gradient approach for each cloud, on resolved and global scales. The [CII] emission peaks in the central (~ 60%) of the emission in this region originates in the molecular gas phase. At larger galactocentric distanc...

Olsen, Karen P; Narayanan, Desika; Thompson, Robert; Toft, Sune; Brinch, Christian
2015-01-01

396

Kepler-4b: Hot Neptune-Like Planet of a G0 Star Near Main-Sequence Turnoff

  CERN Preprints

Summary: Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at RA = 19h02m27.68s, Dec = +50:08:08.7. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 x 10^{-3} and a duration of about 3.95 hours. Radial velocity measurements from the Keck HIRES spectrograph show a reflex Doppler signal of 9.3 (+1.1 -1.9) m/s, consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the radial velocities for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223 (+0.053 -0.091) solar masses and 1.487 (+0.071 -0.084) solar radii. We estimate the planet mass and radius to be 24.5 +/- 3.8 Earth masses and 3.99 +/- 0.21 Earth radi...

Borucki, William J; Brown, Timothy M; Basri, Gibor; Batalha, Natalie; Caldwell, Douglas A; Cochran, William D; Dunham, Edward W; Gautier, Thomas N; Geary, John C; Gilliland, Ronald L; Howell, Steve B; Jenkins, Jon M; Latham, David W; Lissauer, Jack J; Marcy, Geoffrey W; Monet, David; Rowe, Jason F; Sasselov, Dimitar
2010-01-01

397

Evidence for a Significant Intermediate-Age Population in the M31 Halo from Main Sequence Photometry

  Astrophysics (arXiv)

Summary: We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51 arcmin (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at V = 30.7 mag. The CMD, constructed from approximately 300,000 stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as chi-squared fitting to a finely-spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (approximately 30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population, as well as a significant globular-cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo.

Thomas M. Brown; Henry C. Ferguson; Ed Smith; Randy A. Kimble; Allen V. Sweigart; Alvio Renzini; R. Michael Rich; Don A. VandenBerg
2003-05-17

398

Solar Neutrinos: Sensitivity to Pre-Main Sequence Evolution and to the Depth of the Convective Zone

  Astrophysics (arXiv)

Summary: Predicted rates for solar neutrino experiments that are obtained with a modified stellar evolution code originally developed to study the advanced stages of stellar evolution are shown to be in agreement with other recently-calculated precise solar models to about 2% (0.2 SNU for the chlorine experiment). Different scenarios for pre-main sequence evolution are explored and are found to change the predicted rates for solar neutrino experiments by less than or of order 1%. The influence of the depth of the solar convective zone on the predicted solar neutrino fluxes is established by direct calculation. It is shown that a change in the calculated depth of the convective zone that is five times larger than the quoted helioseismological measurement uncertainty determined by Christensen-Dalsgaard, Gough, & Thompson causes a change in the predicted $^8$B neutrino flux of less than 7% and a change in the $^7$Be neutrino flux of less than 4%. Additionally, it is shown that the radiative opacities near the depth of the convective zone cannot differ from the standard OPAL values by more than about 12% without causing the calculated and measured depths of the convective zone to differ by more than five times the helioseismological measurement uncertainty.

John N. Bahcall; Ami Glasner
1994-06-13

399

Asteroseismic Signatures of Helium gradients in Main-Sequence A Stars ; Application to the roAp Star HD60435

  Astrophysics (arXiv)

Summary: Asteroseismology is found to be a excellent tool for detecting diffusion-induced helium gradients inside main-sequence A stars. Models have been computed for 1.6 and 2.0 M$_{\\odot}$ stars with pure helium diffusion, at different ages, so that the helium gradient lies at different depths inside the star. The adiabatic oscillation frequencies have been analysed and compared with those of a model without diffusion. Clear signatures of the diffusion-induced helium gradient are found in the so-called ``second differences" : these frequency differences present modulations due to the partial reflexion of the sound waves on the layer where the helium gradient takes place. A tentative application to the roAp star HD60435, which presents enough detected oscillation frequencies for the test to be possible, is very encouraging. The results suggest the presence of a helium gradient inside the star, which is consistent with the idea that the triggering of the oscillations is due to the hydrogen $\\kappa$-mechanism.

Sylvie Vauclair; Sylvie Theado
2004-06-16

400

How dusty is alpha Centauri? Excess or non-excess over the infrared photospheres of main-sequence stars

  CERN Preprints

Summary: [Abridged] Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby binary aCentauri have higher than solar metallicities, which is thought to promote giant planet formation. We aim to determine the level of emission from debris in the aCen system. Having already detected the temperature minimum, Tmin, of aCenA, we here attempt to do so also for the companion aCenB. Using the aCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. We use Herschel and APEX photometry to determine the stellar spectral energy distributions. In addition, we use APEX for spectral line mapping to study the complex background around aCen seen in the photometric images. Models of stellar atmospheres and discs are used to estimate the amount of debris around these stars. For solar-type stars, a fractional dust luminosity fd 2e-7 could account for SEDs that do not exhibit the Tmin-effect. Slight excesses ...

Wiegert, J; Thébault, P; Olofsson, G; Mora, A; Bryden, G; Marshall, J P; Eiroa, C; Montesinos, B; Ardila, D; Augereau, J C; Aran, A Bayo; Danchi, W C; del Burgo, C; Ertel, S; Fridlund, M C W; Hajigholi, M; Krivov, A V; Pilbratt, G L; Roberge, A; White, G J
2014-01-01

401

Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey

  CERN Preprints

Summary: The Exozodi survey aims to determine the occurrence rate of bright exozodiacal discs around nearby main sequence stars using infrared interferometry. Although the Exozodi survey targets have been carefully selected to avoid the presence of binary stars, the results of this survey can still be biased by the presence of unidentified stellar companions. Using the PIONIER data set collected within the Exozodi survey, we aim to search for the signature of point-like companions around the Exozodi target stars. We use both the closure phases and squared visibilities collected by PIONIER to search for companions within the ~100 mas interferometric field of view. The presence of a companion is assessed by computing the goodness of fit to the data for a series of binary models with various separations and contrasts. Five stellar companions are resolved for the first time around five A-type stars: HD 4150, HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD 16555 was independently resolved by spec...

Marion, Lindsay; Ertel, Steve; Bouquin, Jean-Baptiste Le; Augereau, Jean-Charles; Blind, Nicolas; Defrere, Denis; Lebreton, Jeremy; Milli, Julien
2014-01-01

402

Evolution of Very Massive Population III Stars with Mass Accretion from Pre-Main Sequence to Collapse

  CERN Preprints

Summary: We calculate the evolution of zero-metallicity Population III (Pop III) stars whose mass grows from the initial mass of $\\sim 1M_{\\odot}$ by accreting the surrounding gases. Our calculations cover a whole evolutionary stages from the pre-main sequence, via various nuclear burning stages, through the final core collapse or pair-creation instability phases. We adopt the following stellar mass-dependent accretion rates which are derived from cosmological simulations of early structure formation based on the low mass dark matter halos at redshifts $z \\sim 20$: (1) the accretion rates for the first generation (Pop III.1) stars and (2) the rates for zero-metallicity but the second generation (Pop III.2) stars which are affected by radiation from the Pop III.1 stars. For comparison, we also study the evolution with the mass-dependent accretion rates which are affected by radiatibe feedback. We show that the final mass of Pop III.1 stars can be as large as $\\sim 1000M_{\\odot}$, beyond the mass range ($140 - 300M_{\\od...

Ohkubo, Takuya; Umeda, Hideyuki; Yoshida, Naoki; Tsuruta, Sachiko
2009-01-01

403

Transit detections of extrasolar planets around main-sequence stars - I. A sky map for Hot Jupiters

  CERN Preprints

Summary: The findings of more than 350 extrasolar planets, most of them nontransiting Hot Jupiters, have revealed correlations between the metallicity of the main-sequence (MS) host stars and planetary incidence. This connection can be used to calculate the planet formation probability around other stars, not yet known to have planetary companions. We locate the promising spots for current transit surveys on the celestial plane and strive for absolute values of the expected number of transits in general. We used data of the Tycho catalog for about 1 million objects to locate all the stars with 0m < m_V < 11.5m on the celestial plane. We took several empirical relations between the parameters listed in the Tycho catalog, such as distance to Earth, m_V, and (B-V), and those parameters needed to account for the probability of a star to host an observable, transiting exoplanet. The empirical relations between stellar metallicity and planet occurrence combined with geometrical considerations were used to yield transi...

Heller, René; Antoniadis, John
2009-01-01

404

Decoding the Star-Forming Main Sequence or: How I Learned to Stop Worrying and Love the Central Limit Theorem

  CERN Preprints

Summary: Star-formation rates (SFR) of disk galaxies strongly correlate with stellar mass, with a small dispersion in SSFR at fixed mass, sigma~0.3 dex. With such small scatter this star-formation main sequence (SFMS) has been interpreted as deterministic and fundamental. Here we demonstrate that it is a simple consequence of the central limit theorem. Our derivation begins by approximating in situ stellar mass growth as a stochastic process, much like a random walk (where the expectation of SFR at any time is equal to the SFR at the previous time). We then derive expectation values for median SSFR of star-forming disks and their scatter over time. We generalize the results for stochastic changes in SFR that are not independent of each other but are correlated over time. For unbiased samples of (disk) galaxies, we derive an expectation that should be independent of mass, decline as 1/T, and have a relative scatter that is independent of mass and time. The derived SFMS and its evolution matches published data to z=10 ...

Kelson, Daniel D
2014-01-01

405

On Be Star Candidates and Possible Blue Pre-Main Sequence Objects in the Small Magellanic Cloud

  Astrophysics (arXiv)

Summary: Recently the OGLE experiment has provided accurate light curves and colours for about 2 millions stars in the Small Magellanic Cloud. We have examined this database for its content of Be stars, applying some selection criteria, and we have found a sample of $\\sim$ 1000 candidates. Some of these stars show beautiful light curves with amazing variations never observed in any Galactic variable. We find outbursts in 13% of the sample (Type-1 stars), high and low states in 15%, periodic variations in 7%, and the usual variations seen in Galactic Be stars in 65% of the cases. The Galactic counterparts of Type-1 objects could be the outbursting Be stars found by Hubert & Floquet (1998) after the analysis of Hipparcos photometry. We discuss the possibility that Type-1 stars could correspond to Be stars with accreting white dwarf companions or alternatively, blue pre-main sequence stars surrounded by thermally unstable accretion disks. We provide coordinates and basic photometric information for these stars and some examples of light curves.

R. E. Mennickent; G. Pietrzynski; W. Gieren; O. Szewczyk
2002-06-19

406

Post-common envelope binaries from SDSS - VII: A catalogue of white dwarf-main sequence binaries

  CERN Preprints

Summary: We present a catalogue of 1602 white dwarf-main sequence binaries (WDMS) from the spectroscopic Sloan Digital Sky Survey Data Release 6 (SDSS DR6). Among these we identify 440 as new WDMS binaries. We select WDMS binary candidates from template fitting all 1.27 million DR6 spectra, using combined constraints in both $\\chi^{2}$ and signal-to-noise ratio. In addition, we use Galaxy Evolution Explorer (GALEX) and UKIRT Infrared Sky Survey (UKIDSS) magnitudes to search for objects in which one of the two components dominates the SDSS spectrum. We use a decomposition/fitting technique to measure the effective temperatures, surface gravities, masses and distances to the white dwarfs, as well as the spectral types and distances to the companions in our catalogue. Distributions and density maps obtained from these stellar parameters are then used to study both the general properties and the selection effects of WDMS binaries in SDSS. We also make use of SDSS-GALEX-UKIDSS magnitudes to investigate the distribution of ...

Rebassa-Mansergas, A; Schreiber, M R; Köster, D; Rodríguez-Gil, P
2009-01-01

407

On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

  Astrophysics (arXiv)

Summary: Recent observational studies of intermediate-age star clusters (SCs) in the Large Magellanic Cloud (LMC) have reported that a significant number of these objects show double main-sequence turn-offs (DMSTOs) in their color-magnitude diagrams (CMDs). One plausible explanation for the origin of these DMSTOs is that the SCs are composed of two different stellar populations with age differences of ~ 300 Myr. Based on analytical methods and numerical simulations, we explore a new scenario in which SCs interact and merge with star-forming giant molecular clouds (GMCs) to form new composite SCs with two distinct component populations. In this new scenario, the possible age differences between the two different stellar populations responsible for the DMSTOs are due largely to secondary star formation within GMCs interacting and merging with already-existing SCs in the LMC disk. The total gas masses being converted into new stars (i.e., the second generation of stars) during GMC-SC interaction and merging can be comparable to or larger than the masses of the original SCs (i.e, the first generation of stars) in this scenario. Our simulations show that the spatial distributions of new stars in composite SCs formed from GMC-SC merging are more compact than those of stars initially in the SCs. We discuss both advantages and disadvantages of the new scenario in explaining fundamental properties of SCs with DMSTOs in the LMC and in the Small Magellanic Cloud (SMC). We also discuss the merits of various alternative scenarios for the origin of the DMSTOs.

K. Bekki; A. D. Mackey
2008-12-03

408

Combined stellar structure and atmosphere models for massive stars I. Interior evolution and wind properties on the main sequence

  Astrophysics (arXiv)

Summary: We present the first "combined stellar structure and atmosphere models" (CoStar) for massive stars, which consistently treat the entire mass loosing star from the center out to the asymptotic wind velocity. The models use up-to-date input physics and state-of-the-art techniques to model both the stellar interior and the spherically expanding non--LTE atmosphere including line blanketing. Our models thus yield consistent predictions regarding not only the basic stellar parameters, including abundances, but also theoretical spectra along evolutionary tracks. On the same ground they allow us to study the influence of stellar winds on evolutionary models. In this first paper, we present our method and investigate the wind properties and the interior evolution on the main sequence (MS) at solar metallicity. The wind momentum and energy deposition associated with the MS evolution is given and the adopted wind properties are discussed. From our atmosphere calculations, we also derive theoretical estimates of mass loss driven by radiation pressure. These values are compared with the predictions from recent wind models of the Munich group. We find an overall agreement with most of their results. In addition, our models are better in reproducing the strong wind momentum rates observed in supergiants than those of Puls et al. (1995). A comparison between boundary conditions given by the conventional plane parallel and the new spherically expanding atmosphere approach is made. For the MS evolution the evolutionary tracks and the interior evolution are found to be basically unchanged by the new treatment of the outer layers. Given the small spherical extension of the continuum forming layers in the considered

D. Schaerer; A. de Koter; W. Schmutz; A. Maeder
1995-08-29

409

X-ray Properties of Pre--Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods

  Astrophysics (arXiv)

Summary: We re-analyze all archival Chandra/ACIS observations of the Orion Nebula Cluster (ONC) to study the X-ray properties of a large sample of pre--main-sequence (PMS) stars with optically determined rotation periods. Our goal is to elucidate the origins of X-rays in PMS stars by seeking out connections between the X-rays and the mechanisms most likely driving their production--rotation and accretion. In our sample X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing Lx/Lbol with more rapid rotation, suggesting that these stars are in the "super-saturated" regime of the rotation-activity relationship. However, we also find that stars with optical rotation periods are significantly biased to high Lx. This is not the result of magnitude bias in the optical rotation-period sample but rather to the diminishingly small amplitude of optical variations in stars with low Lx. Evidently, there exists in the ONC a population of stars whose rotation periods are unknown and that possess lower average X-ray luminosities than those of stars with known rotation periods. These stars may sample the linear regime of the rotation-activity relationship. Accretion also manifests itself in X-rays, though in a somewhat counterintuitive fashion: While stars with spectroscopic signatures of accretion show harder X-ray spectra than non-accretors, they show lower X-ray luminosities and no enhancement of X-ray variability. We interpret these findings in terms of a common origin for the X-ray emission observed from both accreting and non-accreting stars, with the X-rays from accreting stars simply being attenuated by magnetospheric accretion columns. This suggests that X-rays from PMS stars have their origins primarily in chromospheres, not accretion.

Keivan G. Stassun; David R. Ardila; Mary Barsony; Gibor Basri; Robert D. Mathieu
2004-03-05

410

Evolutionary models for metal-poor low-mass stars. Lower main sequence of globular clusters and halo field stars

  Astrophysics (arXiv)

Summary: We have performed evolutionary calculations of very-low-mass stars from 0.08 to 0.8 $\\msol$ for different metallicites from [M/H]= -2.0 to -1.0 and we have tabulated the mechanical, thermal and photometric characteristics of these models. The calculations include the most recent interior physics and improved non-grey atmosphere models. The models reproduce the entire main sequences of the globular clusters observed with the Hubble Space Telescope over the afore-mentioned range of metallicity. Comparisons are made in the WFPC2 Flight system including the F555, F606 and F814 filters, and in the standard Johnson-Cousins system. We examine the effects of different physical parameters, mixing-length, $\\alpha$-enriched elements, helium fraction, as well as the accuracy of the photometric transformations of the HST data into standard systems. We derive mass-effective temperature and mass-magnitude relationships and we compare the results with the ones obtained with different grey-like approximations. These latter are shown to yield inaccurate relations, in particular near the hydrogen-burning limit. We derive new hydrogen-burning minimum masses, and the corresponding absolute magnitudes, for the different metallicities. We predict color-magnitude diagrams in the infrared NICMOS filters, to be used for the next generation of the HST observations, providing mass-magnitudes relationships in these colors down to the brown-dwarf limit. We show that the expected signature of the stellar to substellar transition in color-magnitude diagrams is a severe blueshift in the infrared colors, due to the increasing collision-induced absorption of molecular hydrogen with increasing density and decreasing temperature.

Isabelle Baraffe; Gilles Chabrier; France Allard; Peter Hauschildt
1997-04-15

411

A spectral atlas of post-main-sequence stars in omega Centauri: kinematics, evolution, enrichment and interstellar medium

  Astrophysics (arXiv)

Summary: We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, omega Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B-V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre spectrograph at the Anglo Australian Telescope, and cover the approximate range lambda~3840-4940 Angstroem. We measure the radial velocities, effective temperatures, metallicities and surface gravities by fitting ATLAS9 stellar atmosphere models. We analyse the cluster membership and stellar kinematics, interstellar absorption in the Ca II K line at 3933 Angstroem, the RR Lyrae instability strip and the extreme horizontal branch, the metallicity spread and bimodal CN abundance distribution of red giants, nitrogen and s-process enrichment, carbon stars, pulsation-induced Balmer line emission on the asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright stars. Membership is confirmed for the vast majority of stars, and the radial velocities clearly show the rotation of the cluster core. We identify long-period RR Lyrae-type variables with low gravity, and low-amplitude variables coinciding with warm RR Lyrae stars. A barium enhancement in the coolest red giants indicates that 3rd dredge-up operates in AGB stars in omega Cen. This is distinguished from the pre-enrichment by more massive AGB stars, which is also seen in our data. The properties of the AGB, post-AGB and UV-bright stars suggest that RGB mass loss may be less efficient at very low metallicity, [Fe/H]<<-1, increasing the importance of mass loss on the AGB. The catalogue and spectra are made available via CDS.

Jacco Th. van Loon; Floor van Leeuwen; Barry Smalley; Andrew W. Smith; Nicola A. Lyons; Iain McDonald; Martha L. Boyer
2007-09-14

412

Updated Pre-Main Sequence tracks at low metallicities for 0.1< M/Mo<1.5

  Astrophysics (arXiv)

Summary: Young populations at ZMain Sequence (PMS) stars. While it is well known that the mass and age determination of PMS stars is strongly affected by the convection treatment, extending any calibration to metallicities different from solar one is very artificial, in the absence of any calibrators for the convective parameters. For solar abundance, Mixing Lenght Theory models have been calibrated by using the results of 2D radiative-hydrodynamical models (MLTa2D), that result to be very similar to those computed with non-grey ATLAS9 atmosphere boundary condition and full spectrum of turbolence (FST) convection model both in the atmosphere and in the interior (NEMO--FST models). While MLTa2D models are not available for lower metallicities, we extend to lower Z the NEMO--FST models, in the educated guess that in such a way we are simulating also at smaller Z the results of MLTa2D. We present PMS models for low mass stars from 0.1 to 1.5 Mo for metallicities [Fe/H]= -0.5, -1.0 and -2.0. The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models. These evolutionary tracks and isochrones are available in electronic form at a WEB site http://www.mporzio.astro.it/%7Etsa/

Di Criscienzo; P. Ventura; F. D'Antona
2008-12-19

413

The Pisa pre-main sequence tracks and isochrones. A database covering a wide range of Z, Y, mass, and age values

  CERN Preprints

Summary: In recent years new observations of pre-main sequence stars (pre-MS) with Z = 0.008, we also provided models with two different initial deuterium abundances. The characteristics of the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.

Tognelli, E; Degl'Innocenti, S
2011-01-01

414

HST/NICMOS detection of a partially embedded, intermediate-mass pre-main-sequence population in the 30 Doradus Nebula

  Astrophysics (arXiv)

Summary: We present the detection of an intermediate-mass pre-main-sequence population embedded in the nebular filaments surrounding the 30 Doradus region in the Large Magellanic Cloud (LMC) using HST/NICMOS. In addition to four previously known luminous Class I infrared ``protostars,'' the NICMOS data reveal 20 new sources with intrinsic infrared excess similar to Galactic pre-main sequence stars. Based on their infrared brightness, these objects can be identified as the LMC equivalent of Galactic pre-main sequence stars. The faintest LMC Young Stellar Objects in the sample have colors similar to T Tauri and have about the same brightness as T Tauri if placed at the distance of the LMC. We find no evidence for a lower-mass cut-off in the initial mass function. Instead, the whole spectrum of stellar masses from pre-main sequence stars with ~1.5Mo to massive O stars still embedded in dense knots appears to be present in the nebular filaments. The majority of the young stellar objects can be found to the north of the central starburst cluster R136. This region is very likely evolving into an OB association.

Wolfgang Brandner; Eva K. Grebel; Rodolfo H. Barba; Nolan R. Walborn; Andrea Moneti
2001-04-17

415

24e STAR FORMATION V Now let us look in greater detail at the actual observed characteristics of pre-main sequence (PMS)

  Physics Websites

Summary: 1 24e STAR FORMATION V Now let us look in greater detail at the actual observed characteristics of pre-main sequence (PMS) stars. There is a rather big "zoo" of different types of PMS stars, and the classification of these stars has been based mostly observed characteristics such as their appearance

Sitko, Michael L.

416

25 MAIN SEQUENCE AND POST-MS EVOLUTION When the full pp and/or CNO reactions turn on, contraction ceases and the star becomes a stable

  Physics Websites

Summary: , contraction ceases and the star becomes a stable MS star. This cannot continue indefinitely, however, because the very reactions that power the star are altering its chemical composition ­ altering its mean molecular* ! " # $ % & 2.5 years The main sequence lifetime of a one solar mass star is about 10 10 years, and it decreases

Sitko, Michael L.

417

Conclusions We have computed the stellar parameters and their uncertainties for 34 FGK single main sequence Ursa Major MG candidate

  Physics Websites

Summary: sequence Ursa Major MG candidate stars, after that we have obtained the chemical abundances of 20 elements of the Ursa Major Moving Group for kinematically selected FGK stars, based on high-resolution spectroscopic by comparison with a reference star known to be member of the Ursa Major Nucleus (HD 115043, see King et al

Complutense de Madrid, Universidad

418

Gravity modes and mixed modes as probes of stellar cores in main-sequence stars: from solar-like to beta Cep stars

  Astrophysics (arXiv)

Summary: We investigate how the frequencies of gravity modes depend on the detailed properties of the chemical composition gradient that develops near the core of main-sequence stars and, therefore, on the transport processes that are able to modify the \\mu profile in the central regions. We show that in main-sequence models, similarly to the case of white dwarfs, the periods of high-order gravity modes are accurately described by a uniform period spacing superposed to an oscillatory component. The periodicity and amplitude of such component are related, respectively, to the location and sharpness of the \\mu gradient. We briefly discuss and interpret, by means of this simple approximation, the effect of turbulent mixing near the core on the periods of both high-order and low-order g modes, as well as of modes of mixed pressure-gravity character.

A. Miglio; J. Montalban; P. Eggenberger; A. Noels
2007-12-20

419

Nearly-uniform internal rotation of solar-like main-sequence stars revealed by space-based asteroseismology and spectroscopic measurements

  CERN Preprints

Summary: The rotation rates in the deep interior and at the surface of 22 main-sequence stars with masses between $1.0$ and $1.6\\,{\\rm M}_{\\odot}$ are constrained by combining asteroseismological analysis with spectroscopic measurements. The asteroseismic data of each star are taken by the {\\it Kepler} or CoRoT space mission. It is found that the difference between the surface rotation rate and the average rotation rate (excluding the convective core) of most of stars is small enough to suggest that an efficient process of angular momentum transport operates during and/or before the main-sequence stage of stars. If each of the surface convective zone and the underlying radiative zone, for individual stars, is assumed to rotate uniformly, the difference in the rotation rate between the two zones turns out to be no more than a factor of two in most of the stars independently of their ages.

Benomar, O; Shibahashi, H; Ceillier, T; Garcia, R A
2015-01-01

420

MML 53: a new low-mass, pre-main sequence eclipsing binary in the Upper Centarus-Lupus Region discovered by SuperWASP

  CERN Preprints

Summary: We announce the discovery of a new low-mass, pre-main sequence eclipsing binary, MML 53. Previous observations of MML 53 found it to be a pre-main sequence spectroscopic multiple associated with the 15-22 Myr Upper Centaurus Lupus cluster. We identify the object as an eclipsing binary for the first time through the analysis of multiple seasons of time series photometry from the SuperWASP transiting planet survey. Re-analysis of a single archive spectrum shows MML 53 to be a spatially unresolved triple system of young stars which all exhibit significant lithium absorption. Two of the components comprise an eclipsing binary with period, P = 2.097891(6) +- 0.000005 and mass ratio, q~0.8. Here, we present the analysis of the discovery data.

Hebb, L; Aigrain, S; Collier-Cameron, A; Hodgkin, S T; Irwin, J M; Maxted, P F L; Pollacco, D; Street, R A; Wilson, D M; Stassun, K G
2010-01-01

421

Age Dating of Globular Clusters Using UBV(RI) Main-sequence Photometry in the Two-color Diagram: Age of NGC 6397

  Astrophysics (arXiv)

Summary: I propose and apply a method for deriving ages of the metal-poor globular clusters (GCs) in a distance-independent way, which is based on age indicator related to the main sequence in the two-color diagrams with U-B index. Age of the metal-poor GC NGC 6397 has been estimated, using Yale isochrones of Demarque et al. (1996), to be close to 16 Gyr provided the cluster metallicity is near [Fe/H]=-2.0.

Valery V. Kravtsov
2001-06-26

422

The Mass of the Convective Zone in FGK Main Sequence Stars and the Effect of Accreted Planetary Material on Apparent Metallicity Determinations

  CERN Preprints

Summary: The mass of the outer convective zone in FGK main sequence stars decreases dramatically with stellar mass. Therefore, any contamination of a star's atmosphere by accreted planetary material should affect hotter stars much more than cool stars. If recent suggestions that high metal abundances in stars with planets are caused by planetesimal accretion are correct, then metallicity enhancements in earlier-type stars with planets should be very pronounced. No such trend is seen, however.

Pinsonneault, M H; Coffee, M
2001-01-01

423

The Mass of the Convective Zone in FGK Main Sequence Stars and the Effect of Accreted Planetary Material on Apparent Metallicity Determinations

  Astrophysics (arXiv)

Summary: The mass of the outer convective zone in FGK main sequence stars decreases dramatically with stellar mass. Therefore, any contamination of a star's atmosphere by accreted planetary material should affect hotter stars much more than cool stars. If recent suggestions that high metal abundances in stars with planets are caused by planetesimal accretion are correct, then metallicity enhancements in earlier-type stars with planets should be very pronounced. No such trend is seen, however.

M. H. Pinsonneault; D. L. DePoy; M. Coffee
2001-05-15

424

Amplitudes and lifetimes of solar-like oscillations observed by CoRoT* Red-giant versus main-sequence stars

  CERN Preprints

Summary: Context. The advent of space-borne missions such as CoRoT or Kepler providing photometric data has brought new possibilities for asteroseismology across the H-R diagram. Solar-like oscillations are now observed in many stars, including red giants and main- sequence stars. Aims. Based on several hundred identified pulsating red giants, we aim to characterize their oscillation amplitudes and widths. These observables are compared with those of main-sequence stars in order to test trends and scaling laws for these parameters for both main-sequence stars and red giants. Methods. An automated fitting procedure is used to analyze several hundred Fourier spectra. For each star, a modeled spectrum is fitted to the observed oscillation spectrum, and mode parameters are derived. Results. Amplitudes and widths of red-giant solar-like oscillations are estimated for several hundred modes of oscillation. Amplitudes are relatively high (several hundred ppm) and widths relatively small (very few tenths of a {\\mu}Hz). Conclus...

Baudin, F; Belkacem, K; Hekker, S; Morel, T; Samadi, R; Benomar, O; Goupil, M -J; Carrier, F; Ballot, J; Deheuvels, S; De Ridder, J; Hatzes, A P; Kallinger, T; Weiss, W W
2011-01-01

425

Three-dimensional simulations of near-surface convection in main-sequence stars. III. The structure of small-scale magnetic flux concentrations

  CERN Preprints

Summary: The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars. Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity. We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500G. We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced...

Beeck, Benjamin; Cameron, Robert H; Reiners, Ansgar
2015-01-01

426

Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff

  CERN Preprints

Summary: The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the previously observed broad main sequence turnoff region for "contamination" by field stars and (evolved) binary star systems. We find that while these impact the number of objects in this region, none can fully account for the large color spread. Our results therefore solidify the recent finding that stars in the main sequence turnoff region of this cluster have a large spread in color which is unrelated to measurement errors or contamination by field stars, and likely due to a ~300 Myr range in the ages of cluster stars. An unbiased estimate of the stellar density distribution across the main sequence turnoff region shows that the spread is fairly continuous rather than strongly bimodal as suggested previously. We f...

Goudfrooij, Paul; Kozhurina-Platais, Vera; Chandar, Rupali
2009-01-01

427

uvby-Hbeta CCD photometry and membership segregation of the open cluster NGC 2548; Gaps in the Main Sequence of open clusters

  Astrophysics (arXiv)

Summary: Deep CCD photometry in the uvby-Hbeta intermediate-band system is presented for the cluster NGC 2548 (M 48). A complete membership analysis based on astrometric and photometric criteria is applied. The photometric analysis of a selected sample of stars yields a reddening value of E(b-y)=0.06\\pm0.03, a distance modulus of V_0-M_V=9.3\\pm0.5 (725 pc) and a metallicity of [Fe/H]= -0.24\\pm0.27. Through isochrone fitting we find an age of log t = 8.6\\pm0.1 (400 Myr). Our optical photometry and JHK from 2MASS are combined to derive effective temperatures of cluster member stars. The effective temperature distribution along the main sequence of the cluster shows several gaps. A test to study the significance of these gaps in the main sequence of the HR diagram has been applied. The method is also applied to several other open clusters (Pleiades, Hyades, NGC 1817 and M 67) to construct a sequence of metallicities and ages. The comparison of the results of each cluster gives four gaps with high significance (one of them, centred at 4900 K, has not been previously reported).

L. Balaguer-Nunez; C. Jordi; D. Galadi-Enriquez
2005-04-06

428

Evolution of liquid crystal microstructure during shape memory sequence in two main-chain polydomain smectic-C elastomers

  Condensed Matter (arXiv)

Summary: Structural studies by synchrotron x-ray diffraction on two main-chain smectic-C elastomers reveal the presence of two different relaxation mechanisms in these systems at low and high strains. At low strains, the smectic layers are reoriented with layer-normals distributed in a plane perpendicular to the stretch direction. The system relaxes relatively slowly (time-constant ~ 45 minutes) which is attributed to the flow properties of the LC layers embedded in the elastomer network. At high strains, the equilibration time (~ 4 - 8 minutes) conforms to a faster relaxation and appears to have its origin in the polymer components of the system. Due to misaligned microdomains at small strains, the value of global orientational order parameter S for the mesogenic parts is initially small (~ 0.15). With increasing strain, the local domain-directors, the mesogens, and the polymer chains, all tend to align parallel to the stretch direction giving rise to a higher measured value of S ~ 0.83 at a strain of 4.0. The siloxane segments remain less ordered, attaining a value of S ~ 0.4 for a strain of 4.0. The layers gradually become oblique to the stretch direction conforming to the structural property of the smectic-C phase. The system finally assumes a chevron-like optically monodomain structure. Both elastomers are locked-in this state even after removal of the external stress giving rise to strain retention and the shape memory effect. The presence of a transverse component in the main-chain leads to higher strain retention in the second elastomer. A preference for the orientation of the smectic layer-normals toward the stretch direction persists after removal of external stress. Upon thermal annealing, the chevron-like microstructure gradually melts via a different path to the initial polydomain structure.

Sonal Dey
2014-09-18

429

Mon. Not. R. Astron. Soc. 000, 18 (2014) Printed 18 September 2014 (MN LATEX style file v2.2) A probable pre-main sequence chemically peculiar star in the open

  Physics Websites

Summary: .2) A probable pre-main sequence chemically peculiar star in the open cluster Stock 16 M. Netopil1 , L. Fossati2 likely a member of the young Stock 16 open cluster (age 3­8Myr). The probable cluster membership ­ stars: pre-main-sequence ­ open clusters and associations: individual: Stock 16 1 INTRODUCTION


2014-01-01

430

Mon. Not. R. Astron. Soc. 000, 114 (2009) Printed 3 August 2009 (MN LATEX style file v2.2) The main-sequence rotation-colour relation in the Coma

  Physics Websites

Summary: .2) The main-sequence rotation-colour relation in the Coma Berenices open cluster A. Collier Cameron1 , V. A We present the results of a photometric survey of rotation rates in the Coma Berenices (Melotte 111. The goal of the Coma survey was to measure precise rotation periods for main-sequence F, G and K dwarfs

Norton, Andrew J.
2009-01-01

431

Investigation of the magnetic field characteristics of Herbig Ae/Be stars: Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars

  Astrophysics (arXiv)

Summary: We are investigating the magnetic characteristics of pre-main sequence Herbig Ae/Be stars, with the aim of (1) understanding the origin and evolution of magnetism in intermediate-mass stars, and (2) exploring the influence of magnetic fields on accretion, rotation and mass-loss at the early stages of evolution of A, B and O stars. We have begun by conducting 2 large surveys of Herbig Ae/Be stars, searching for direct evidence of photospheric magnetic fields via the longitudinal Zeeman effect. From observations obtained using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, we report the confirmed detection of magnetic fields in 4 pre-main sequence A- and B-type stars, and the apparent (but as yet unconfirmed) detection of fields in 2 other such stars. We do not confirm the detection of magnetic fields in several stars reported by other authors to be magnetic: HD 139614, HD 144432 or HD 31649. One of the most evolved stars in the detected sample, HD 72106A, shows clear evidence of strong photospheric chemical peculiarity, whereas many of the other (less evolved) stars do not. The magnetic fields that we detect appear to have surface intensities of order 1 kG, seem to be structured on global scales, and appear in about 10% of the stars studied. Based on these properties, these magnetic stars appear to be pre-main sequence progenitors of the magnetic Ap/Bp stars.

G. A. Wade; D. Drouin; S. Bagnulo; J. D. Landstreet; E. Mason; J. Silvester; E. Alecian; T. Bohm; J. -C. Bouret; C. Catala; J. -F. Donati; C. Folsom; K. Bale
2006-01-30

432

X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars

  Astrophysics (arXiv)

Summary: A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075 sources (Feigelson et al. 2002), providing a uniquely large and well-defined sample to study the dependence of magnetic activity on bulk properties for stars descending the Hayashi tracks. The following results are obtained: (1) X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with bolometric luminosity with = -3.8 for stars with masses 0.7main sequence saturation level; (2) the X-ray emission drops rapidly below this level in some or all stars with 2main sequence stars. The absence of a strong X-ray/rotation relationship in PMS stars, and particularly the high X-ray values seen in some very slowly rotating stars, is a clear indication that the mechanisms of magnetic field generation differ from those operating in main sequence stars. The most promising possibility is a turbulent dynamo distributed throughout the deep convection zone, but other models such as alpha-Omega dynamo with `supersaturation' or relic core fields are not immediately excluded. The drop in magnetic activity in intermediate-mass stars may reflect the presence of a significant radiative core. The evidence does not support X-ray production in large-scale star-disk magnetic fields.

Eric D. Feigelson; James A. Gaffney III; Gordon Garmire; Lynne A. Hillenbrand; Leisa Townsley
2002-11-04

433

The origin of the double main sequence in Omega Centauri: Helium enrichment due to gas fueling from its ancient host galaxy ?

  Astrophysics (arXiv)

Summary: Recent observational studies of $\\omega$ Centauri by {\\it Hubble Space Telescope} have discovered a double main sequence in the color magnitude diagrams (CMDs) of its stellar populations. The stellar population with the blue main sequence (bMS) is observationally suggested to have a helium abundance much larger, by $\\Delta Y\\sim 0.12$, than that of the red main sequence (rMS). By using somewhat idealized models in which stars of the bMS are formed from gas ejected from those of the rMS, we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars, from mass loss of very massive young stars, or from type II supernovae within $\\omega$ Cen. We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS, none of the above three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs. The common, serious problem in all cases is that the observed number fraction of the bMS can not be explained without assuming unusually top-heavy IMFs. This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources. We thus suggest a new scenario that most of the second generation of stars (i.e., the bMS) in $\\omega$ Cen could be formed from gas ejected from field stellar populations that surrounded $\\omega$ Cen when it was a nucleus of an ancient dwarf galaxy.

Kenji Bekki; John E. Norris
2005-12-15

434

Massive-Star Forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post Main-Sequence Triggered Star Formation?

  Astrophysics (arXiv)

Summary: We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least eleven young stellar objects (YSOs) embedded in a ring-like diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of class II YSOs in [3.6]-[5.8] but significantly redder in [8]-[24], i.e., 0<[3.6]-[5.8]<1.2 and 5<[8]-[24]<9. Most of them have near-infrared counterparts in the 2MASS JHKs images, and some of them have an optical counterpart too. Their JHKs colors and magnitudes indicate that the YSOs are massive (<= 10 Msun) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to eraly Herbig Be stars, which are early B-type pre-main-sequence stars with inner disks that have been destroyed. The confinement to a loop structure indicates that the YSOs are young, i.e., <= 2 Myr. We propose that their formation is triggered by the progenitor star of G54.1+0.3, which has a mass of <= 15 Msun. The triggering must have occurred near the end of the progenitor's life, possibly after it had evolved off the main sequence.

Bon-Chul Koo; Christopher F. McKee; Jae-Joon Lee; Ho-Gyu Lee; Jeong-Eun Lee; Dae-Sik Moon; Seung Soo Hong; Hidehiro Kaneda; Takashi Onaka
2008-01-02

435

Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence in M71: I. Sample Selection, Observing Strategy and Stellar Parameters

  Astrophysics (arXiv)

Summary: We present the sample for an abundance analysis of 25 members of M71 with luminosities ranging from the red giant branch tip to the upper main sequence. The spectra are of high dispersion and of high precision. We describe the observing strategy and determine the stellar parameters for the sample stars using both broad band colors and fits of H$\\alpha$ profiles. The derived stellar parameters agree with those from the Yale$^2$ stellar evolutionary tracks to within 50 -- 100K for a fixed log g, which is within the level of the uncertainties.

Judith G. Cohen; Bradford B. Behr; Michael M. Briley
2001-05-22

436

ISOCAM observations of the rho Ophiuchi cloud Luminosity and mass functions of the pre-main sequence embedded cluster

  CERN Preprints

Summary: We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this ...

Bontemps, S; Kaas, A A; Nordh, L H; Olofsson, G; Huldtgren, M; Abergel, A; Blommaert, J; Boulanger, F; Burgdorf, M J; Césarsky, C J; Cesarsky, D; Copet, E; Davies, J; Falgarone, E; Lagache, G; Montmerle, T; Perault, M; Persi, P; Prusti, T; Puget, J L; Sibille, F
2001-01-01

437

The Discovery of a Population of Pre-Main Sequence Stars in N66/NGC346 from Deep HST/ACS Image

  Astrophysics (arXiv)

Summary: We report the discovery of a rich population of low mass stars in the young, massive star forming region N66/NGC346 in the Small Magellanic Cloud, from deep V, I and H alpha images taken with the HST/ACS. These stars have likely formed together with the NGC346 cluster, ~3-5 Myr ago. Their magnitude and colors are those of pre-main sequence stars in the mass range 0.6-3 Mo, mostly concentrated in the main cluster, but with secondary subclusters spread over a region across ~45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground based and ISO observations. We show that N66/NGC346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step towards the understanding of how star formation occurred and has progressed in this low metallicity environment.

A. Nota; M. Sirianni; E. Sabbi; M. Tosi; M. Clampin; J. Gallagher; M. Meixner; S. Oey; A. Pasquali; L. J. Smith; R. Walterbos; J. Mack
2006-02-09

438

The clustered nature of star formation. Pre--main-sequence clusters in the star-forming region NGC 602/N90 in the Small Magellanic Cloud

  CERN Preprints

Summary: Located at the tip of the wing of the Small Magellanic Cloud (SMC), the star-forming region NGC602/N90 is characterized by the HII nebular ring N90 and the young cluster of pre--main-sequence (PMS) and early-type main sequence stars NGC602. We present a thorough cluster analysis of the stellar sample identified with HST/ACS camera in the region. We show that apart from the central cluster, low-mass PMS stars are congregated in thirteen additional small compact sub-clusters at the periphery of NGC602. We find that the spatial distribution of the PMS stars is bimodal, with an unusually large fraction (~60%) of the total population being clustered, while the remaining is diffusely distributed in the inter-cluster area. From the corresponding color-magnitude diagrams we disentangle an age-difference of ~2.5Myr between NGC602 and the compact sub-clusters which appear younger. The diffuse PMS population appears to host stars as old as those in NGC602. Almost all detected PMS sub-clusters appear to be centrally conc...

Gouliermis, Dimitrios A; Dolphin, Andrew E; Gennaro, Mario; Tognelli, Emanuele; Moroni, Pier Giorgio Prada
2012-01-01

439

Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348

  Astrophysics (arXiv)

Summary: We present time series observations of intermediate mass PMS stars belonging to the young star cluster IC 348. The new data reveal that a young member of the cluster, H254, undergoes periodic light variations with delta Scuti-like characteristics. This occurrence provides an unambiguous evidence confirming the prediction that intermediate-mass pre-main sequence (PMS) stars should experience this transient instability during their approach to the main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able to constrain the intrinsic stellar parameters of H254 by means of linear, non adiabatic, radial pulsation models. The range of the resulting luminosity and effective temperature permitted by the models is narrower than the observational values. In particular, the pulsation analysis allows to derive an independent estimate of the distance to IC 348 of about 320 pc. Further observations could either confirm the monoperiodic nature of H254 or reveal the presence of other frequencies.

V. Ripepi; F. Palla; M. Marconi; S. Bernabei; A. Arellano Ferro; L. Terranegra; J. M. Alcalá
2002-06-07

440

Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region

  Astrophysics (arXiv)

Summary: We report the discovery of a pre-main-sequence, low-mass, double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We present our observations including radial velocities derived from optical high-resolution spectroscopy, and present an orbit solution that permits the determination of precise empirical masses for both components of the system. We measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03 Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the system exhibiting strong Li lines and a center-of-mass velocity consistent with cluster membership, that this system is a member of the Orion star-forming region and quite possibly the Orion Nebula Cluster, and therefore has an age of only a few million years. As there are currently only a few empirical mass and radius measurements for low-mass, PMS stars, this system presents an interesting test for the predictions of current theoretical models of pre-main sequence stellar evolution.

P. A. Cargile; K. G. Stassun; R. D. Mathieu
2008-02-08

441

Massive-Star Forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post Main-Sequence Triggered Star Formation?

  CERN Preprints

Summary: We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least eleven young stellar objects (YSOs) embedded in a ring-like diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of class II YSOs in [3.6]-[5.8] but significantly redder in [8]-[24], i.e., 0<[3.6]-[5.8]<1.2 and 5<[8]-[24]<9. Most of them have near-infrared counterparts in the 2MASS JHKs images, and some of them have an optical counterpart too. Their JHKs colors and magnitudes indicate that the YSOs are massive (<= 10 Msun) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to eraly Herbig Be stars, which are early B-type pre-main-sequence ...

Koo, Bon-Chul; Lee, Jae-Joon; Lee, Ho-Gyu; Lee, Jeong-Eun; Moon, Dae-Sik; Hong, Seung Soo; Kaneda, Hidehiro; Onaka, Takashi
2007-01-01

442

Detection of the Main Sequence Turn-off of a Newly Discovered Milky Way Halo Structure in the Triangulum-Andromeda Region

  Astrophysics (arXiv)

Summary: An upper main sequence (MS) and main-sequence turn-off (MSTO) feature appears in the color-magnitude diagram (CMD) of a large area photometric survey of the southern half of M31 stretching to M33. Imaging in the Washington M,T_2,DDO51 photometric system allows us to remove the background M31/M33 giants from our CMD and more clearly define the dwarf star feature, which has an MSTO near M ~ 20.5. The corresponding stellar population shows little density variation over the 12 X 6 square degree area of the sky sampled and is of very low surface brightness, >32 mag/arcsec^2. We show that this feature is not the same as a previously identified, MS+MSTO in the foreground of the Andromeda Galaxy that has been associated with the tidal stream ringing the Milky Way disk at less than half the distance. Thus, the new stellar system is a separate, more distant entity, perhaps a segment of tidal debris from a disrupted satellite galaxy. It is most likely related to the structure with similar distance, location and density uniformity seen as an excess of K and M giants in the Two Micron All-Sky Survey reported in the companion paper by Rocha-Pinto et al. (2004).

Steven R. Majewski; James C. Ostheimer; Helio J. Rocha-Pinto; Richard J. Patterson; Puragra Guhathakurta; David Reitzel
2004-06-08

443

Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

  Astrophysics (arXiv)

Summary: There is no straightforward explanation for intrinsic X-ray emission from intermediate-mass main-sequence stars. Therefore the observed emission is often interpreted in terms of (hypothesized) late-type magnetically active companion stars. We use Chandra imaging observations to spatially resolve in X-rays a sample of main-sequence B-type stars with recently discovered companions at arcsecond separation. We find that all spatially resolved companions are X-ray emitters, but seven out of eleven intermediate-mass stars are also X-ray sources. If this emission is interpreted in terms of additional sub-arcsecond or spectroscopic companions, this implies a high multiplicity of B-type stars. Firm results on B star multiplicity pending, the alternative, that B stars produce intrinsic X-rays, can not be discarded. The appropriate scenario in this vein is might be a magnetically confined wind, as suggested for the X-ray emission of the magnetic Ap star IQ Aur. However, the only Ap star in the Chandra sample is not detected in X-rays, and therefore does not support this picture.

B. Stelzer; N. Huelamo; G. Micela; S. Hubrig
2006-02-23

444

A Turnover in the Galaxy Main Sequence of Star Formation at $M_{*} \\sim 10^{10} M_{\\odot}$ for Redshifts $z < 1.3$

  CERN Preprints

Summary: The relationship between galaxy star formation rates (SFR) and stellar masses ($M_\\ast$) is re-examined using a mass-selected sample of $\\sim$62,000 star-forming galaxies at $z \\le 1.3$ in the COSMOS 2-deg$^2$ field. Using new far-infrared photometry from $Herschel$-PACS and SPIRE and $Spitzer$-MIPS 24 $\\mu$m, along with derived infrared luminosities from the NRK method based on galaxies' locations in the restframe color-color diagram $(NUV - r)$ vs. $(r - K)$, we are able to more accurately determine total SFRs for our complete sample. At all redshifts, the relationship between median $SFR$ and $M_\\ast$ follows a power-law at low stellar masses, and flattens to nearly constant SFR at high stellar masses. We describe a new parameterization that provides the best fit to the main sequence and characterizes the low mass power-law slope, turnover mass, and overall scaling. The turnover in the main sequence occurs at a characteristic mass of about $M_{0} \\sim 10^{10} M_{\\odot}$ at all redshifts. The low mass power...

Lee, Nicholas; Casey, Caitlin M; Toft, Sune; Scoville, N Z; Hung, Chao-Ling; Floc'h, Emeric Le; Ilbert, Olivier; Zahid, H Jabran; Aussel, Herve; Capak, Peter; Kartaltepe, Jeyhan S; Kewley, Lisa J; Li, Yanxia; Schawinski, Kevin; Sheth, Kartik; Xiao, Quanbao
2015-01-01

445

Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS1 at the VLT

  Astrophysics (arXiv)

Summary: To properly understand the physics of Ap and Bp stars it is particularly important to identify the origin of their magnetic fields. For that, an accurate knowledge of the evolutionary state of stars that have a measured magnetic field is an important diagnostic. Previous results based on a small and possibly biased sample suggest that the distribution of magnetic stars with mass below 3 M_sun in the H-R diagram differs from that of normal stars in the same mass range (Hubrig et al. 2000). In contrast, higher mass magnetic Bp stars may well occupy the whole main-sequence width (Hubrig, Schoeller & North 2005). In order to rediscuss the evolutionary state of upper main sequence magnetic stars, we define a larger and bias-free sample of Ap and Bp stars with accurate Hipparcos parallaxes and reliably determined longitudinal magnetic fields. We used FORS1 at the VLT in its spectropolarimetric mode to measure the magnetic field in chemically peculiar stars where it was unknown or poorly known as yet. In this first paper we present our results of the mean longitudinal magnetic field measurements in 136 stars. Our sample consists of 105 Ap and Bp stars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPB stars. A magnetic field was for the first time detected in 57 Ap and Bp stars, in four HgMn stars, one PGa star, one normal B-type star and four SPB stars.

S. Hubrig; P. North; M. Schoeller; G. Mathys
2006-03-22

446

The Monitor project: JW 380 -- a 0.26, 0.15 Msol pre main sequence eclipsing binary in the Orion Nebula Cluster

  Astrophysics (arXiv)

Summary: We report the discovery of a low-mass (0.26 +/- 0.02, 0.15 +/- 0.01 Msol) pre-main-sequence eclipsing binary with a 5.3 day orbital period. JW 380 was detected as part of a high-cadence time-resolved photometric survey (the Monitor project) using the 2.5m Isaac Newton Telescope and Wide Field Camera for a survey of a single field in the Orion Nebula Cluster (ONC) region in V and i bands. The star is assigned a 99 per cent membership probability from proper motion measurements, and radial velocity observations indicate a systemic velocity within 1 sigma of that of the ONC. Modelling of the combined light and radial velocity curves of the system gave stellar radii of 1.19 +0.04 -0.18 Rsol and 0.90 +0.17 -0.03 Rsol for the primary and secondary, with a significant third light contribution which is also visible as a third peak in the cross-correlation functions used to derive radial velocities. The masses and radii appear to be consistent with stellar models for 2-3 Myr age from several authors, within the present observational errors. These observations probe an important region of mass-radius parameter space, where there are currently only a handful of known pre-main-sequence eclipsing binary systems with precise measurements available in the literature.

Jonathan Irwin; Suzanne Aigrain; Simon Hodgkin; Keivan G. Stassun; Leslie Hebb; Mike Irwin; Estelle Moraux; Jerome Bouvier; Aude Alapini; Richard Alexander; D. M. Bramich; Jon Holtzman; Eduardo L. Martin; Mark J. McCaughrean; Frederic Pont; P. E. Verrier; Maria Rosa Zapatero Osorio
2007-06-15

447

Three-dimensional simulations of near-surface convection in main-sequence stars - II. Properties of granulation and spectral lines

  CERN Preprints

Summary: The atmospheres of cool main-sequence stars are structured by convective flows from the convective envelope that penetrate the optically thin layers and lead to structuring of the stellar atmospheres analogous to solar granulation. The flows have considerable influence on the 3D structure of temperature and pressure and affect the profiles of spectral lines formed in the photosphere. For the set of six 3D radiative (M)HD simulations of cool main-sequence stars described in the first paper of this series, we analyse the near-surface layers. We aim at describing the properties of granulation of different stars and at quantifying the effects on spectral lines of the thermodynamic structure and flows of 3D convective atmospheres. We detected and tracked granules in brightness images from the simulations to analyse their statistical properties, as well as their evolution and lifetime. We calculated spatially resolved spectral line profiles using the line synthesis code SPINOR. To enable a comparison to stellar obs...

Beeck, Benjamin; Reiners, Ansgar; Schüssler, Manfred
2013-01-01

448

Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

  CERN Preprints

Summary: There is no straightforward explanation for intrinsic X-ray emission from intermediate-mass main-sequence stars. Therefore the observed emission is often interpreted in terms of (hypothesized) late-type magnetically active companion stars. We use Chandra imaging observations to spatially resolve in X-rays a sample of main-sequence B-type stars with recently discovered companions at arcsecond separation. We find that all spatially resolved companions are X-ray emitters, but seven out of eleven intermediate-mass stars are also X-ray sources. If this emission is interpreted in terms of additional sub-arcsecond or spectroscopic companions, this implies a high multiplicity of B-type stars. Firm results on B star multiplicity pending, the alternative, that B stars produce intrinsic X-rays, can not be discarded. The appropriate scenario in this vein is might be a magnetically confined wind, as suggested for the X-ray emission of the magnetic Ap star IQ Aur. However, the only Ap star in the Chandra sample is not dete...

Stelzer, B; Micela, G; Hubrig, S
2006-01-01

449

Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations

  CERN Preprints

Summary: We have measured the present accretion rate of roughly 800 low-mass (~1-1.4 Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental parameter for star formation is determined for low-mass stars outside our Galaxy. The Balmer continuum emission used to derive the accretion rate positively correlates with the Halpha excess. Both these phenomena are believed to originate from accretion from a circumstellar disk so that their simultaneous detection provides an important confirmation of the pre-Main Sequence nature of the Halpha and UV excess objects, which are likely to be the LMC equivalent of Galactic Classical TTauri stars. The stars with statistically significant excesses are measured to have accretion rates larger than 1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover, the oldest TTauri star known in the Milky Way ...

Romaniello, M; Panagia, N; Romaniello, Martino; Robberto, Massimo; Panagia, Nino
2004-01-01

450

Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348

  CERN Preprints

Summary: We present time series observations of intermediate mass PMS stars belonging to the young star cluster IC 348. The new data reveal that a young member of the cluster, H254, undergoes periodic light variations with delta Scuti-like characteristics. This occurrence provides an unambiguous evidence confirming the prediction that intermediate-mass pre-main sequence (PMS) stars should experience this transient instability during their approach to the main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able to constrain the intrinsic stellar parameters of H254 by means of linear, non adiabatic, radial pulsation models. The range of the resulting luminosity and effective temperature permitted by the models is narrower than the observational values. In particular, the pulsation analysis allows to derive an independent estimate of the distance to IC 348 of about 320 pc. Further observations could either confirm the monoperiodic nature of H254 or reveal the presence of other frequencies.

Ripepi, V; Marconi, M; Bernabei, S; Ferro, A A; Terranegra, L; Alcalá, J M
2002-01-01

451

A Canis Major over-density imaging survey. I. Stellar content and star-count maps : A distinctly elongated body of main sequence stars

  Astrophysics (arXiv)

Summary: [Abridged] We present first results from a large-area (~80degx20deg), sparsely sampled two-filter (B,R) imaging survey towards the Canis Major stellar over-density, claimed to be a disrupting Milky Way satellite galaxy. Utilizing stellar colour-magnitude diagrams reaching to B ~ 22 mag, we provide a first delineation of its surface density distribution using main sequence stars. Its projected shape is highly elongated, nearly parallel to the Galactic plane, with an axis ratio of >~ 5:1, substantially more so than what Martin et al. originally found. We also provide a first map of a prominent over-density of blue, presumably younger main sequence stars, which appears to have a maximum near [l,b = 240,-7 deg]. The young population is markedly more localized. We estimate an upper limit on the line-of-sight (l.o.s.) depth of the old population based on the main sequence width, obtaining sigma_los < 1.8 +/- 0.3 kpc, at an adopted D_helio = 7.5 +/- 1 kpc. For the young stellar population, we find sigma_los < 1.5 kpc. There are different explanations for the CMa over-density: (a) a partially disrupting dwarf galaxy on a low-latitude orbit, (b) a projection of the warped outer Galactic disk, and (c) a projection of an out-of-plane spiral arm. While the data provide no firm arguments against the less well-defined third scenario, they have clear implications for each of the others: (a) We infer from the strong elongation in longitude, and simulations in the literature, that the over-density is unlikely to be a gravitationally bound system at the present epoch, but may well be just a recently disrupted satellite remnant. (b) Based on modeling, the line-of-sight depth of the over-density in old stars is clearly inconsistent with published locally axi-symmetric descriptions of the warped Galactic disk.

D. J. Butler; D. Martinez-Delgado; H-W. Rix; J. Penarrubia; J. T. A. de Jong
2006-09-12

452

Revisiting the Blazar Main Sequence

  CERN Preprints

Summary: A discussion of the FSRQ -- BL Lac unification. All distinctive features marking these two Blazar subclasses find an unifying explanation if the sources are powered by central engines constituted by similar Kerr holes, but fueled at high and low accretion rates, respectively. The connection need not be a genetic one, but evidence toward some FSRQs switching into BL Lacs at lower z will be provided by moderately negative BL Lac evolution. Then an extrapolation will be warranted toward ultra-high energy particle accelerators operating at very low accretion rates.

Cavaliere, A
2002-01-01

453

Revisiting the Blazar Main Sequence

  Astrophysics (arXiv)

Summary: A discussion of the FSRQ -- BL Lac unification. All distinctive features marking these two Blazar subclasses find an unifying explanation if the sources are powered by central engines constituted by similar Kerr holes, but fueled at high and low accretion rates, respectively. The connection need not be a genetic one, but evidence toward some FSRQs switching into BL Lacs at lower z will be provided by moderately negative BL Lac evolution. Then an extrapolation will be warranted toward ultra-high energy particle accelerators operating at very low accretion rates.

A. Cavaliere; V. D'Elia
2002-11-08

454

The very low mass multiple system LHS\\,1070 -- a testbed for model atmospheres for the lower end of the main sequence

  CERN Preprints

Summary: LHS1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres.This work aims at determining the fundamental stellar parameters of LHS1070 and to test recent model atmospheres: BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have performed a chi^2-minimization comparing well calibrated optical and infrared spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters Teff, radius, and log g for each of the three components of LHS1070.

Rajpurohit, A S; Schultheis, M; Leinert, Ch; Allard, F; Homeier, D; Ratzka, T; Abraham, P; Moster, B; Witte, S; Ryde, N
2012-01-01

455

Limits on eclipses of the pre-main-sequence star KH 15D in the first half of the 20th century

  Astrophysics (arXiv)

Summary: Over the last decade, the pre-main sequence star KH 15D has exhibited periodic eclipses that are surprisingly deep (~3 mag) and long-lasting (~40% of the 48.4-day period). The cause of the eclipses is unknown, but it could be a feature in a nearly edge-on protoplanetary disk. Here we report on an analysis of archival photographs of KH 15D from the Harvard College Observatory plate collection, most of which were taken during the years 1913-1951. During this time range, the data are consistent with no eclipses; the duty cycle of 1 mag eclipses was less than 20%. The decadal timescale of this change in eclipse behavior is compatible with the expected timescale of protoplanet/disk interactions. Archival images from more recent epochs should reveal the onset of the eclipses.

Joshua N. Winn; Peter M. Garnavich; K. Z. Stanek; Dimitar D. Sasselov
2003-06-25

456

The environmental impacts on the star formation main sequence: an H-alpha study of the newly discovered rich cluster at z=1.52

  CERN Preprints

Summary: We report the discovery of a strong over-density of galaxies in the field of a radio galaxy at z=1.52 (4C65.22) based on our broad-band and narrow-band (H-alpha) photometry with Subaru Telescope. We find that H-alpha emitters are located in the outskirts of the density peak (cluster core) dominated by passive red-sequence galaxies. This resembles the situation in lower-redshift clusters, suggesting that the newly discovered structure is a well-evolved rich galaxy cluster at z=1.5. Our data suggest that the color-density and stellar mass-density relations are already in place at z~1.5, mostly driven by the passive red massive galaxies residing within Rc<200 kpc from the cluster core. These environmental trends almost disappear when we consider only star-forming (SF) galaxies. We do not find SFR-density or SSFR-density relations amongst SF galaxies, and the location of the SF main sequence does not significantly change with environment. Nevertheless, we find a tentative hint that star-bursting galaxies (up-s...

Koyama, Yusei; Tadaki, Ken-ichi; Hayashi, Masao; Tanaka, Ichi; Shimakawa, Rhythm
2014-01-01

457

The influence of dynamic tides on the apsidal-motion rate in close binaries with an evolved main-sequence star

  Astrophysics (arXiv)

Summary: The validity of the classical formula for the rate of secular apsidal motion in close binaries is investigated for a sequence of models of a 5 solar mass star ranging from the last stages of the C12 -> N14 reaction to the phase where hydrogen is exhausted in the core. For binaries with short orbital periods, the apsidal-motion rates predicted by the classical formula deviate from the rates determined within the framework of the theory of dynamic tides due to the effects of the compressibility of the stellar fluid and due to resonances of dynamic tides with free oscillation modes of the component stars (Smeyers & Willems 2001). As the star evolves on the main sequence, the deviations caused by the compressibility of the stellar fluid increase with increasing radius of the star. The additional deviations caused by the resonances are largest near the end of the core-hydrogen burning phase. Both of these deviations increase with increasing values of the orbital eccentricity.

B. Willems; A. Claret
2001-11-29

458

The influence of dynamic tides on the apsidal-motion rate in close binaries with an evolved main-sequence star

  CERN Preprints

Summary: The validity of the classical formula for the rate of secular apsidal motion in close binaries is investigated for a sequence of models of a 5 solar mass star ranging from the last stages of the C12 -> N14 reaction to the phase where hydrogen is exhausted in the core. For binaries with short orbital periods, the apsidal-motion rates predicted by the classical formula deviate from the rates determined within the framework of the theory of dynamic tides due to the effects of the compressibility of the stellar fluid and due to resonances of dynamic tides with free oscillation modes of the component stars (Smeyers & Willems 2001). As the star evolves on the main sequence, the deviations caused by the compressibility of the stellar fluid increase with increasing radius of the star. The additional deviations caused by the resonances are largest near the end of the core-hydrogen burning phase. Both of these deviations increase with increasing values of the orbital eccentricity.

Willems, B
2002-01-01

459

Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M71: III. Abundance Ratios

  Astrophysics (arXiv)

Summary: We present abundance ratios for 23 elements with respect to Fe in a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff, in the globular cluster M71. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R=35,000). We find that the neutron capture, the iron peak and the alpha-element abundance ratios show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff. The alpha-elements Mg, Ca, Si and Ti are overabundant relative to Fe. The anti-correlation between O and Na abundances, observed in other metal poor globular clusters, is detected in our sample and extends to the main sequence. A statistically significant correlation between Al and Na abundances is observed among the M71 stars in our sample, extending to Mv = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is varying, as expected, and Zr may be varying from star to star as well. M71 appears to have abundance ratios very similar to M5 whose bright giants were studied by Ivans et al. (2001), but seems to have a smaller amplitude of star-to-star variations at a given luminosity, as might be expected from its higher metallicity. The results of our abundance analysis of 25 stars in M71 provide sufficient evidence of abundance variations at unexpectedly low luminosities to rule out the mixing scenario. Either alone or, even more powerfully, combined with other recent studies of C and N abundances in M71 stars, the existence of such abundance variations cannot be reproduced within the context of our current understanding of stellar evolution.

Solange V. Ramirez; Judith G. Cohen
2002-03-21

460

Absolute dimensions of the M-type eclipsing binary YY Geminorum (Castor C): a challenge to evolutionary models in the lower main-sequence

  Astrophysics (arXiv)

Summary: We present new spectroscopic observations of the detached late-type double-lined eclipsing binary YY Gem, a member of the Castor sextuple system and one of the benchmarks for the comparison between observations and stellar evolution theory in the lower main-sequence. In addition, we have re-analyzed existing light curves in several passbands using modern techniques that account for the conspicuous presence of spots. This, combined with the spectroscopy, has yielded a very precise determination of the absolute dimensions of the components, which are virtually identical to each other. We obtain mean values of M=0.5992+/-0.0047 Mo, R=0.6191+/-0.0057 Ro, and Teff=3820+/-100 K. Both the mass and the radius determinations are good to better than 1%. A re-analysis of the Hipparcos transit data for Castor AB has yielded an improved parallax for the system of 66.90+/-0.63 mas. With this, we have estimated the age (~370 Myr) and metal abundance ([Fe/H]~0.0) of YY Gem from isochrone fits to Castor A and B. We have compared the observations of YY Gem with a large number of recent theoretical calculations, and we show that all models underestimate the radius by up to 20%, and most overestimate the effective temperature by 150 K or more. Both of these trends are confirmed by observations of another similar system in the Hyades cluster (V818 Tau). Consequently, theoretical ages for relatively low-mass objects such as T Tauri stars derived from the H-R diagram may be considerably biased. If the radius is used directly as a measure of evolution, ages could be underestimated by as much as a factor of 10 in this mass regime. In view of these discrepancies, absolute ages from essentially all current models for the lower main sequence must be viewed with at least some measure of skepticism. (abridged)

Guillermo Torres; Ignasi Ribas
2001-11-08

461

New cooling sequences for old white dwarfs

  CERN Preprints

Summary: We present full evolutionary calculations appropriate for the study of hydrogen-rich DA white dwarfs. This is done by evolving white dwarf progenitors from the zero age main sequence, through the core hydrogen burning phase, the helium burning phase and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. Complete evolutionary sequences are computed for a wide range of stellar masses and for two different metallicities: Z=0.01, which is representative of the solar neighborhood, and Z=0.001, which is appropriate for the study of old stellar systems, like globular clusters. During the white dwarf cooling stage we compute self-consistently the phase in which nuclear reactions are still important, the diffusive evolution of the elements in the outer layers and, finally, we also take into account all the relevant energy sources in the deep interior of the white dwarf, like the release of latent heat and the release of gravitational energy due to carbon-oxygen phase separation upon crystall...

Renedo, Isabel; Bertolami, Marcelo M Miller; Romero, Alejandra D; Corsico, Alejandro H; Rohrmann, Rene D; Garcia-Berro, Enrique
2010-01-01

462

Where stars form: inside-out growth and coherent star formation from HST Halpha maps of 2676 galaxies across the main sequence at z~1

  CERN Preprints

Summary: We present Ha maps at 1kpc spatial resolution for star-forming galaxies at z~1, made possible by the WFC3 grism on HST. Employing this capability over all five 3D-HST/CANDELS fields provides a sample of 2676 galaxies. By creating deep stacked Halpha (Ha) images, we reach surface brightness limits of 1x10^-18\\erg\\s\\cm^2\\arcsec^2, allowing us to map the distribution of ionized gas out to >10kpc for typical L* galaxies at this epoch. We find that the spatial extent of the Ha distribution increases with stellar mass as r(Ha)[kpc]=1.5(Mstars/10^10Msun)^0.23. Furthermore, the Ha emission is more extended than the stellar continuum emission, consistent with inside-out assembly of galactic disks. This effect, however, is mass dependent with r(Ha)/r(stars)=1.1(M/10^10Msun)^0.054, such that at low masses r(Ha)~r(stars). We map the Ha distribution as a function of SFR(IR+UV) and find evidence for `coherent star formation' across the SFR-M plane: above the main sequence, Ha is enhanced at all radii; below the main sequen...

Nelson, Erica June; Schreiber, Natascha M Förster; Franx, Marijn; Brammer, Gabriel B; Momcheva, Ivelina G; Wuyts, Stijn; Whitaker, Katherine E; Skelton, Rosalind E; Fumagalli, Mattia; Kriek, Mariska; Labbé, Ivo; Leja, Joel; Rix, Hans-Walter; Tacconi, Linda J; van der Wel, Arjen; Bosch, Frank C van den; Oesch, Pascal A; Dickey, Claire; Lange, Johannes Ulf
2015-01-01

463

Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)

  CERN Preprints

Summary: We present the photometric analysis of the external regions of three Galactic Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the characterization of the multiple stellar populations along the main sequence (MS) and the study of the radial trend of the different populations hosted by the target clusters. The data have been collected using FORS2 mounted at the ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color-magnitude diagrams. We exploit appropriate combination of colors and magnitudes which are powerful tools to identify multiple stellar populations, like B versus U-B and V versus c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121 we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction ...

Nardiello, D; Piotto, G; Marino, A F; Bellini, A; Cassisi, S
2014-01-01

464

Discovery of the Pre-Main Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations

  Astrophysics (arXiv)

Summary: We report the discovery of an extraordinary number of pre-main sequence (PMS) stars in the vicinity of the stellar association LH 95 in the Large Magellanic Cloud (LMC). Using the {\\em Advanced Camera for Surveys} on-board the {\\em Hubble} Space Telescope in wide-field mode we obtained deep high-resolution imaging of the main body of the association and of a nearby representative LMC background field. These observations allowed us to construct the color-magnitude diagram (CMD) of the association in unprecedented detail, and to decontaminate the CMD for the average LMC stellar population. The most significant result is the direct detection of a substantial population of PMS stars and their clustering properties with respect to the distribution of the higher mass members of the association. Although LH 95 represents a rather modest star forming region, our photometry, with a detection limit $V$ \\lsim 28 mag, reveals in its vicinity more than 2,500 PMS stars with masses down to $\\sim 0.3$ M{\\solar}. Our observations offer, thus, a new perspective of a typical LMC association: The stellar content of LH 95 is found to extend from bright OB stars to faint red PMS stars, suggesting a fully populated Initial Mass Function (IMF) from the massive blue giants down to the sub-solar mass regime.

Dimitrios A. Gouliermis; Thomas Henning; Wolfgang Brandner; Andrew E. Dolphin; Michael Rosa; Bernhard Brandl
2007-06-29

465

Discovery of the Pre-Main Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations

  CERN Preprints

Summary: We report the discovery of an extraordinary number of pre-main sequence (PMS) stars in the vicinity of the stellar association LH 95 in the Large Magellanic Cloud (LMC). Using the {\\em Advanced Camera for Surveys} on-board the {\\em Hubble} Space Telescope in wide-field mode we obtained deep high-resolution imaging of the main body of the association and of a nearby representative LMC background field. These observations allowed us to construct the color-magnitude diagram (CMD) of the association in unprecedented detail, and to decontaminate the CMD for the average LMC stellar population. The most significant result is the direct detection of a substantial population of PMS stars and their clustering properties with respect to the distribution of the higher mass members of the association. Although LH 95 represents a rather modest star forming region, our photometry, with a detection limit $V$ \\lsim 28 mag, reveals in its vicinity more than 2,500 PMS stars with masses down to $\\sim 0.3$ M{\\solar}. Our observat...

Gouliermis, Dimitrios A; Brandner, Wolfgang; Dolphin, Andrew E; Rosa, Michael; Brandl, Bernhard
2007-01-01

466

Chandra Observation of the Trifid Nebula: X-ray emission from the O star complex and actively forming pre-main sequence stars

  Astrophysics (arXiv)

Summary: The Trifid Nebula, a young star-forming HII region, was observed for 16 hours by the Chandra X-ray Observatory. We detected 304 X-ray sources, thirty percent of which are hard sources and seventy percent of of which have near-infrared counterparts. Chandra resolved the HD164492 multiple system into a number of discrete X-ray sources. X-ray emission is detected from components HD164492A (an O7.5III star which ionizes the nebula), B and C (a B6V star), and possibly D (a Be star). HD164492A has a soft spectrum (kT ~ 0.5 keV) while the component C blend shows much harder emission (kT ~ 6 keV). The soft spectrum of the O star is similar to emission seen from other single O stars and is probably produced by shocks within its massive stellar wind. Lack of hard emission suggests that neither a magnetically confined wind shock nor colliding wind emission is important in HD164492A. A dozen stars are found to have flares in the field and most of them are pre-main sequence stars (PMS). Six sources with flares have both optical and 2MASS counterparts. These counterparts are not embedded and thus it is likely that these sources are in a later stage of PMS evolution, possibly Class II or III. Two flare sources did not have any near-IR, optical, or radio counterparts. We suggest these X-ray flare stars are in an early pre-main sequence stage (Class I or earlier). We also detected X-ray sources apparently associated with two massive star forming cores, TC1 and TC4. The spectra of these sources show high extinction and X-ray luminosities of 2 - 5 x 10^{31} erg/s. If these sources are Class 0 objects, it is unclear if their X-ray emission is due to solar-type magnetic activities as in Class I objects, or some other mechanism.

J. Rho; S. Ramirez; M. F. Corcoran; K. Hamaguchi; B. Lefloch
2004-01-19

467

Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856

  CERN Preprints

Summary: Recent studies have shown that the extended main-sequence turn off (eMSTO) is a common feature of intermediate-age star clusters in the Magellanic Clouds (MCs). The most simple explanation is that these stellar systems harbor multiple generations of stars with an age difference of a few hundred Myrs. However, while an eMSTO has been detected in a large number of clusters with ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in nearby galaxies do not find any evidence for a prolonged star-formation history, i.e. for multiple stellar generations. These results have suggested alternative interpretation of the eMSTOs observed in intermediate-age star clusters. The eMSTO could be due to stellar rotation mimicking an age spread or to interacting binaries. In these scenarios, intermediate-age MC clusters would be simple stellar populations, in close analogy with younger clusters. Here we provide the first evidence for an eMSTO in a young stellar cluster. We exploit multi-band Hubble Space Te...

Milone, A P; Piotto, G; Marino, A F; Cassisi, S; Bellini, A; Jerjen, H; Pietrinferni, A; Aparicio, A; Rich, R M
2015-01-01

468

On the effect of overshooting as predicted by the modelling of the pre-main sequence evolution of a 2 solar-mass star

  Astrophysics (arXiv)

Summary: We discuss the effects of convective overshooting in the PMS evolution of intermediate mass stars, by analysing in detail the early evolution towards the main sequence of a 2 M_sun stellar model. These effects can be extremely important in the end of the PMS, when the abundances in CNO elements approach the equilibrium in the centre. We provide a possible physical explanation on why a moderate amount of overshooting produces, as the star approaches the ZAMS, an extra loop in the evolutionary tracks on the HR diagram. An interesting feature is that there is a very well defined amount of overshooting (for a given stellar mass and chemical composition) beyond which a loop is produced. For smaller amounts of overshooting such a loop does not take place and the evolutionary tracks are similar to the ones obtained by Iben (1965). The amount of overshooting needed to produce the loop decreases with stellar mass. We discuss the underlining physical reasons for the behaviour predicted by the evolution models and argue that it provides a crucial observational test for convective overshooting in the core of intermediate mass stars.

J. P. Marques; M. J. P. F. G. Monteiro; J. Fernandes
2006-06-01

469

Helium-Abundance and Other Composition Effects on the Properties of Stellar Surface Convection in Solar-like Main-sequence Stars

  CERN Preprints

Summary: We investigate the effect of helium abundance and $\\alpha$-element enhancement on the properties of convection in envelopes of solar-like main-sequence stars stars using a grid of 3D radiation hydrodynamic simulations. Helium abundance increases the mean molecular weight of the gas, and alters opacity by displacing hydrogen. Since the scale of the effect of helium may depend on the metallicity, the grid consists of simulations with three helium abundances ($Y=0.1, 0.2, 0.3$), each with two metallicities ($Z=0.001, 0.020)$. We find that changing the helium mass fraction generally affects structure and convective dynamics in a way opposite to that of metallicity. Furthermore, the effect is considerably smaller than that of metallicity. The signature of helium differs from that of metallicity in the manner in which the photospheric velocity distribution is affected. \\rev{We also find that helium abundance and surface gravity behave largely in similar ways, but differ in the way they affect the mean molecular wei...

Tanner, Joel D; Demarque, Pierre
2013-01-01

470

Rotational mixing in low-mass stars : I Effect of the mu-gradients in main sequence and subgiant Pop I stars

  Astrophysics (arXiv)

Summary: We present a first set of results concerning stellar evolution of rotating low-mass stars. Our models include fully consistent transport of angular momentum and chemicals due to the combined action of rotation induced mixing (according to Maeder & Zahn 1998) and element segregation. The analysis of the effects of local variations of molecular weight due to the meridional circulation on the transport of angular momentum and chemicals are under the scope of this study. We apply this mechanism to low mass main sequence and subgiant stars of population I. We show that the so-called $\\mu$-currents are of major importance in setting the shape of the rotation profile, specially near the core. Furthermore, as shown by Talon & Charbonnel (1998) and Charbonnel & Talon (1999) using models without $\\mu$-currents, we confirm that rotation-induced mixing in stars braked via magnetic torquing can explain the blue side of the Li dip, as well as the low Li abundances observed in subgiants even when $\\mu$-currents are taken into account. We emphasize that $\\mu$ variations are not to be neglected when treating rotation-induced mixing, and that they could be of great importance for latter evolutionary stages.

Ana Palacios; Suzanne Talon; Corinne Charbonnel; Manuel Forestini
2002-10-23

471

The stellar content of the Hamburg/ESO survey VI. The metallicity distribution of main-sequence turnoff stars in the Galactic halo

  CERN Preprints

Summary: To investigate the metallicity distribution function (MDF) of the Galactic halo, a metal-poor main-sequence turnoff-star (MSTO) sample was selected from the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up moderate-resolution observations (R ~ 2000) of some 682 stars (among which 617 were accepted program stars) were carried out with the 2.3m telescope at the Siding Spring Observatory (SSO). Corrections for the survey volume covered by the sample stars were quantitatively estimated and applied to the observed MDF. The corrections are quite small, when compared with those for a previously studied sample of metal-poor giants. The corrected observational MDF of the turnoff sample was then compared with that of the giants, as well as with a number of theoretical predictions of Galactic chemical evolution, including the mass-loss modified simple model. We show that, though the survey-volume corrected MDFs of the metal-poor turnoff and the halo giants notably differ in the region of [Fe/H]...

Li, H N; Sch{ö}rck, T; Norris, J E; Bessell, M S; Yong, D; Beers, T C; Lee, Y S; Frebel, A; Zhao, G
2010-01-01

472

Searching for young Jupiter analogs around AP Col: L-band high-contrast imaging of the closest pre-main sequence star

  CERN Preprints

Summary: The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The combination of its youth, distance, and intrinsically low luminosity make it an ideal target to search for extrasolar planets using direct imaging. We report deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2 in the L-band. Using aggressive speckle suppression and background subtraction techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for projected separations a>4.5 AU, and m >= 2 M_Jup for projected separations as small as 3 AU, assuming an age of 40 Myr using the COND theoretical evolutionary models. Using a different set of models the mass limits increase by a factor of ~2. The observations presented here are the deepest mass-sensitivity limits yet achieved within 20 AU on a star with direct imaging. While Doppler radial velocity surveys have shown that Jovian bodies with close-in orbits are rare ar...

Quanz, Sascha P; Janson, Markus; Avenhaus, Henning; Meyer, Michael R; Hillenbrand, Lynne A
2012-01-01

473

Low-resolution spectroscopy of main sequence stars belonging to 12 Galactic globular clusters. I. CH and CN band strength variations

  CERN Preprints

Summary: Globular clusters show abundance variations for light elements that are not yet well understood. The preferred explanation involves a self-enrichment scenario, with two subsequent generations of stars. Observations of main sequence stars allow us to investigate the signature of this chemically processed material without the complicating effects of internal mixing. Our goal is to investigate the C-N anti-correlation with low-resolution spectroscopy of 20-50 stars fainter than the first dredge-up in seven globular clusters (NGC288, NGC1851, NGC5927, NGC6352, NGC6388, and Pal12) with different properties. We complemented our observations with 47~Tuc archival data, with four additional clusters from the literature (M15, M22, M55, NGC362), and with additional literature data on NGC288. In this first paper, we measured the strength of CN and CH band indices, and we investigated the anti-correlation and bimodality of these indices. We compared r_CN, the ratio of stars belonging to the CN-strong and weak groups, with...

Pancino, E; Zoccali, M; Carrera, R
2010-01-01

474

A higher efficiency of converting gas to stars push galaxies at z ~ 1.6 well above the star-forming main sequence

  CERN Preprints

Summary: Local starbursts have a higher efficiency of converting gas into stars, as compared to typical star-forming galaxies at a given stellar mass, possibly indicative of different modes of star formation. With the peak epoch of galaxy formation occurring at z > 1, it remains to be established whether such an efficient mode of star formation is occurring at high-redshift. To address this issue, we measure the CO molecular gas content of seven high-redshift starburst galaxies with ALMA and IRAM/PdBI. Our sample is selected from the FMOS-COSMOS near-infrared spectroscopic survey of star-forming galaxies at z ~ 1.6 with Subaru. All galaxies have star formation rates (~300-800 Msolar/yr) elevated, by at least four times, above the star-forming main sequence. We detect CO emission in all cases at high significance, indicative of plentiful gas supplies (f_gas ~ 30-50%). Even more compelling, we firmly establish for the first time that starbursts at high redshift systematically have a lower ratio of CO to total infrared l...

Silverman, J D; Rodighiero, G; Rujopakarn, W; Sargent, M; Renzini, A; Liu, D; Feruglio, C; Kashino, D; Sanders, D; Kartaltepe, J; Nagao, T; Arimoto, N; Berta, S; Bethermin, M; Lutz, D; Magdis, G; Mancini, C; Onodera, M; Zamorani, G
2015-01-01

475

Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample

  CERN Preprints

Summary: The chemical compositions of seven Carbon-Enhanced Metal-Poor (CEMP) turn-off stars are determined from high-resolution spectroscopy. Five of them are selected from the SDSS/SEGUE sample of metal-poor stars. The effective temperatures of these objects are all higher than 6000 K, while their metallicities, parametrized by [Fe/H], are all below -2. Six of our program objects exhibit high abundance ratios of barium ([Ba/H]> +1), suggesting large contributions of the products of former AGB companions via mass transfer across binary systems. Combining our results with previous studies provides a total of 20 CEMP main-sequence turn-off stars for which the abundances of carbon and at least some neutron-capture elements are determined. Inspection of the [C/H] ratios for this sample of CEMP turn-off stars show that they are generally higher than those of CEMP giants; their dispersion in this ratio is also smaller. We take these results to indicate that the carbon-enhanced material provided from the companion AGB star ...

Aoki, Wako; Sivarani, Thirupathi; Marsteller, Brian; Lee, Young Sun; Honda, Satoshi; Norris, John E; Ryan, Sean G; Carollo, Daniela
2008-01-01

476

A New Method for the Assessment of Age and Age-Spread of Pre-Main Sequence Stars in Young Stellar Associations of the Magellanic Clouds

  CERN Preprints

Summary: We present a new method for the evaluation of the age and age-spread among pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams using PMS evolutionary models. We convert each isochrone into 2D probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age, as well as the best age for the entire cluster. We apply our method to the photometric catalog of ~2000 PMS stars in the young association LH 95 in the LMC, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V~28, corresp...

Da Rio, Nicola; Gennaro, Mario
2010-01-01

477

Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\\,II region NGC\\,2174 (Sh2-252)

  CERN Preprints

Summary: We investigate 5 embedded clusters (ECs) and the extended stellar group itself of the prominent H\\,II region NGC\\,2174 (Sh2-252), which presents scarce and heterogeneous information, coming from the optical and infrared. Considering the discrepant values of distance and age, the clusters and the H\\,II region appear to be physically unrelated. The analysis is based on field-star decontaminated 2MASS photometry, which allows sampling the pre-main sequence (PMS). We find that Sh2-252A, C, E, NGC\\,2175s, and Teu\\,136 are small ECs (radius within $1.0 - 2.3$\\,pc) characterised by a similar age ($\\sim5$\\,Myr), reddening ($\\aV\\sim1$), distance from the Sun ($\\ds\\sim1.4$\\,kpc), and low mass ($60-200\\,\\ms$). This age is consistent with the H\\,II region, the presence of O and B stars still in the MS, and the dominance ($\\ga95%$ in number) of PMS stars in colour-magnitude diagrams (CMDs). NGC\\,2175 is not a star cluster, but an extended stellar group that encompasses the ECs Sh2-252\\,A and C. It contains $\\sim36%$ of th...

Bonatto, Charles
2011-01-01

478

Chandra Observation of the Trifid Nebula: X-ray emission from the O star complex and actively forming pre-main sequence stars

  CERN Preprints

Summary: The Trifid Nebula, a young star-forming HII region, was observed for 16 hours by the Chandra X-ray Observatory. We detected 304 X-ray sources, thirty percent of which are hard sources and seventy percent of of which have near-infrared counterparts. Chandra resolved the HD164492 multiple system into a number of discrete X-ray sources. X-ray emission is detected from components HD164492A (an O7.5III star which ionizes the nebula), B and C (a B6V star), and possibly D (a Be star). HD164492A has a soft spectrum (kT ~ 0.5 keV) while the component C blend shows much harder emission (kT ~ 6 keV). The soft spectrum of the O star is similar to emission seen from other single O stars and is probably produced by shocks within its massive stellar wind. Lack of hard emission suggests that neither a magnetically confined wind shock nor colliding wind emission is important in HD164492A. A dozen stars are found to have flares in the field and most of them are pre-main sequence stars (PMS). Six sources with flares have both o...

Rho, J; Corcoran, M F; Hamaguchi, K; Le Floc'h, B
2004-01-01

479

Low Mass Pre-Main Sequence stars in the Large Magellanic Cloud - II: HST-WFPC2 observations of two fields in the 30 Doradus region

  Astrophysics (arXiv)

Summary: As a part of an ongoing effort to characterise the young stellar populations in the Large Magellanic Cloud, we present HST-WFPC2 broad and narrow band imaging of two fields with recent star formation activity in the Tarantula region. A population of objects with Halpha and/or Balmer continuum excess was identified. On account of the intense Halpha emission (equivalent widths up to several tens of Angstroms), its correlation with the Balmer continuum excess and the stars' location on the HR diagram, we interpret them as low mass (~1-2 Mo) Pre-Main Sequence stars. In this framework, the data show that coeval high and low mass stars have significantly different spatial distributions, implying that star formation processes for different ranges of stellar masses are rather different and/or require different initial conditions. We find that the overall slope of the mass function of the young population is somewhat steeper than the classical Salpeter value and that the star formation density of this young component is 0.2-0.4 Mo/yr/kpc2, i.e. intermediate between the value for an active spiral disk and that of a starburst region. The uncertainties associated with the determination of the slope of the mass function and the star formation density are thoroughly discussed.

Martino Romaniello; Salvatore Scuderi; Nino Panagia; Rosa Maria Salerno; Carlo Blanco
2005-10-06

480

Low Mass Pre-Main Sequence stars in the Large Magellanic Cloud - II: HST-WFPC2 observations of two fields in the 30 Doradus region

  CERN Preprints

Summary: As a part of an ongoing effort to characterise the young stellar populations in the Large Magellanic Cloud, we present HST-WFPC2 broad and narrow band imaging of two fields with recent star formation activity in the Tarantula region. A population of objects with Halpha and/or Balmer continuum excess was identified. On account of the intense Halpha emission (equivalent widths up to several tens of Angstroms), its correlation with the Balmer continuum excess and the stars' location on the HR diagram, we interpret them as low mass (~1-2 Mo) Pre-Main Sequence stars. In this framework, the data show that coeval high and low mass stars have significantly different spatial distributions, implying that star formation processes for different ranges of stellar masses are rather different and/or require different initial conditions. We find that the overall slope of the mass function of the young population is somewhat steeper than the classical Salpeter value and that the star formation density of this young component ...

Romaniello, M; Panagia, N; Salerno, R M; Blanco, C; Romaniello, Martino; Scuderi, Salvatore; Panagia, Nino; Salerno, Rosa Maria; Blanco, Carlo
2005-01-01