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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

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
2007-01-19

3

Optically observable zero-age main-sequence O stars

  CERN Preprints

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.

Walborn, N R
2007-01-01

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

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

6

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

7

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

8

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

9

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

  CERN Preprints

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.

Wiersma, R; Zwart, S P
2006-01-01

10

Convection, Protostellar Collapse, Deuterium Burning and PreMain Sequence Tracks

  Physics Websites

Summary: ­burning ignites and mostly occurs during the main accretion phase; (2) Solar mass stars are partially radiative contract to the (zero age) main­sequence where the en­ ergy radiated into space from the photospheres of solar mass stars is cal­ culated starting with an unstable isothermal cloud fragment. This allows

Wuchterl, Günther

11

Are pre-main-sequence stars older than we thought?

  CERN Preprints

Summary: We fit the colour-magnitude diagrams of stars between the zero-age main-sequence and terminal-age main sequence in young clusters and associations. The ages we derive are a factor 1.5 to 2 longer than the commonly used ages for these regions, which are derived from the positions of pre-main-sequence stars in colour-magnitude diagrams. From an examination of the uncertainties in the main-sequence and pre-main-sequence models, we conclude that the longer age scale is probably the correct one, which implies we must revise upwards the commonly used ages for young clusters and associations. Such a revision would explain the discrepancy between the observational lifetimes of proto-planetary discs and theoretical calculations of the time to form planets. It would also explain the absence of clusters with ages between 5 and 30Myr. We use the $\\tau^2$ statistic to fit the main-sequence data, but find that we must make significant modifications if we are to fit sequences which have vertical segments in the colour-magni...

Naylor, Tim
2009-01-01

12

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

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...

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

18

Pre-main-sequence isochrones -- II. Revising star and planet formation timescales

  CERN Preprints

Summary: We have derived ages for 13 young (<30 Myr) star-forming regions and find they are up to a factor two older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (~10-12 Myr) and that the average Class I lifetime is greater (~1 Myr) than currently believed. For each star-forming region we derived two ages from colour-magnitude diagrams. First we fitted models of the evolution between the zero-age main-sequence and terminal-age main-sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50 Myr) we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us c...

Bell, Cameron P M; Mayne, N J; Jeffries, R D; Littlefair, S P
2013-01-01

19

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

20

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

21

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

22

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

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

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

25

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

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

Probing the Upper Mass Limit of the Main Sequence Observationally

  Physics Websites

Summary: of the stars through the observed Keplarian motion. For observations of a double-line spectroscopic binary, you that the measured Msin3 i values are the minimum values for the mass of each star. If the double-line spectroscopic of the Main Sequence. A significant piece of this is the question of how massive the most massive star

Wiita, Paul J.

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

Pre-main sequence stars in the Cepheus flare region

  CERN Preprints

Summary: We present results of optical spectroscopic and BVR_CI_C photometric observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A total of 64 of these are newly confirmed PMS stars, originally selected from various published candidate lists. We estimate effective temperatures and luminosities for the PMS stars, and comparing the results with pre-main sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data, we construct their spectral energy distributions and classify 5% of the stars as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17 PMS stars associated with L125...

Kun, M; Kenyon, S J; Mamajek, E E; Gutermuth, R A
2009-01-01

52

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

53

The shortest period detached white dwarf + main-sequence binary

  CERN Preprints

Summary: We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94 minute orbital period eclipsing white dwarf / main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of 85.5 +/- 0.2 deg. The white dwarf has a mass of 0.51 +/- 0.05 Msun and a radius of 0.0247 +/- 0.0008 Rsun. With a temperature of 35,300 +/- 400K the white dwarf is highly over-inflated if it has a carbon-oxygen core, however if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 +/- 0.01 Msun and a radius of 0.110 +/- 0.004 Rsun placing it close to the hydrogen burning limit. The system emerged from a...

Parsons, S G; Gnsicke, B T; Dhillon, V S; Copperwheat, C M; Littlefair, S P; Pyrzas, S; Drake, A J; Koester, D; Schreiber, M R; Rebassa-Mansergas, A
2011-01-01

54

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

55

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

56

Dust and Gas Debris Around Main Sequence Stars

  CERN Preprints

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 terrestri...

Chen, C H
2005-01-01

57

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

58

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

59

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

60

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

61

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

62

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

63

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; Jordn, A; Olszewski, E; Walker, M; Zoccali, M; Kroupa, P
2013-01-01

64

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

65

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

66

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; Lpez-Morales, Mercedes; Ridgway, Stephen; Gies, Douglas; Jao, Wei-Chun; Rojas-Ayala, Brbara; Parks, J Robert; Sturmann, Laszlo; Sturmann, Judit; Turner, Nils H; Farrington, Chris; Goldfinger, P J; Berger, David H
2012-01-01

67

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

68

Spectroscopy of Main Sequence Stars in Globular Clusters

  CERN Preprints

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.

Cannon, R; Norris, J; Stanford, L; Croke, B F W; Cannon, Russell; Costa, Gary Da; Norris, John; Stanford, Laura; Croke, Barry
2002-01-01

69

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

70

The Main Sequence Luminosity Function of Palomar 5 from HST

  CERN Preprints

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 sug...

Grillmair, C J; Grillmair, Carl J.; Smith, Graeme H.
2001-01-01

71

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
2006-10-18

72

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

73

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

74

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

75

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

76

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 Edsj
2008-09-10

77

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

  CERN Preprints

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 determ...

Venn, K A; Lambert, D L; Lemke, M; Langer, N; Lennon, D J; Keenan, F P
2001-01-01

78

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

79

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

80

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

81

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

82

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

83

Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars

  CERN Preprints

Summary: One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the red giant branch (RGB) and is followed by a series of secondary flashes before the star settles into the zero-age horizontal branch (ZAHB). Yet, no stars have been positively identified in this key evolutionary phase, mainly for two reasons: first, this pre-ZAHB phase is very short compared to other major evolutionary phases in the life of a star; and second, these pre-ZAHB stars are expected to overlap the loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in the color-magnitude diagram (CMD). We investigate the possibility of detecting these stars through stellar pulsations, since some of them are expected to rapidly cross the Cepheid/RR Lyrae instability strip in their route from the RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a consequence of their very high evolutionary speed, some of these stars may present anomalously large period change rat...

Aguirre, V Silva; Weiss, A; Valcarce, A A R
2009-01-01

84

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

85

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

86

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

87

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 in all spectral types from late B to early K. Subject headinggs: circumstellar matter -- open clusters around other main-sequence stars, including #12; Pictoris. Coronographic imaging identified a resolved

Clemens, Dan

88

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' gradually raises the core's temperature. With a higher temperature, more energy flows out, and the star gets photosphere hydrogen- `burning' core A Main-Sequence Star... photosphere hydrogen- `burning' shell inert

Barnes, Joshua Edward

89

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

90

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-Caldern, M.

91

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

92

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

93

The Observational Signatures of Convectively Excited Internal Gravity Waves in Main Sequence Stars

  CERN Preprints

Summary: We predict the flux and surface velocity perturbations produced by convectively excited internal gravity waves (IGWs or g-modes) in main sequence stars. Core convection in massive stars can excite IGWs to sufficient amplitudes to be detectable with high precision photometry by Kepler and CoRoT, if the thickness of the convective overshoot region is 5 solar masses, but are potentially detectable down to M ~ 2 - 3 solar masses . During the main sequence evolution, radiative damping decreases such that ever lower frequency modes reach the stellar surface and flux perturbations reach up to ~ 100 micromagnitudes at the terminal-age main sequence. Using the same convective excitation model, we confirm previous predictions that solar IGWs produce surface velocity perturbations of < 0.3 mm/s. This implies that stochastically excited IGWs are more easily detectable in the photometry of massive main sequence stars than in the Sun.

Shiode, Joshua H; Cantiello, Matteo; Bildsten, Lars
2012-01-01

94

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

95

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

96

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

97

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

98

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

99

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

100

Deep FORS1 observations of the double Main Sequence of omega Centauri

  Astrophysics (arXiv)

Summary: We present the results of a deep photometric survey performed with FORS1@VLT aimed at investigating the complex Main Sequence structure of the stellar system omega Centauri. We confirm the presence of a double Main Sequence and identify its blue component (bMS) over a large field of view up to 26' from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other "normal" MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.

A. Sollima; F. R. Ferraro; M. Bellazzini; L. Origlia; O. Straniero; E. Pancino
2006-09-25

101

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

102

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

103

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

  CERN Preprints

Summary: [abbreviated] Long-lived, large-scale magnetic field configurations exist in upper main sequence, white dwarf, and neutron stars. Externally, these fields are roughly dipolar, 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 appear 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 be...

Reisenegger, Andreas
2008-01-01

104

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

105

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

106

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

107

A MULTIRESOLUTION APPROACH FOR MAIN MOBILE OBJECT LOCALIZATION IN VIDEO SEQUENCES

  Biology and Medicine Websites

Summary: . of Electrical and Computer Eng., National Technical University of Athens, 9 Iroon Polytechniou str., 15773 Athens, Greece. ABSTRACT Main mobile object localization is a task that emerges in research fields, indoor and outdoor sequences, when either a static or a mobile camera is utilized. The proposed


108

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

109

Identification of Main Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: Beta-Pictoris Analogs?

  CERN Preprints

Summary: Spitzer IRAC 3.6-8 micron photometry obtained as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral types in a 1.2 sq. degree field centered on the southern Galactic HII region RCW49. These stars comprise a subset of 184 stars with known spectral classification, most of which were pre-selected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars based on optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post main-sequence stars. The lone O star is an O4V((f)) at a spectrophotometric distance of 3233+ 540- 535 pc and may be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have SED...

Uzpen, B; Olsen, K A G; Clemens, D P; Laurance, T L; Meade, M R; Babler, B L; Indebetouw, R; Whitney, B A; Watson, C; Wolfire, M G; Wolff, M J; Benjamin, R A; Bania, T M; Cohen, M; Devine, K E; Dickey, J M; Heitsch, F; Jackson, J M; Marston, A P; Mathis, J S; Mercer, E P; Stauffer, J R; Stolovy, S R; Backman, D E; Churchwell, E
2005-01-01

110

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

111

A Massive Star Odyssey, from Main Sequence to Supernova Proceedings IAU Symposium No. 212, c

  Computer Technologies and Information Sciences Websites

Summary: A Massive Star Odyssey, from Main Sequence to Supernova Proceedings IAU Symposium No. 212, c # 2002 is believed to be the power source of some of the largest explosions in the universe: supernovae, hypernovae of massive stars can help constrain these mechanisms. 1. Introduction Supernovae (SNe) and Gamma­Ray Bursts


112

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

113

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

114

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

115

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.

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

116

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

117

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

118

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

119

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

120

Identification of Main Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: Beta-Pictoris Analogs?

  Astrophysics (arXiv)

Summary: Spitzer IRAC 3.6-8 micron photometry obtained as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral types in a 1.2 sq. degree field centered on the southern Galactic HII region RCW49. These stars comprise a subset of 184 stars with known spectral classification, most of which were pre-selected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars based on optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post main-sequence stars. The lone O star is an O4V((f)) at a spectrophotometric distance of 3233+ 540- 535 pc and may be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have SEDs that are consistent with hot dusty debris disks, a possible signature of planet formation. Modeling the excesses as blackbodies demonstrates that the blackbody components have fractional bolometric disk-to-star luminosity ratios, LIR/L*, ranging from 10^-3 to 10^-2 with temperatures ranging from 220 to 820 K. The inferred temperatures are more consistent with asteroid belts rather than the cooler temperatures expected for Kuiper belts. Mid-IR excesses are found in all spectral types from late B to early K.

B. Uzpen; H. A. Kobulnicky; K. A. G. Olsen; D. P. Clemens; T. L. Laurance; M. R. Meade; B. L. Babler; R. Indebetouw; B. A. Whitney; C. Watson; M. G. Wolfire; M. J. Wolff; R. A. Benjamin; T. M. Bania; M. Cohen; K. E. Devine; J. M. Dickey; F. Heitsch; J. M. Jackson; A. P. Marston; J. S. Mathis; E. P. Mercer; J. R. Stauffer; S. R. Stolovy; D. E. Backman; E. Churchwell
2005-04-20

121

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

122

Abundances on the Main Sequence of the Globular Cluster Omega Centauri

  CERN Preprints

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...

Costa, G S D; Norris, J E; Cannon, R D; Stanford, Laura; Norris, John E.
2005-01-01

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

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

  CERN Preprints

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 clai...

Wade, G A; Bagnulo, S; Landstreet, J D; Mason, E; Silvester, J; Alecian, E; Bhm, T; Bouret, J C; Catala, C; Donati, J F
2005-01-01

131

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

132

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

133

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

134

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 quantitatively for the ob- served lower limit of X-ray surface flux in solar-type main-sequence stars. Our flux on the surface of nonrotating reference stars. We find agreement between our G0 star calculations

Abbett, Bill

135

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

136

A search for H2 around pre-main-sequence stars

  Astrophysics (arXiv)

Summary: We present the results of a search for pure rotational molecular hydrogen emission from two pre-main-sequence stars, AB Aur and CQ Tau. Observations were made using MICHELLE, the mid-IR echelle spectrometer at the UK Infrared Telescope. We found some evidence for emission in the J = 4 --> 2 line in AB Aur, but no J = 3 --> 1 line from either star. We derive upper limits on line flux which are significantly smaller than previous line flux estimates based on ISO observations. This suggests that the emission detected by ISO is extended on scales of at least 6 arcsec, and does not come from the disk as previously thought.

I. Sheret; S. K. Ramsay Howat; W. R. F. Dent
2003-06-12

137

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

138

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

139

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

140

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.

Lpez-Valdivia, R; Chvez, M; Tapia-Schiavon, C; Hernndez-guila, J B; Valds, J R; Chavushyan, V
2014-01-01

141

Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars

  CERN Preprints

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. T...

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

142

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

143

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
2008-09-02

144

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

145

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. Peri; J. R. D. Lpine; P. Benevides-Soares
2000-07-10

146

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; Peri, J P; Lpine, J R D; Benevides-Soares, P
2000-01-01

147

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

148

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

149

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

150

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

151

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

152

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

153

The role of magnetic fields in pre-main sequence stars

  CERN Preprints

Summary: Strong, kilo-Gauss, magnetic fields are required to explain a range of observational properties in young, accreting pre-main sequence (PMS) systems. We review the techniques used to detect magnetic fields in PMS stars. Key results from a long running campaign aimed at characterising the large scale magnetic fields in accreting T Tauri stars are presented. Maps of surface magnetic flux in these systems can be used to build 3-D models exploring the role of magnetic fields and the efficiency with which magnetic fields can channel accretion from circumstellar disks on to young stars. Long-term variability in T Tauri star magnetic fields strongly point to a dynamo origin of the magnetic fields. Studies are underway to quantify how changes in magnetic fields affect their accretion properties. We also present the first results from a new programme that investigates the evolution of magnetic fields in intermediate mass (1.5-3 Msun) pre-main sequence stars as they evolve from being convective T Tauri stars to fully ra...

Hussain, Gaitee A J
2014-01-01

154

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

155

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

156

Evolution of Primordial Stars Powered by Dark Matter Annihilation up to the Main-Sequence Stage

  CERN Preprints

Summary: Primordial stars formed in the early universe are thought to be hosted by compact dark matter (DM) halos. If DM consists of Weakly Interacting Massive Particles (WIMPs), such stars may be powered by DM annihilation during the early phases of their evolutions. We study the pre-main sequence evolutions of the primordial star using a detailed stellar evolution code under the assumption that the annihilation of adiabatically contracted WIMPs DM within the star provides a sufficient energy to sustain the stellar equilibrium. We follow the evolution of accreting stars using several gas mass accretion rates derived from cosmological simulations. We show that the stellar mass becomes very large, up to 900 - 1000 M_sun when the star reaches the main-sequence phase for a reasonable set of model parameters such as DM particle mass and the annihilation cross section. During the dark star phase, the star expands over a thousand solar-radii, while the surface temperature remains below 10^4 K. The energy generated by nuclea...

Hirano, Shingo; Yoshida, Naoki
2011-01-01

157

gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT

  CERN Preprints

Summary: Pulsations in pre-main sequence stars have been discovered several times within the last years. But nearly all of these pulsators are of delta Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by theory, but lack observational evidence. We present the investigation of variability caused by rotation and (gammaDoradus-type) pulsation in two pre-main sequence members of the young open cluster NGC2264 using high-precision time series photometry from the CoRoT satellite and dedicated high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC 2264VAS87 was obtained by the CoRoT satellite during the dedicated short run SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run SRa05 in December 2011 and January 2012. Frequency analysis was conducted using Period04 and SigSpec. The spectral analysis was performed using equivalent widths and spectral synthesis. The frequency analysis yielded 10 and 14 intrinsic frequencies for NGC2264VAS20 and NGC2264VAS ...

Zwintz, K; Ryabchikova, T; Kaiser, A; Gruberbauer, M; Barnes, T G; Baglin, A; Chaintreuil, S
2013-01-01

158

Main-sequence variable stars in young open cluster NGC 1893

  CERN Preprints

Summary: In this paper we present time series photometry of 104 variable stars in the cluster region NGC 1893. The association of the present variable candidates to the cluster NGC 1893 has been determined by using $(U-B)/(B-V)$ and $(J-H)/(H-K)$ two colour diagrams, and $V/(V-I)$ colour magnitude diagram. Forty five stars are found to be main-sequence variables and these could be B-type variable stars associated with the cluster. We classified these objects as $\\beta$ Cep, slowly pulsating B stars and new class variables as discussed by Mowlavi et al. (2013). These variable candidates show $\\sim$0.005 to $\\sim$0.02 mag brightness variations with periods of $<$ 1.0 d. Seventeen new class variables are located in the $H-R$ diagram between the slowly pulsating B stars and $\\delta$ Scuti variables. Pulsation could be one of the causes for periodic brightness variations in these stars. The X-ray emission of present main-sequence variables associated with the cluster lies in the saturated region of X-ray luminosity vers...

Lata, Sneh; Pandey, A K; Richichi, Andrea; Eswaraiah, C; Kumar, Brajesh; Kappelmann, Norbert; Sharma, Saurabh
2014-01-01

159

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

160

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

161

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

162

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

163

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

  CERN Preprints

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+-...

Catala, C; Donati, J F; Wade, G A; Landstreet, J D; Bhm, T; Bouret, J C; Bagnulo, S; Folsom, C; Silvester, J
2006-01-01

164

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 Edsj
2008-10-30

165

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

166

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; Rgulo, 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

167

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

168

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

169

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

170

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; Alcal, J M; Hernndez, J I Gonzlez; 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; Jofr, 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

171

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

172

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

173

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

  CERN Preprints

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) an...

Ushomirsky, G; Brown, E F; Bildsten, L; Hilliard, V G; Schroeder, P C; Ushomirsky, Greg; Matzner, Christopher D.; Brown, Edward F.; Bildsten, Lars; Hilliard, Vadim G.; Schroeder, Peter C.
1997-01-01

174

Deep HST-WFPC2 photometry of NGC 288. II. The Main Sequence Luminosity Function

  Astrophysics (arXiv)

Summary: The Main Sequence Luminosity Function (LF) of the Galactic globular cluster NGC 288 has been obtained using deep WFPC2 photometry. We have employed a new method to correct for completeness and fully account for bin-to-bin migration due to blending and/or observational scatter. The effect of the presence of binary systems in the final LF is quantified and is found to be negligible. There is a strong indication of the mass segregation of unevolved single stars and clear signs of a depletion of low mass stars in NGC 288 with respect to other clusters. The results are in good agreement with the prediction of theoretical models of the dynamical evolution of NGC 288 that take into account the extreme orbital properties of this cluster.

M. Bellazzini; Flavio Fusi Pecci; Paolo Montegriffo; Maria Messineo; L. Monaco; R. T. Rood
2002-02-08

175

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. Alcal
2008-09-26

176

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

177

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; Bhm, T; Bouret, J C; Donati, J F; Landstreet, J D
2006-01-01

178

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

179

Multi-Planet Destabilisation and Escape in Post-Main Sequence Systems

  CERN Preprints

Summary: Discoveries of exoplanets orbiting evolved stars motivate critical examinations of the dynamics of $N$-body systems with mass loss. Multi-planet evolved systems are particularly complex because of the mutual interactions between the planets. Here, we study the underlying dynamical mechanisms which can incite planetary escape in two-planet post-main sequence systems. Stellar mass loss alone is unlikely to be rapid and high enough to eject planets at typically-observed separations. However, the combination of mass loss and planet-planet interactions can prompt a shift from stable to chaotic regions of phase space. Consequently, when mass loss ceases, the unstable configuration may cause escape. By assuming a constant stellar mass loss rate, we utilize maps of dynamical stability to illustrate the distribution of regular and chaotic trajectories in phase space. We show that chaos can drive the planets to undergo close encounters, leading to the ejection of one planet. Stellar mass loss can trigger the transition...

Voyatzis, George; Veras, Dimitri; Varvoglis, Harry
2013-01-01

180

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

  CERN Preprints

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.

Jura, M; Furlan, E; Green, J; Sargent, B; Forrest, W J; Watson, D M; Barry, J; Hall, P; Herter, T L; Houck, J R; Sloan, G C; Uchida, K; D'Alessio, P; Brandl, B R; Keller, L D; Kemper, F; Morris, P; Najita, J R; Calvet, N; Hartmann, L; Myers, P C
2004-01-01

181

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

182

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
2007-11-11

183

Absolute properties of BG Ind - a bright F3 system just leaving the Main Sequence

  CERN Preprints

Summary: We present photometric and spectroscopic analysis of the bright detached eclipsing binary BG Ind. The masses of the components are found to be 1.428 +- 0.008 and 1.293 +- 0.008 Msun and the radii to be 2.290+-0.017 and 1.680+-0.038 Rsun for primary and secondary stars, respectively. Spectra- and isochrone-fitting coupled with color indices calibration yield [Fe/H]=-0.2+-0.1. At an age of 2.65+-0.20 Gyr BG Ind is well advanced in the main-sequence evolutionary phase - in fact, its primary is at TAMS or just beyond it. Together with three similar systems (BK Peg, BW Aqr and GX Gem) it offers an interesting opportunity to test the theoretical description of overshooting in the critical mass range 1.2-1.5 Msun.

,
2010-01-01

184

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

185

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

186

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. Gnsicke; T. R. Marsh; A. Aungwerojwit; A. Rebassa-Mansergas; P. Rodrguez-Gil; J. Southworth; M. R. Schreiber; A. Nebot Gomez-Moran; D. Koester
2008-12-15

187

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

188

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

189

A comparison of hydrodynamics techniques for modelling collisions between main sequence stars

  Astrophysics (arXiv)

Summary: An Eulerian TVD code and a Lagrangian SPH code are used to simulate the off-axis collision of equal-mass main sequence stars in order to address the question of whether stellar mergers can produce a remnant star where the interior has been replenished with hydrogen due to significant mixing. Each parent main sequence star is chosen to be found near the turnoff, with hydrogen depleted in the core, and is modelled with a M=0.8 solar mass realistic stellar model and as a n=3 polytrope. An ideal fluid description with adiabatic index gamma=5/3 is used for all hydrodynamic calculations. We found good agreement between the simulations for the polytropic case, with the remnant showing strong, non-local mixing throughout. In the interior quarter of the mass, ~35% is mixed in from larger radii and on average the remnant is ~50% fully mixed. For the realistic model, we found less mixing, particularly in the interior and in the SPH simulation. In the inner quarter, ~20% of the contained mass in the TVD case, but only ~3% in the SPH one is mixed in from outside. The simulations give consistent results for the overall profile of the merger remnant and the amount of mass loss, but the differences in mixing suggests that the intrinsic difference between grid and particle based schemes remains a possible artifact. We conclude that both the TVD and SPH schemes can be used equally well for problems that are best suited to their strengths and that care should be taken in interpreting results about fluid mixing.

Hy Trac; Alison Sills; Ue-Li Pen
2006-05-31

190

Differential rotation on both components of the pre main-sequence binary system HD 155555

  Astrophysics (arXiv)

Summary: We present the first measurements of surface differential rotation on a pre-main sequence binary system. Using intensity (Stokes I) and circularly polarised (Stokes V) timeseries spectra, taken over eleven nights at the Anglo-Australian Telescope (AAT), we incorporate a solar-like differential rotation law into the surface imaging process. We find that both components of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show significant differential rotation. The equator-pole laptimes as determined from the intensity spectra are 80 days for the primary star and 163 days for the secondary. Similarly for the magnetic spectra we obtain equator-pole laptimes of 44 and 71 days respectively, showing that the shearing timescale of magnetic regions is approximately half that found for stellar spots. Both components are therefore found to have rates of differential rotation similar to those of the same spectral type main sequence single stars. The results for HD 155555 are therefore in contrast to those found in other, more evolved, binary systems where negligible or weak differential rotation has been discovered. We discuss two possible explanations for this; firstly that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation, secondly that the weak differential rotation previously observed on evolved binaries is a consequence of their large convection zone depths. We suggest that the latter is the more likely solution and show that both temperature and convection zone depth (from evolutionary models) are good predictors of differential rotation strength. Finally, we also examine the possible consequences of the measured differential rotation on the interaction of binary star coronae.

N. J. Dunstone; G. A. J. Hussain; A. Collier Cameron; S. C. Marsden; M. Jardine; J. R. Barnes; J. C. Ramirez Vlex; J. -F. Donati
2008-04-15

191

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

192

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

193

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

194

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

  Physics Websites

Summary: high the star is fully convective. The star in the HR diagram moves at tracks of almost constant-main sequence stars. Their location on the HR diagram is shown below: The arrows indicate how the T-Tauri stars

Peletier, Reynier

195

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

196

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

197

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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

  CERN Preprints

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...

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

206

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

207

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

208

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

209

Determining the main-sequence mass of Type II supernova progenitors

  CERN Preprints

Summary: We present radiation-hydrodynamics simulations of core-collapse supernova (SN) explosions, artificially generated by driving a piston at the base of the envelope of a rotating or non-rotating red-supergiant progenitor star. We search for trends in ejecta kinematics in the resulting Type II-Plateau (II-P) SN, exploring dependencies with explosion energy and pre-SN stellar-evolution model. We recover the trivial result that larger explosion energies yield larger ejecta velocities in a given progenitor. However, we emphasise that for a given explosion energy, the increasing helium-core mass with main-sequence mass of such Type II-P SN progenitors leads to ejection of core-embedded oxygen-rich material at larger velocities. We find that the photospheric velocity at 15d after shock breakout is a good and simple indicator of the explosion energy in our selected set of pre-SN models. This measurement, combined with the width of the nebular-phase OI6303-6363A line, can be used to place an upper-limit on the progenito...

Dessart, Luc; Waldman, Roni
2010-01-01

210

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

211

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

212

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

213

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
2003-12-11

214

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

215

Near-Infrared Photometric Monitoring of a Pre-Main-Sequence Object KH 15D

  Astrophysics (arXiv)

Summary: An extensive photometric monitoring of KH 15D, an enigmatic variable in the young star cluster NGC 2264, has been conducted. Simultaneous and accurate near-infrared (JHKs-bands) photometry is presented between 2003 December and 2005 March covering most of the variable phase. The infrared variability is characterized by large-amplitude and long-lasting eclipse, as observed at optical. The period of variability is 48.3 +/- 0.2 days, the maximum photometric amplitude of variability is ~4.2 mag, and the eclipse duration is \\~0.5 in phase units. These are consistent with the most recent period, amplitude, and duration at optical. The blueing of the J-H color (~0.16 mag) during the eclipse, which has been suggested before, is unambiguously confirmed; a similar blueing at H-Ks is less clear but is probably present at a similar level. The overall shape of the JHKs light curves is very similar to the optical one, including a fair time-symmetry and a less stable flux during the eclipse with a slight hump near the zero phase. Most of these variability features of KH 15D observed at near-infrared wavelengths can be explained with the recent model employing an eclipse by the inclined, precessing disk and an outer scattering region around a pre-main-sequence binary.

Nobuhiko Kusakabe; Motohide Tamura; Yasushi Nakajima; Ryo Kandori; Akika Ishihara; Tetsuya Nagata; Takahiro Nagayama; Shogo Nishiyama; Daisuke Baba; Shuji Sato; Koji Sugitani; Edwin L. Turner; Lyu Abe; Hiroshi Kimura; Tetsuo Yamamoto
2005-09-14

216

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

217

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

218

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

219

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

220

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...

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

221

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

222

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

223

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

224

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
2004-03-26

225

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

226

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

227

The main-sequence rotation-colour relation in the Coma Berenices open cluster

  CERN Preprints

Summary: We present the results of a photometric survey of rotation rates in the Coma Berenices (Melotte 111) open cluster, using data obtained as part of the SuperWASP exoplanetary transit-search programme. The goal of the Coma survey was to measure precise rotation periods for main-sequence F, G and K dwarfs in this intermediate-age (~600 Myr) cluster, and to determine the extent to which magnetic braking has caused the stellar spin periods to converge. We find a tight, almost linear relationship between rotation period and J-K colour with a root-mean square scatter of only 2 percent. The relation is similar to that seen among F, G and K stars in the Hyades. Such strong convergence can only be explained if angular momentum is not at present being transferred from a reservoir in the deep stellar interiors to the surface layers. We conclude that the coupling timescale for angular momentum transport from a rapidly-spinning radiative core to the outer convective zone must be substantially shorter than the cluster age, a...

Cameron, A Collier; Hebb, L; Skinner, G; Anderson, D R; Christian, D J; Clarkson, W I; Enoch, B; Irwin, J; Joshi, Y; Haswell, C A; Hellier, C; Horne, K D; Kane, S R; Lister, T A; Maxted, P F L; Norton, A J; Parley, N; Pollacco, D; Ryans, R; Scholz, A; Skillen, I; Smalley, B; Street, R A; West, R G; Wilson, D M; Wheatley, P J
2009-01-01

228

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

229

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

230

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; Rgulo, C; Belkacem, K; Houdek, G; Garca, R A; Chaplin, W J
2014-01-01

231

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

232

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

233

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

234

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

235

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

236

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
2001-05-23

237

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

238

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

239

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

240

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

241

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

  CERN Preprints

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...

Montalban, J; Kupka, F; Heiter, U
2003-01-01

242

V1647 Ori: The X-ray Evolution of a Pre-main Sequence Accretion Burst

  Astrophysics (arXiv)

Summary: We present Chandra X-ray Observatory monitoring observations of the recent accretion outburst displayed by the pre-main sequence (pre-MS) star V1647 Ori. The X-ray observations were obtained over a period beginning prior to outburst onset in late 2003 and continuing through its apparent cessation in late 2005, and demonstrate that the mean flux of the spatially coincident X-ray source closely tracked the near-infrared luminosity of V1647 Ori throughout its eruption. We find negligible likelihood that the correspondence between X-ray and infrared light curves over this period was the result of multiple X-ray flares unrelated to the accretion burst. The recent Chandra data confirm that the X-ray spectrum of V1647 Ori hardened during outburst, relative both to its pre-outburst state and to the X-ray spectra of nearby pre-MS stars in the L1630 cloud. We conclude that the observed changes in the X-ray emission from V1647 Ori over the course of its 2003-2005 eruption were generated by a sudden increase and subsequent decline in its accretion rate. These results for V1647 Ori indicate that the flux of hard X-ray emission from erupting low-mass, pre-MS stars, and the duration and intensity of such eruptions, reflect the degree to which star-disk magnetic fields are reorganized before and during major accretion events.

J. H. Kastner; M. Richmond; N. Grosso; D. A. Weintraub; T. Simon; A. Henden; K. Hamaguchi; A. Frank; H. Ozawa
2006-07-28

243

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

244

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

  CERN Preprints

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, ...

Chen, Y Q; Benoni, T; Zhao, G
2001-01-01

245

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

246

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

247

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

248

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

249

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

250

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

251

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

  CiteSeer

Summary: Abstract. 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? 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. 1.

Bhavya B; Blesson Mathew; Annapurni Subramaniam
2008-01-01

252

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

253

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

254

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

255

Mon. Not. R. Astron. Soc. 000, 117 (2012) Printed 13 August 2013 (MN LATEX style file v2.2) The Main Sequence of three Red Supergiant Clusters

  Physics Websites

Summary: . There are in excess of 100 stars with more than 8 M identified in each cluster. These main sequence members of the bright main sequence stars to determine the K-band extinction towards the clusters. The differential and RSGC 3. Spectroscopic follow up of the cluster main sequence stars should lead to more precise distance

Froebrich, Dirk

256

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

257

MAD Science Demonstration Proposal Title: Deep into the lower main sequence of the globular cluster NGC 3201

  Physics Websites

Summary: MAD Science Demonstration Proposal Title: Deep into the lower main sequence of the globular cluster-band data collected with MAD. Fig. 2 shows the optical-NIR CMD of Cen based on data collected with two different MAD pointings. To our knowledge this is the deepest optical-NIR diagram ever collected for a GC

Liske, Jochen

258

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

  CERN Preprints

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 ...

Buccino, Andrea P
2008-01-01

259

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

260

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

  CERN Preprints

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 ob...

Amado, P J; Surez, J C; Martin-Ruiz, S; Garrido, R; Rodrguez, E; Catala, C; Goupil, M J
2004-01-01

261

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

262

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

263

Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars

  Astrophysics (arXiv)

Summary: We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3 magnitudes of the white dwarf cooling sequence, and main sequence stars eight magnitudes and more below the turn-off. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/- 0.18 mag. We couple our distance estimates with extensive photometry of the cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement. (Abridged)

Andrew C. Layden; Ata Sarajedini; Ted von Hippel; Adrienne M. Cool
2005-07-14

264

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

265

Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M5

  CERN Preprints

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, obse...

Ramrez, S V; Ramirez, Solange V.; Cohen, Judith G.
2003-01-01

266

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

267

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

268

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
2007-12-14

269

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

270

Stellar ages and convective cores in field main-sequence stars: first asteroseismic application to two Kepler targets

  CERN Preprints

Summary: Using asteroseismic data and stellar evolution models we make the first detection of a convective core in a Kepler field main-sequence star, putting a stringent constraint on the total size of the mixed zone and showing that extra mixing beyond the formal convective boundary exists. In a slightly less massive target the presence of a convective core cannot be conclusively discarded, and thus its remaining main-sequence life time is uncertain. Our results reveal that best-fit models found solely by matching individual frequencies of oscillations corrected for surface effects do not always properly reproduce frequency combinations. Moreover, slightly different criteria to define what the best-fit model is can lead to solutions with similar global properties but very different interior structures. We argue that the use of frequency ratios is a more reliable way to obtain accurate stellar parameters, and show that our analysis in field main-sequence stars can yield an overall precision of 1.5%, 4%, and 10% in rad...

Aguirre, V Silva; Brando, I M; Christensen-Dalsgaard, J; Deheuvels, S; Do?an, G; Metcalfe, T S; Serenelli, A M; Ballot, J; Chaplin, W J; Cunha, M S; Weiss, A; Appourchaux, T; Casagrande, L; Cassisi, S; Creevey, O L; Garcia, R A; Lebreton, Y; Noels, A; Sousa, S G; Stello, D; White, T R; Kawaler, S D; Kjeldsen, H
2013-01-01

271

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
2007-09-10

272

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

  CERN Preprints

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 SEG...

Brown, Warren R; Kenyon, Scott J; Kurtz, Michael J; Bromley, Benjamin C
2007-01-01

273

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

274

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

275

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

276

A Short Wavelength IUE Atlas of Pre--Main--Sequence Christopher M. Johns--Krull, Jeff A. Valenti, and Jeffrey L. Linsky

  Physics Websites

Summary: A Short Wavelength IUE Atlas of Pre--Main--Sequence Stars Christopher M. Johns--Krull, Jeff A­0440 Abstract. We present an atlas of IUE low--dispersion short--wavelength spectra of pre--main--sequence (PMS). Our objective is to obtain the highest quality spectra of these stars and to extract time

Johns-Krull, Christopher M.

277

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

278

PSR J1903+0327 : A Unique Milli-Second Pulsar with a Main-Sequence Companion Star

  CERN Preprints

Summary: PSR J1903+0327 is a mili-second pulsar with a mass of 1.67 M_{\\odot} in a highly eccentric orbit (e=0.44) around a main-sequence star. This unique system cannot be reconciled with current observations where milli-second pulsars are generally seen to orbit white dwarfs in almost exactly circular orbits. Current theoretical models of binary and stellar formation and evolution cannot explain the high eccentricity of this system either. In this work, we present three new epochs of optical spectroscopy for the companion to PSR J1903+0327, obtained to confirm the association of the main-sequence star with the pulsar. These 3 new epochs together with the 2 previous ones obtained by Freire et al. (2011) firmly establish the high eccentricity of the companion's orbit as predicted by pulsar timing. Using all five epochs of optical data, we have provided an independent estimate of the mass-ratio, R=1.56\\pm0.15 as well as the systemic radial velocity of the binary, \\gamma= 42.1\\pm2.5 km/s. We constrain the spectral type ...

Khargharia, Juthika; Froning, Cynthia S; Gopakumar, Achamveedu; Joshi, Bhal Chandra
2011-01-01

279

Testing Scaling Relations for Solar-Like Oscillations from the Main Sequence to Red Giants using Kepler Data

  CERN Preprints

Summary: We have analyzed solar-like oscillations in ~1700 stars observed by the Kepler Mission, spanning from the main-sequence to the red clump. Using evolutionary models, we test asteroseismic scaling relations for the frequency of maximum power (nu_max), the large frequency separation (Delta_nu) and oscillation amplitudes. We show that the difference of the Delta_nu-nu_max relation for unevolved and evolved stars can be explained by different distributions in effective temperature and stellar mass, in agreement with what is expected from scaling relations. For oscillation amplitudes, we show that neither (L/M)^s scaling nor the revised scaling relation by Kjeldsen & Bedding (2011) are accurate for red-giant stars, and demonstrate that a revised scaling relation with a separate luminosity-mass dependence can be used to calculate amplitudes from the main-sequence to red-giants to a precision of ~25%. The residuals show an offset particularly for unevolved stars, suggesting that an additional physical dependency ...

Huber, D; Stello, D; Hekker, S; Mathur, S; Mosser, B; Verner, G A; Bonanno, A; Buzasi, D L; Campante, T L; Elsworth, Y P; Hale, S J; Kallinger, T; Aguirre, V Silva; Chaplin, W J; De Ridder, J; Garcia, R A; Appourchaux, T; Frandsen, S; Houdek, G; Molenda-Zakowicz, J; Monteiro, M J P F G; Christensen-Dalsgaard, J; Gilliland, R L; Kawaler, S D; Kjeldsen, H; Broomhall, A M; Corsaro, E; Salabert, D; Sanderfer, D T; Seader, S E; Smith, J C
2011-01-01

280

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

281

Stellar Diameters and Temperatures III. Main Sequence A, F, G, & K Stars: Additional high-precision measurements and empirical relations

  CERN Preprints

Summary: Based on CHARA Array measurements, we present the angular diameters of 23 nearby, main- sequence stars, ranging from spectral type A7 to K0, five of which are exoplanet host stars. We derive linear radii, effective temperatures, and absolute luminosities of the stars using HIPPARCOS parallaxes and measured bolometric fluxes. The new data are combined with previously published values to create an Angular Diameter Anthology of measured angular diameters to main-sequence stars (luminosity class V and IV). This compilation consists of 125 stars with diameter uncertainties of less than 5%, ranging in spectral types from A to M. The large quantity of empirical data are used to derive color-temperature relations to an assortment of color indices in the Johnson (BVRIJHK), Cousins (RI), Kron (RI), Sloan (griz), and WISE (W3W4) photometric systems. These relations have an average standard deviation of ~3% and are valid for stars with spectral types A0 to M4. To derive even more accurate relations for Sun-like stars, we...

Boyajian, Tabetha S; van Belle, Gerard; Farrington, Chris; Schaefer, Gail; Jones, Jeremy; White, Russel; McAlister, Harold A; Brummelaar, Theo A ten; Ridgway, Stephen; Gies, Douglas; Sturmann, Laszlo; Sturmann, Judit; Turner, Nils H; Goldfinger, P J; Vargas, Norm
2013-01-01

282

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

283

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

284

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

285

Evolution of very low mass pre-main sequence stars and young brown dwarfs under accretion: A phenomenological approach

  CERN Preprints

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).

Gallardo, Jose; Chabrier, Gilles
2008-01-01

286

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

287

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

288

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

289

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

290

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

291

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

292

A new Bohm-Vitense gap in the temperature range 5560 to 5610K in the Main Sequence

  CERN Preprints

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.

Kovtyukh, V V; Belik, S I
2004-01-01

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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
2006-10-21

301

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

302

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, Nadge; De Bondt, Katrien
2012-01-01

303

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...

Martnez-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

304

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

305

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

306

Polarimetric variations of binary stars. V. Pre-main-sequence spectroscopic binaries located in Ophiuchus and Scorpius

  Astrophysics (arXiv)

Summary: We present polarimetric observations of 7 pre-main-sequence (PMS) spectroscopic binaries located in the rho Oph and Upper Sco star forming regions (SFRs). The average observed polarizations at 7660A are between 0.5% and 3.5%. After estimates of the interstellar polarization are removed, all binaries have an intrinsic polarization above 0.4%. Two binaries, NTTS162814-2427 and NTTS162819-2423S, present high levels of intrinsic polarization between 1.5% and 2.1%. All 7 PMS binaries have a statistically variable or possibly variable polarization. Combining these results with our previous sample of binaries located in the Tau, Aur and Ori SFRs, 68% of the binaries have an intrinsic polarization above 0.5%, and 90% of the binaries are polarimetrically variable or possibly variable. NTTS160814-1857, NTTS162814-2427, and NTTS162819-2423S are clearly polarimetrically variable. The first two also exhibit phase-locked variations over ~10 and ~40 orbits respectively. NTTS160905-1859 shows periodic variations that are not phased-locked and only present for short intervals of time. The amplitudes of the variations reach a few tenths of a percent. The high-eccentricity system NTTS162814-2427 shows single-periodic variations, in agreement with our previous numerical simulations. Non-periodic events introduce stochastic noise that partially masks the periodic variations and prevents the Brown, McLean, & Emslie (1978) formalism from finding a reasonable estimate of the inclination.

N. Manset; P. Bastien
2003-02-24

307

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

308

The Clustering Behavior of Pre-Main Sequence Stars in NGC 346 in the Small Magellanic Cloud

  Astrophysics (arXiv)

Summary: We present evidence that the star-forming region NGC 346/N66 in the Small Magellanic Cloud is the product of hierarchical star formation, probably from more than one star formation event. We investigate the spatial distribution and clustering behavior of the pre-main sequence (PMS) stellar population in the region, using data obtained with Hubble Space Telescope's Advanced Camera for Surveys. By applying the nearest neighbor and minimum spanning tree methods on the rich sample of PMS stars previously discovered in the region we identify ten individual PMS clusters in the area and quantify their structures. The clusters show a wide range of morphologies from hierarchical multi-peak configurations to centrally condensed clusters. However, only about 40 per cent of the PMS stars belong to the identified clusters. The central association NGC 346 is identified as the largest stellar concentration, which cannot be resolved into subclusters. Several PMS clusters are aligned along filaments of higher stellar density pointing away from the central part of the region. The PMS density peaks in the association coincide with the peaks of [OIII] and 8 micron emission. While more massive stars seem to be concentrated in the central association when considering the entire area, we find no evidence for mass segregation within the system itself.

S. Schmeja; D. A. Gouliermis; R. S. Klessen
2008-12-16

309

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

310

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

311

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

312

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

313

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

314

Dynamical Masses for Low-Mass Pre-Main Sequence Stars: A Preliminary Physical Orbit for HD 98800 B

  CERN Preprints

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 comp...

Boden, A F; Akeson, R L; Carpenter, J M; Torres, G; Latham, D W; Soderblom, D R; Nelan, E; Franz, O G; Wasserman, L H
2005-01-01

315

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

316

Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars

  CERN Preprints

Summary: We study the impact of internal gravity waves (IGW), meridional circulation, shear turbulence, and stellar contraction on the internal rotation profile and surface velocity evolution of solar metallicity low-mass pre-main sequence stars. We compute a grid of rotating stellar evolution models with masses between 0.6 and 2.0Msun taking these processes into account for the transport of angular momentum, as soon as the radiative core appears and assuming no more disk-locking from that moment on.IGW generation along the PMS is computed taking Reynolds-stress and buoyancy into account in the bulk of the stellar convective envelope and convective core (when present). Redistribution of angular momentum within the radiative layers accounts for damping of prograde and retrograde IGW by thermal diffusivity and viscosity in corotation resonance. Over the whole mass range considered, IGW are found to be efficiently generated by the convective envelope and to slow down the stellar core early on the PMS. In stars more massi...

Charbonnel, C; Amard, L; Palacios, A; Talon, S
2013-01-01

317

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

318

A Highly Consistent Framework for the Evolution of the Star-Forming "Main Sequence" from z~0-6

  CERN Preprints

Summary: Using a compilation of 25 studies from the literature, we investigate the evolution of the star-forming galaxy (SFG) Main Sequence (MS) in stellar mass and star formation rate (SFR) out to $z \\sim 6$. These studies encompass both stacked and non-stacked data as well as many SFR indicators/timescales and selection methods. After converting all observations to a common set of calibrations, we find a remarkable consensus among MS observations ($\\sim 0.1$ dex 1$\\sigma$ interpublication scatter). By fitting for time evolution of the MS in bins of constant mass, we are able to deconvolve the observed scatter about the MS within each observed redshift bins. After accounting for observed scatter between different SFR indicators, we find the width of the MS distribution is $\\sim 0.2$ dex and remains constant over cosmic time. Our fits indicate the slope of the MS is likely time-dependent, with our best fit $\\log\\textrm{SFR}(M_*,t) = \\left(0.83 \\pm 0.03 - 0.027 \\pm 0.004 \\times t\\right) \\log M_* - \\left(6.38 \\pm 0.27 -...

Speagle, Joshua S; Capak, Peter L; Silverman, John D
2014-01-01

319

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

320

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

321

The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones

  Astrophysics (arXiv)

Summary: We continue our series of papers on open cluster distances with a critical assessment of the accuracy of main-sequence fitting using isochrones that employ empirical corrections to the color-temperature relations. We use four nearby open clusters with multicolor photometry and accurate metallicities and present a new metallicity for Praesepe ([Fe/H] = +0.11 +/- 0.03) from high-resolution spectra. The internal precision of distance estimates is about a factor of 5 better than the case without the color calibrations. After taking into account all major systematic errors, we obtain distances accurate to about 2%-3% when there exists a good metallicity estimate. Metallicities accurate to better than 0.1 dex may be obtained from BVIcKs photometry alone. We also derive a helium abundance for the Pleiades of Y = 0.279 +/- 0.015, which is equal within the errors to the Sun's initial helium abundance and that of the Hyades. Our best estimates of distances are (m - M)_0 = 6.33 +/- 0.04, 8.03 +/- 0.04, and 9.61 +/- 0.03 to Praesepe, NGC 2516, and M67, respectively. Our Pleiades distance at the spectroscopic metallicity, (m - M)_0 = 5.66 +/- 0.01 (internal) +/- 0.05 (systematic), is in excellent agreement with several geometric distance measurements. We have made calibrated isochrones for -0.3 <= [Fe/H] <= +0.2 available at http://www.astronomy.ohio-state.edu/iso/ .

Deokkeun An; Donald M. Terndrup; Marc H. Pinsonneault; Diane B. Paulson; Robert B. Hanson; John R. Stauffer
2007-01-13

322

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

323

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

324

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

325

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

326

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

327

Polarimetric variations of binary stars. IV. Pre-main-sequence spectroscopic binaries located in Taurus, Auriga, and Orion

  Astrophysics (arXiv)

Summary: We present polarimetric observations of 14 pre-main-sequence (PMS) binaries located in the Taurus, Auriga, and Orion star forming regions. The majority of the average observed polarizations are below 0.5%, and none are above 0.9%. After removal of estimates of the interstellar polarization, about half the binaries have an intrinsic polarization above 0.5%, even though most of them do not present other evidences for the presence of circumstellar dust. Various tests reveal that 77% of the PMS binaries have or possibly have a variable polarization. LkCa3, Par1540, and Par2494 present detectable periodic and phase-locked variations. The periodic polarimetric variations are noisier and of a lesser amplitude (~0.1%) than for other types of binaries, such as hot stars. This could be due to stochastic events that produce deviations in the average polarization, a non-favorable geometry (circumbinary envelope), or the nature of the scatterers (dust grains are less efficient polarizers than electrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dust in its environment to produce detectable levels of polarization and variations. A fourth interesting case is W134, which displays rapid changes in polarization that could be due to eclipses. We compare the observations with some of our numerical simulations, and also show that an analysis of the periodic polarimetric variations with the Brown, McLean, & Emslie (BME) formalism to find the orbital inclination is for the moment premature: non-periodic events introduce stochastic noise that partially masks the periodic low-amplitude variations and prevents the BME formalism from finding a reasonable estimate of the orbital inclination.

N. Manset; P. Bastien
2002-06-04

328

The Distances to Open Clusters from Main-Sequence Fitting. IV. Galactic Cepheids, the LMC, and the Local Distance Scale

  Astrophysics (arXiv)

Summary: We derive the basic properties of seven Galactic open clusters containing Cepheids and construct their period-luminosity (P-L) relations. For our cluster main-sequence fitting we extend previous Hyades-based empirical color-temperature corrections to hotter stars using the Pleiades as a template. We use BVI_{C}JHK_{s} data to test the reddening law, and include metallicity effects to perform a more comprehensive study for our clusters than prior efforts. The ratio of total to selective extinction R_V that we derive is consistent with expectations. Assuming the LMC P-L slopes, we find = -3.93 +/- 0.07 (statistical) +/- 0.14 (systematic) for 10-day period Cepheids, which is generally fainter than those in previous studies. Our results are consistent with recent HST and Hipparcos parallax studies when using the Wesenheit magnitudes W(VI). Uncertainties in reddening and metallicity are the major remaining sources of error in the V-band P-L relation, but a higher precision could be obtained with deeper optical and near-infrared cluster photometry. We derive distances to NGC4258, the LMC, and M33 of (m - M)_0 = 29.28 +/- 0.10, 18.34 +/- 0.06, and 24.55 +/- 0.28, respectively, with an additional systematic error of 0.16 mag in the P-L relations. The distance to NGC4258 is in good agreement with the geometric distance derived from water masers [\\Delta (m - M)_0 = 0.01 +/- 0.24]; our value for M33 is less consistent with the distance from an eclipsing binary [\\Delta (m - M)_0 = 0.37 +/- 0.34]; our LMC distance is moderately shorter than the adopted distance in the HST Key Project, which formally implies an increase in the Hubble constant of 7% +/- 8%.

Deokkeun An; Donald M. Terndrup; Marc H. Pinsonneault
2007-07-22

329

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

330

VLBI Observations of Single MainSequence M Stars A.O. Benz 1 , W. Alef 2 and M. Gudel 3;4

  Physics Websites

Summary: ; radiation. Also, magnetically trapped particles have a loss­cone velocity distribution prone to instabilityVLBI Observations of Single Main­Sequence M Stars A.O. Benz 1 , W. Alef 2 and M. G¨udel 3;4 1; Abstract Single dMe stars have been observed at 18 cm wavelength by intercontinental very long baseline

Guedel, Manuel

331

EINSTEIN Observations of T Tauri Stars in TaurusAuriga. II: Relationships between Xray Emission and PreMainSequence Activity

  Physics Websites

Summary: Einstein IPC X­ray data on Tau­Aur pre­main­sequence stars, presented in a companion paper, we have studied) in Tau­Aur; they claim that an anticorrelation exists between the line equivalent width and the X


332

Parameters of a Stromgren Sphere Let's assume that we have a 40,000 K B-main sequence star sur-

  Physics Websites

Summary: to the density of free electrons, the density of hydrogen atoms need- ing an electron, and the recombination- 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

333

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

334

PRE-MAIN SEQUENCE DWARFS NEAR VELORUM. K.W. Smith 1;2 , M.Gudel 1 , M. Audard 1 , and R.D. Je ries 3

  Physics Websites

Summary: of the star would increase the strength of magnetic #12;elds and force the disc edge outwards, re- ducing association. We detect 107 sources which are suspected to correspond with optically detected PMS stars activity. Key words: Stars: active - stars: T Tauri 1. Introduction Low mass pre-main sequence stars evolve

Guedel, Manuel

335

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

336

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

337

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

338

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

339

A detailed study of the main sequence of the Globular Cluster NGC 6397: can we derive constraints on the existence of multiple population?

  CERN Preprints

Summary: If NGC 6397 contains a large fraction of "second generation" stars (>70% according to recent analysis), the helium abundance of its stars might also be affected, show some star-to-star variation, and be larger than the standard Big Bang abundance Y~0.24. Can we derive constraints on this issue from the analysis of the main sequence width and from its luminosity function? We build up new models for the turnoff masses and the main sequence down to the hydrogen burning minimum mass, adopting two versions of an updated equation of state (EOS) including the OPAL EOS. Models consider different initial helium and CNO abundances to cover the range of possible variations between the first and second generation stars. We compare the models with the observational main sequence. We also make simulations of the theoretical luminosity functions, for different choices of the mass function and of the mixture of first and second generation stars, and compare them with the observed luminosity function, by means of the Kolmogor...

Di Criscienzo, M; Ventura, P
2009-01-01

340

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

341

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

342

uvby-Hbeta CCD photometry and membership segregation of the open cluster NGC 2548; Gaps in the Main Sequence of open clusters

  CERN Preprints

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 the...

Balaguer-Nuez, L; Galadi-Enriquez, D
2004-01-01

343

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

344

Linear relation for wind-blown bubble sizes of main-sequence OB stars in a molecular environment and implication for supernova progenitors

  CERN Preprints

Summary: We find a linear relationship between the size of a massive star's main-sequence bubble in a molecular environment and the star's initial mass: R_b \\approx 1.21M/Msun - 8.98 pc, assuming a constant interclump pressure. Since stars in the mass range of 8 to 25-30 Msun will end their evolution in the red supergiant phase without launching a Wolf-Rayet wind, the main-sequence wind-blown bubbles are mainly responsible for the extent of molecular gas cavities, while the effect of the photoionization is comparatively small. This linear relation can thus be used to infer the masses of the massive star progenitors of supernova remnants (SNRs) that are discovered to evolve in molecular cavities, while few other means are available for inferring properties of SNR progenitors. We have used this method to estimate the initial masses of the progenitors of eight SNRs: Kes 69, Kes 75, Kes 78, 3C 396, 3C 397, HC 40, Vela, and RX J1713-3946.

Chen, Yang; Chu, You-Hua
2013-01-01

345

Old Main Sequence Turnoff Photometry in the Small Magellanic Cloud. II. Star Formation History and Its Spatial Gradients

  CERN Preprints

Summary: We present a quantitative analysis of the SFH of 12 fields in the SMC. We find that there are four main periods of enhancement of star formation: a young one peaked at around 0.2-0.5 Gyr old, only present in the eastern and in the central-most fields; two at intermediate ages present in all fields (a conspicuous one peaked at 4-5 Gyr old, and a less significant one peaked at 1.5-2.5); and an old one, peaked at 10 Gyr in all fields but the western ones. In the western fields, this old enhancement splits into two, one peaked at around 8 Gyr old and another at around 12 Gyr old. This "two-enhancement" zone seems to be a robust feature since it is unaffected by our choice of stellar evolutionary library but more data covering other fields of the SMC are necessary in order to ascertain its significancy. Some correlation could exist with encounters taken from the orbit determination of Kallivayalil et al. (2006). But our results would be also fit in a first pericenter passage scenario like the one claimed by Besla ...

Noel, Noelia E D; Gallart, Carme; Hidalgo, Sebastian L; Costa, Edgardo; Mendez, Rene A
2009-01-01

346

The Effect of Diffusion on Pulsations of Stars on the Upper Main Sequence. delta Scuti and Metallic A Stars

  Astrophysics (arXiv)

Summary: Recent dramatic improvements in the modeling of abundance evolution due to diffusion in A stars have been achieved with the help of monochromatic opacity tables from the OPAL group. An important result in the context of stellar pulsations is the substantial helium abundance shown to be left over in the driving region of delta Scuti-type pulsations in chemically peculiar Am stars. An accurate opacity profile in the entire stellar envelope including the full effect of heavy elements is also now available for the first time. Pulsations are shown to be excluded for young Am stars but occur naturally when these stars evolve off the ZAMS. The predicted variable metallic A stars all lie towards the red edge of the instability strip, in qualitative agreement with the observed variable delta Delphini and mild Am stars. Results show little direct excitation from iron-peak elements in A-type stars. The main abundance effect is due to the settling of helium, along with a marginal effect due to the enhancement of hydrogen.

S. Turcotte; J. Richer; G. Michaud; J. Christensen-Dalsgaard
2000-06-20

347

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

348

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

349

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

350

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

351

Mixing at young ages: Beryllium abundances in cool main-sequence stars of the open clusters IC 2391 and IC 2602

  CERN Preprints

Summary: The determination of lithium abundances in stars of young clusters have shown that they deplete Li by different degrees during their pre-main sequence phase. Beryllium abundances are complementary to the lithium ones, and can help tracing the mixing processes in the stellar interiors. Our aim is to derive beryllium abundances in a sample of G- and K-type stars of two young pre-main sequence open clusters, IC 2391 and IC 2602. The Be abundances are used to investigate the mixing of internal material in these stars. The reliability of the Be lines as abundance indicators in low-temperatures is also investigated in detail. We derived Be abundances from high-resolution, high signal-to-noise UVES/VLT spectra using spectrum synthesis and model atmospheres. Atmospheric parameters and other elemental abundances are adopted from a previous work. The sample stars have masses in the range between 0.80 < M/Msun < 1.20. They have been shown to differ in lithium abundance by about 0.60 dex, with lower A(Li) in cooler...

Smiljanic, Rodolfo; Pasquini, Luca
2011-01-01

352

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

353

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

354

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

355

Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence Turn Off in M71 III. Abundance Ratios

  CERN Preprints

Summary: We present abundance ratios for 22 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 ...

Ramrez, S V; Ramirez, Solange V.; Cohen, Judith G.
2002-01-01

356

X-rays in the Orion Nebula Cluster Constraints on the origins of magnetic activity in pre-main sequence stars

  CERN Preprints

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 partic...

Feigelson, E D; Hillenbrand, L A; Townsley, L K; Feigelson, Eric D.; III, James A. Gaffney; Garmire, Gordon; Hillenbrand, Lynne A.; Townsley, Leisa
2003-01-01

357

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

358

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

359

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

360

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
2007-09-21

361

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

362

Investigation of the magnetic field characteristics of Herbig Ae/Be stars: Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars

  CERN Preprints

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 stron...

Wade, G A; Bagnulo, S; Landstreet, J D; Mason, E; Silvester, J; Alecian, E; Bhm, T; Bouret, J C; Catala, C; Donati, J F; Folsom, C; Bale, K
2006-01-01

363

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

364

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

365

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

366

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

367

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

368

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

369

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

370

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
2001-11-29

371

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

372

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

373

Lithium and sodium in the globular cluster M4. A Main Sequence star with Li compatible with the cosmological value: nature or nurture?

  CERN Preprints

Summary: Context. The abundance inhomogeneities of light elements observed in Globular Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li-Na anti-correlation in the GC M 4, which is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R=17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a well defined Li-Na anti-correlation among un-evolved MS stars, albeit with a shallow slope d(A(Li))/d(A(Na)) - 0.2. One star in the sample, # 37934, shows the remarkably high lithium a...

Monaco, L; Bonifacio, P; Caffau, E; Geisler, D; Marconi, G; Momany, Y; Ludwig, H -G
2011-01-01

374

The Monitor project: JW 380 -- a 0.26, 0.15 Msol pre main sequence eclipsing binary in the Orion Nebula Cluster

  CERN Preprints

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 presen...

Irwin, Jonathan; Hodgkin, Simon; Stassun, Keivan G; Hebb, Leslie; Irwin, Mike; Moraux, Estelle; Bouvier, Jerome; Alapini, Aude; Alexander, Richard; Bramich, D M; Holtzman, Jon; Martin, Eduardo L; McCaughrean, Mark J; Pont, Frederic; Verrier, P E; Osorio, Maria Rosa Zapatero
2007-01-01

375

Old Main-Sequence Turnoff Photometry in the Small Magellanic Cloud. I. Constraints on the Star Formation History in Different Fields

  CERN Preprints

Summary: We present ground-based B and R-band color-magnitude diagrams (CMDs), reaching the oldest main-sequence turnoffs with good photometric accuracy for twelve fields in the Small Magellanic Cloud (SMC). Our fields, located between ~1 and ~4 degrees from the center of the galaxy, are situated in different parts of the SMC such as the "Wing'' area, and towards the West and South. In this paper we perform a first analysis of the stellar content in our SMC fields through comparison with theoretical isochrones and color functions (CFs). We find that the underlying spheroidally distributed population is composed of both intermediate-age and old stars and that its age composition does not show strong galacto-centric gradients. The three fields situated toward the east, in the Wing region, show very active current star formation. However, only in the eastern field closest to the center do we find an enhancement of recent star formation with respect to a constant SFR(t). The fields corresponding to the western side of the...

Noel, Noelia E D; Costa, Edgardo; Mendez, Rene A
2007-01-01

376

Measurement of Spin-Orbit Misalignment and Nodal Precession for the Planet around Pre-Main-Sequence Star PTFO 8-8695 From Gravity Darkening

  CERN Preprints

Summary: PTFO 8-8695b represents the first transiting exoplanet candidate orbiting a pre-main-sequence star. We find that the unusual lightcurve shapes of PTFO 8-8695 can be explained by transits of a planet across an oblate, gravity-darkened stellar disk. We simultaneously and self-consistently fit two separate lightcurves observed in 2009 December and 2010 December. Our two self-consistent fits yield M_p = 3.0 M_Jup and M_p = 3.6 M_Jup for assumed stellar masses of M_* = 0.34 M_Sun and M_* = 0.44 M_Sun respectively. The two fits have precession periods of 293 days and 581 days. These mass determinations (consistent with previous upper limits) along with the strength of the gravity-darkened precessing model together validate PTFO 8-8695b as just the second Hot Jupiter known to orbit an M-dwarf. Our fits show a high degree of spin-orbit misalignment in the PTFO 8-8695 system: 69 +/- 2 or 73.1 +/- 0.5 degrees, in the two cases. The large misalignment is consistent with the hypothesis that planets become Hot Jupiters wi...

Barnes, Jason W; Jackson, Brian K; Ciardi, David R; Fortney, Jonathan J
2013-01-01

377

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

378

CoRoT 223992193: A new, low-mass, pre-main sequence eclipsing binary with evidence of a circumbinary disk

  CERN Preprints

Summary: We present the discovery of CoRoT 223992193, a double-lined, detached eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the CoRoT space mission during a 23-day observation of the 3 Myr old NGC 2264 star-forming region. Using multi-epoch optical and near-IR follow-up spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William Herschel Telescope we obtain a full orbital solution and derive the fundamental parameters of both stars by modelling the light curve and radial velocity data. The orbit is circular and has a period of $3.8745745 \\pm 0.0000014$ days. The masses and radii of the two stars are $0.67 \\pm 0.01$ and $0.495 \\pm 0.007$ $M_{\\odot}$ and $1.30 \\pm 0.04$ and $1.11 ~^{+0.04}_{-0.05}$ $R_{\\odot}$, respectively. This system is a useful test of evolutionary models of young low-mass stars, as it lies in a region of parameter space where observational constraints are scarce; comparison with these models indicates an apparent age of $\\sim$3.5-6 Myr. The systemic ve...

Gillen, Edward; McQuillan, Amy; Bouvier, Jerome; Hodgkin, Simon; Alencar, Silvia H P; Terquem, Caroline; Southworth, John; Gibson, Neale P; Cody, Ann Marie; Lendl, Monika; Morales-Caldern, Maria; Favata, Fabio; Stauffer, John; Micela, Giuseppina
2013-01-01

379

On a transition from solar-like coronae to rotation-dominated jovian-like magnetospheres in ultracool main-sequence stars

  CERN Preprints

Summary: For main-sequence stars beyond spectral type M5 the characteristics of magnetic activity common to warmer solar-like stars change into the brown-dwarf domain: the surface magnetic field becomes more dipolar and the evolution of the field patterns slows, the photospheric plasma is increasingly neutral and decoupled from the magnetic field, chromospheric and coronal emissions weaken markedly, and the efficiency of rotational braking rapidly decreases. Yet, radio emission persists, and has been argued to be dominated by electron-cyclotron maser emission instead of the gyrosynchrotron emission from warmer stars. These properties may signal a transition in the stellar extended atmosphere. Stars warmer than about M5 have a solar-like corona and wind-sustained heliosphere in which the atmospheric activity is powered by convective motions that move the magnetic field. Stars cooler than early-L, in contrast, may have a jovian-like rotation-dominated magnetosphere powered by the star's rotation in a scaled-up analog of...

Schrijver, C J
2009-01-01

380

Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: comparison with the SDSS DR7 observed sample

  CERN Preprints

Summary: Detached white dwarf + main sequence (WD+MS) systems represent the simplest population of post-common envelope binaries (PCEBs). Since the ensemble properties of this population carries important information about the characteristics of the common-envelope (CE) phase, it deserves close scrutiny. However, most population synthesis studies do not fully take into account the effects of the observational selection biases of the samples used to compare with the theoretical simulations. Here we present the results of a set of detailed Monte Carlo simulations of the population of WD+MS binaries in the Sloan Digital Sky Survey (SDSS) Data Release 7. We used up-to-date stellar evolutionary models, a complete treatment of the Roche lobe overflow episode, and a full implementation of the orbital evolution of the binary systems. Moreover, in our treatment we took into account the selection criteria and all the known observational biases. Our population synthesis study allowed us to make a meaningful comparison with the a...

Camacho, J; Garca-Berro, E; Zorotovic, M; Schreiber, M R; Rebassa-Mansergas, A; Gmez-Morn, A Nebot; Gnsicke, B T
2014-01-01

381

Astronomy & Astrophysics manuscript no. (will be inserted by hand later) On Be Star Candidates and Possible Blue Pre-Main Sequence Objects in the Small Magellanic Cloud

  CiteSeer

Summary: Abstract. 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 ? 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. Pietrzy?ski; W. Gieren O. Szewczyk
2002-01-01

382

Planetary Construction Zones in Occultation: Eclipses by Circumsecondary and Circumplanetary Disks and a Candidate Eclipse of a Pre-Main Sequence Star in Sco-Cen

  CERN Preprints

Summary: The large relative sizes of circumstellar and circumplanetary disks imply that they might be seen in eclipse in stellar light curves. We present photometric and spectroscopic data for a pre-main sequence K5 star (1SWASP J140747.93-394542.6 = ASAS J140748-3945.7), a newly discovered ~0.9 Msun member of the ~16 Myr-old Upper Centaurus-Lupus subgroup of Sco-Cen at a distance of 128+-13 pc. This star exhibited a remarkably long, deep, and complex eclipse event centered on 29 April 2007 (as discovered in SuperWASP photometry, and with portions of the dimming confirmed by ASAS data). At least 5 multi-day dimming events of >0.5 mag are identified, with a >3.3 mag deep eclipse bracketed by two pairs of ~1 mag eclipses symmetrically occurring +-12 days and +-26 days before and after. Hence, significant dimming of the star was taking place on and off over at least a ~54 day period in 2007, and a strong >1 mag dimming event occurring over a ~12 day span. We place a firm lower limit on the period of 850 days (i.e. the o...

Mamajek, Eric E; Pecaut, Mark; Moolekamp, Fred; Scott, Erin L; Kenworthy, Matthew; Cameron, Andrew Collier; Parley, Neil
2011-01-01

383

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

384

On the Structure and Properties of Differentially Rotating Main-Sequence Stars in the 1-2 M_sun Range

  CERN Preprints

Summary: We conduct a systematic examination of the properties of models for chemically homogeneous, differentially rotating, main-sequence stars of mass 1-2 M_sun. The models were constructed using a code based on a reformulation of the self-consistent field method of computing the equilibrium stellar structure for a specified conservative internal rotation law. [abridged] Relative to nonrotating stars of the same mass, these models all have reduced luminosities and effective temperatures, and flattened photospheric shapes (i.e., decreased polar radii) with equatorial radii that can be larger or smaller, depending on the degree of differential rotation. For a fixed ratio of the axial rotation rate to the surface equatorial rotation rate, increasingly rapid rotation generally deepens convective envelopes, shrinks convective cores, and can lead to the presence of a convective core (envelope) in a 1 M_sun (2 M_sun) model, a feature that is absent in a nonrotating star of the same mass. The positions of differentially ro...

MacGregor, K B; Skumanich, Andrew; Metcalfe, T S
2007-01-01

385

Can we predict the global magnetic topology of a pre-main sequence star from its position in the Hertzsprung-Russell diagram?

  CERN Preprints

Summary: ZDI studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (HR) diagrams for the stars in the sample. Intriguingly, the large scale field topology of a given pre-main sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we argue that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the HR diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the HR diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core...

Gregory, S G; Morin, J; Hussain, G A J; Mayne, N J; Hillenbrand, L A; Jardine, M
2012-01-01

386

Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance

  CERN Preprints

Summary: Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence (MS) stars of a Globular Cluster. At the same time we observed Li in a sample of Sub-Giant (SG) 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. We derived the effective temperatures, from H$\\alpha$ fitting, and NLTE Li abundances of the stars in our the sample, using 3D and 1D models. We find that SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature. These results provide an unambiguous evidence that the Li abundance changes wit...

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

387

New Clues to the Cause of Extended Main Sequence Turn-Offs in Intermediate-Age Star Clusters in the Magellanic Clouds

  CERN Preprints

Summary: We use the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) to obtain deep, high resolution images of two intermediate-age star clusters in the Large Magellanic Cloud of relatively low mass ($\\approx$ $10^4$ $M_{\\odot}$) and significantly different core radii, namely NGC2209 and NGC2249. For comparison purposes, we also re-analyzed archival HST images of NGC1795 and IC2146, two other relatively low mass star clusters. From the comparison of the observed color-magnitude diagrams with Monte Carlo simulations, we find that the main sequence turnoff (MSTO) regions in NGC2209 and NGC2249 are significantly wider than that derived from simulations of simple stellar populations, while those in NGC1795 and IC2146 are not. We determine the evolution of the clusters' masses and escape velocities from an age of 10 Myr to the present age. We find that the differences among these clusters can be explained by dynamical evolution arguments if the currently extended clusters (NGC2209 and IC2146) experienced...

Correnti, Matteo; Kalirai, Jason S; Girardi, Leo; Puzia, Thomas H; Kerber, Leandro
2014-01-01

388

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

389

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

390

The production of strong broad He II emission after the tidal disruption of a main-sequence star by a supermassive black hole

  CERN Preprints

Summary: The tidal disruption event (TDE) PS1-10jh lacked strong Balmer lines but showed strong, broad, He II emission both before maximum light and for at least 8 months thereafter. Gezari et al. interpreted this as evidence for the disruption of a rare hydrogen-deficient star. However, Guillochon et al. have argued instead that the disrupted star was a normal main-sequence star and that the strength of the He II emission compared with the Balmer lines is a result the emission being similar to the broad-line region (BLR) of an AGN, but lacking the outer, lower-ionization BLR gas. We show that the profile of He II 4686 in PS1-10jh is similar to the blueshifted profiles of high-ionization lines in AGNs. We find an He II 4686/Halpha ratio for PS1-10jh of ~ 3.7. We show that both the high-velocity gas of the inner BLR of normal AGNs and the spectra of type II-P supernovae right after shock break out also produce very high He II 4686/Halpha ratios. A high He II 4686/Halpha ratio can thus be produced with a solar H/He abun...

Gaskell, C Martin
2013-01-01

391

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

392

Blue and IR Light Curves of the Mysterious Pre-Main Sequence Star V582 Mon (KH 15D) from 1955 to 1970

  Astrophysics (arXiv)

Summary: In recent years, an increasing number of publications have been addressed to the peculiar and mysterious pre-main sequence star V582 Mon, also known as KH 15D. This extraordinary T Tauri star, located in the young star cluster NGC 2264, appears as to be an eclipsing variable. In the present paper, we report a unique and self-consistent set of light curves in the blue and near-infrared bands, spanning a 15-year interval (epoch 1955-1970). Our photometric data show clearly the beginning of the eclipse stage occurred in early 1958 in the blue, and perhaps around four years later in the infrared. The light curve period turns out to be the same reported by recent observations (about 48.3 days), so that no evidence for a period change results. On the other hand, in our data the light curve shape appears as sinusoidal and is therefore different from the one displayed today. The photometric behaviour, determined with time-series and colour-index analysis, suggests that V582 Mon (KH 15D) could be initially surrounded by an accretion disk/torus seen edge-on, with subsequent thin dust formation at the beginning of the blue radiation absorption. The dust could then aggregate into larger particles providing the transition between selective and total absorption, accompanied with eclipsing variability in the infrared. The minima of the periodic light curve become deeper due to the increasing dimension and number of dust grains, and then flattens due to a contraction in the disk.

Paolo Maffei; Stefano Ciprini; Gino Tosti
2004-12-10

393

The transiting exoplanet host star GJ 436: a test of stellar evolution models in the lower main sequence, and revised planetary parameters

  Astrophysics (arXiv)

Summary: Knowledge of the stellar parameters for the parent stars of transiting exoplanets is pre-requisite for establishing the planet properties themselves, and often relies on stellar evolution models. GJ 436, which is orbited by a transiting Neptune-mass object, presents a difficult case because it is an M dwarf. Stellar models in this mass regime are not as reliable as for higher mass stars, and tend to underestimate the radius. Here we use constraints from published transit light curve solutions for GJ 436 along with other spectroscopic quantities to show how the models can still be used to infer the mass and radius accurately, and at the same time allow the radius discrepancy to be estimated. Similar systems should be found during the upcoming Kepler mission, and could provide in this way valuable constraints to stellar evolution models in the lower main sequence. The stellar mass and radius of GJ 436 are M = 0.452 [-0.012,+0.014] M(Sun) and R = 0.464 [-0.011,+0.009] R(Sun), and the radius is 10% larger than predicted by the standard models, in agreement with previous results from well studied double-lined eclipsing binaries. We obtain an improved planet mass and radius of M = 23.17 +/- 0.79 M(Earth) and R = 4.22 [-0.10,+0.09] R(Earth), a density of rho = 1.69 [-0.12,+0.14] g/cm3, and an orbital semimajor axis of a = 0.02872 +/- 0.00027 AU.

Guillermo Torres
2007-10-25

394

Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample

  Astrophysics (arXiv)

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 is preserved at the surface of turn-off stars with no significant dilution. In contrast, a large dispersion in the observed [Ba/H] is found for the sample of CEMP turn-off stars, suggesting that the efficiency of the s-process in very metal-poor AGB stars may differ greatly from star to star. Four of the six stars from the SDSS/SEGUE sample exhibit kinematics that are associated with membership in the outer-halo population, a remarkably high fraction.

Wako Aoki; Timothy C. Beers; Thirupathi Sivarani; Brian Marsteller; Young Sun Lee; Satoshi Honda; John E. Norris; Sean G. Ryan; Daniela Carollo
2008-01-28

395

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds.X. The Chamaeleon II Pre-Main Sequence Population as Observed With IRAC and MIPS

  Astrophysics (arXiv)

Summary: We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and near infrared data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population of Cha II, in a mapped area of ~1.75 square degrees, and study the spatial distribution and properties of the cloud members and candidate pre-main sequence (PMS) objects and their circumstellar matter. From the analysis of the volume density of the PMS objects and candidates we find two tight groups of objects with volume densities higher than 25 solar masses per cubic parsec and 5-10 members each. These groups correlate well in space with the regions of high extinction. A multiplicity fraction of about 13% is observed for objects with separations between 0.8" and 6.0". Using the results of masses and ages from a companion paper, we estimate the star formation efficiency to be 1-4% significantly lower than for Cha I. This might mean that different star-formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that the Cha II cloud is turning some 6-7 solar masses into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. On the other hand, the disk fraction of 70-80% that we estimate in Cha II is much higher than in other star forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed, with particular regard to the discovery of a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50.

Juan M. Alcala'; Loredana Spezzi; Nicholas Chapman; Neal J. Evans II; Tracy L. Huard; Jes K. Jorgens; Bruno Merin; Karl R. Stapelfel; Elvira Covino; Antonio Frasca; Davide Gandolfi; Isa Oliveira
2008-02-11

396

Angular momentum transport by internal gravity waves. IV - Wave generation by surface convection zone, from the pre-main sequence to the early-AGB in intermediate mass stars

  Astrophysics (arXiv)

Summary: This is the fourth in a series of papers that deal with angular momentum transport by internal gravity waves in stellar interiors. Here, we want to examine the potential role of waves in other evolutionary phases than the main sequence. We study the evolution of a 3Msun Population I model from the pre-main sequence to the early-AGB phase and examine whether waves can lead to angular momentum redistribution and/or element diffusion at the external convection zone boundary. We find that, although waves produced by the surface convection zone can be ignored safely for such a star during the main sequence, it is not the case for later evolutionary stages. In particular, angular momentum transport by internal waves could be quite important at the end of the sub-giant branch and during the early-AGB phase. Wave-induced mixing of chemicals is expected during the early-AGB phase.

Suzanne Talon; Corinne Charbonnel
2008-01-30

397

The SeqSaw method we developed involves four main steps. First, SeqSaw splits sequencing reads into shorter segments with overlaps (step 1 in Fig. S1). For example, a 75 nt

  Biotechnology Websites

Summary: Method The SeqSaw method we developed involves four main steps. First, SeqSaw splits sequencing. S1). Finally, on the basis of the mapping results, SeqSaw predicts junctions according to a series of optional filters (step 4 in Fig. S1). In step 2, SeqSaw first employs Bowtie [1] to accelerate the process

Gu, Jin

398

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 Telescope to obtain a high resolution and high signal-to-noise ratio spectrum of Stock 16-12, an early- type likely a member of the young Stock 16 open cluster (age 3­8Myr). The probable cluster membership


399

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 of Chemistry and Physics, Keele University, Staffordshire, ST5 5BG, UK. 3Astrophysics Research Centre, School and J - K colour with a root-mean square scatter of only 2 percent. The relation is similar to that seen

Norton, Andrew J.

400

Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 Msun Primary and an 0.7 Msun Secondary

  Astrophysics (arXiv)

Summary: We report the discovery of a double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We analyze the system spectroscopically and photometrically to empirically determine precise, distance-independent masses, radii, effective temperatures, and luminosities for both components. The measured masses for the primary and secondary, accurate to ~1%, are 1.01 Msun and 0.73 Msun, respectively; thus the primary is a definitive pre-main-sequence solar analog, and the secondary is the lowest-mass star yet discovered among pre-main-sequence eclipsing binary systems. We use these fundamental measurements to test the predictions of pre-main-sequence stellar evolutionary tracks. None of the models we examined correctly predict the masses of the two components simultaneously, and we implicate differences between the theoretical and empirical effective temperature scales for this failing. All of the models predict the observed slope of the mass-radius relationship reasonably well, though the observations tend to favor models with low convection efficiencies. Indeed, considering our newly determined mass measurements together with other dynamical mass measurements of pre-main-sequence stars in the literature, as well as measurements of Li abundances in these stars, we show that the data strongly favor evolutionary models with inefficient convection in the stellar interior, even though such models cannot reproduce the properties of the present-day Sun.

Keivan G. Stassun; Robert D. Mathieu; Luiz Paulo R. Vaz; Nicholas Stroud; Frederick J. Vrba
2003-12-22

401

GOODS-HERSCHEL: star formation, dust attenuation and the FIR-radio correlation on the Main Sequence of star-forming galaxies up to z~4

  CERN Preprints

Summary: We use the deep panchromatic dataset available in the GOODS-N field, spanning all the way from GALEX ultra-violet to VLA radio continuum data, to select a star-forming galaxy sample at z~[0.5-4] and robustly measure galaxy photometric redshifts, star formation rates, stellar masses and UV rest-frame properties. We quantitatively explore, using mass-complete samples, the evolution of the star formation activity and dust attenuation properties of star-forming galaxies up to z~4. Our main results can be summarized as follows: i) we find that the slope of the SFR-M correlation is consistent with being constant, and equal to ~0.8 at least up to z~1.5, while the normalization keeps increasing to the highest redshift, z~4, we are able to explore; ii) for the first time in this work, we are able to explore the FIR-radio correlation for a mass-selected sample of star-forming galaxies: the correlation does not evolve up to z~4; iii) we confirm that galaxy stellar mass is a robust proxy for UV dust attenuation in star-f...

Pannella, Maurilio; Daddi, Emanuele; Dickinson, Mark E; Hwang, Ho Seong; Schreiber, Corentin; Strazzullo, Veronica; Aussel, Herve; Bethermin, Matthieu; Buat, Veronique; Charmandaris, Vassilis; Cibinel, Anna; Juneau, Stephanie; Ivison, Rob; Borgne, Damien Le; Floc'h, Emeric Le; Leiton, Roger; Lin, Lihwai; Magdis, Georgios; Morrison, Glenn E; Mullaney, James R; Onodera, Masato; Renzini, Alvio; Salim, Samir; Sargent, Mark T; Scott, Douglas; Shu, Xinwen; Wang, Tao
2014-01-01

402

Universite du Maine Universitdu Maine

  Computer Technologies and Information Sciences Websites

Summary: Schwenk Professeur LIUM, Universit´e du Maine Co-encadrant : Dr. Lo¨ic Barrault Ma^itre de Conf imagined to finnish my PhD without his continuous supervision. My sincere thanks also goes to Dr. Lo¨ic

Boyer, Edmond

403

Magnetic fields and differential rotation on the pre-main sequence II: The early-G star HD 141943 - coronal magnetic field, H-alpha emission and differential rotation

  CERN Preprints

Summary: Spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at three observing epochs (2007, 2009 and 2010). The observations were obtained using the 3.9-m Anglo-Australian telescope with the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. The brightness and surface magnetic field topologies (given in Paper I) were used to determine the star's surface differential rotation and reconstruct the coronal magnetic field of the star. The coronal magnetic field at the 3 epochs shows on the largest scales that the field structure is dominated by the dipole component with possible evidence for the tilt of the dipole axis shifting between observations. We find very high levels of differential rotation on HD 141943 (~8 times the solar value for the magnetic features and ~5 times solar for the brightness features) similar to that evidenced by another young early-G star, HD 171488. These results indicate that a significant increase in the level of differe...

Marsden, S C; Vlez, J C Ramrez; Alecian, E; Brown, C J; Carter, B D; Donati, J F; Dunstone, N; Hart, R; Semel, M; Waite, I A
2011-01-01

404

The VLT-FLAMES Tarantula Survey XIX. B-type Supergiants - Atmospheric Parameters and Nitrogen Abundances to Investigate the Role of Binarity and the Width of the Main Sequence

  CERN Preprints

Summary: TLUSTY non-LTE model atmosphere calculations have been used to determine atmospheric parameters and nitrogen (N) abundances for 34 single and 18 binary B-type supergiants (BSGs). The effects of flux contribution from an unseen secondary were considered for the binary sample. We present the first systematic study of the incidence of binarity for a sample of BSGs across the theoretical terminal age main sequence (TAMS). To account for the distribution of effective temperatures of the BSGs it may be necessary to extend the TAMS to lower temperatures. This is consistent with the derived distribution of mass discrepancies, projected rotational velocities (vsini) and N abundances, provided that stars cooler than this temperature are post RSG objects. For the BSGs in the Tarantula and previous FLAMES surveys, most have small vsini. About 10% have larger vsini (>100 km/s) but surprisingly these show little or no N enhancement. All the cooler BSGs have low vsini of <70km/s and high N abundance estimates, implying t...

McEvoy, C M; Evans, C J; Kalari, V M; Markova, N; Simn-Daz, S; Vink, J S; Walborn, N R; Crowther, P A; de Koter, A; de Mink, S E; Dunstall, P R; Hnault-Brunet, V; Herrero, A; Langer, N; Lennon, D J; Apellniz, J Maz; Najarro, F; Puls, J; Sana, H; Schneider, F R N; Taylor, W D
2014-01-01

405

Preliminaries Main Theorem

  Mathematics Websites

Summary: Preliminaries Main Theorem W-Graphs and their -Invariants Proof of the Main Theorem Type;Preliminaries Main Theorem W-Graphs and their -Invariants Proof of the Main Theorem Type An The other classical Jackson Todor MilevTau Signatures and Characters of Weyl Groups #12;Preliminaries Main Theorem W

Nevins, Monica

406

Introduction Main Result

  Mathematics Websites

Summary: Introduction Main Result Enumeration Crossings and Nestings of Two Edges in Set Partitions Yan Crossings and Nestings of Set Partitions #12;Introduction Main Result Enumeration Definition} Catherine Yan Crossings and Nestings of Set Partitions #12;Introduction Main Result Enumeration Definition

Yan, Catherine Huafei

407

Introduction Main result

  Mathematics Websites

Summary: Introduction Main result Applications A method to fully discretize vibrating systems using vibrating systems #12;Introduction Main result Applications Example problem - wave equation Wave equation discretize vibrating systems #12;Introduction Main result Applications Example problem - wave equation One

Sart, Remi

408

Some Claims Main Theorem

  Computer Technologies and Information Sciences Websites

Summary: Some Claims Main Theorem Zeckendrof's Theorem Sajjad Department of Computer Science Swansea University December 08, 2008 Sajjad Department of Computer Science Swansea University Zeckendrof's Theorem #12;Some Claims Main Theorem Sajjad Department of Computer Science Swansea University Zeckendrof

Berger, Ulrich

409

Main results An application

  Mathematics Websites

Summary: Motivation Main results An application Beyond the profinite fundamental gerbe The Nori fundamental on the fundamental group scheme, 5.5.2014 Niels Borne, Angelo Vistoli The Nori fundamental gerbe #12;Motivation Main results An application Beyond the profinite fundamental gerbe Plan Motivation The étale fundamental group

Borne, Niels

410

Orange Main Green North

  Environmental Sciences and Ecology Websites

Summary: Brown Main Orange Main Green North Blue South Red Downtown Bowling Green State University Shuttle ANNEX THE OAKS DINING CARILLON PLACE DINING WOLFE CENTER STROH CENTER 15 13 127 6 2 6 1 5 10 9 8 7 5 4 3

Moore, Paul A.

411

FACILITIES MAINTENANCE MAIN WAREHOUSE

  Chemistry Websites

Summary: . OBJECTIVES: The main study objectives are to identify and document key processes; measure and document charted, and critical steps documented for workload measurement and process improvement (Attach 2). A ManFACILITIES MAINTENANCE MAIN WAREHOUSE Workload Measurement, Validation, and Process Analysis

Hemmers, Oliver

412

NA57 main results

  Nuclear Experiment (arXiv)

Summary: The CERN NA57 experiment was designed to study the production of strange and multi-strange particles in heavy ion collisions at SPS energies; its physics programme is essentially completed. A review of the main results is presented.

G. E. Bruno; for the NA57 Collaboration
2007-10-15

413

Chemical analysis of 24 dusty (pre-)main-sequence stars

  Astrophysics (arXiv)

Summary: We have analysed the chemical photospheric composition of 24 Herbig Ae/Be and Vega-type stars in search for the lambda Bootis phenomenon. We present the results of the elemental abundances of the sample stars. Some of the stars were never before studied spectroscopically at optical wavelengths. We have determined the projected rotational velocities of our sample stars. Furthermore, we discuss stars that depict a (selective) depletion pattern in detail. HD 4881 and HD 139614 seem to display an overall deficiency. AB Aur and possibly HD 126367 have subsolar values for the iron abundance, but are almost solar in silicon. HD 100546 is the only clear lambda Bootis star in our sample.

Bram Acke; Christoffel Waelkens
2004-08-12

414

Chemical analysis of 24 dusty (pre-)main-sequence stars

  CERN Preprints

Summary: We have analysed the chemical photospheric composition of 24 Herbig Ae/Be and Vega-type stars in search for the lambda Bootis phenomenon. We present the results of the elemental abundances of the sample stars. Some of the stars were never before studied spectroscopically at optical wavelengths. We have determined the projected rotational velocities of our sample stars. Furthermore, we discuss stars that depict a (selective) depletion pattern in detail. HD 4881 and HD 139614 seem to display an overall deficiency. AB Aur and possibly HD 126367 have subsolar values for the iron abundance, but are almost solar in silicon. HD 100546 is the only clear lambda Bootis star in our sample.

Acke, B; Acke, Bram; Waelkens, Christoffel
2004-01-01

415

Magnetic Interactions in Pre-Main-Sequence Binaries

  CERN Preprints

Summary: Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy $E_{\\rm int}$ as a function of the stellar radii, the stellar magnetic field strengths, the binary semi-major axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variab...

Adams, Fred C; Galli, Daniele; Lizano, Susana; Shu, Frank H
2011-01-01

416

Pre-main sequence spectroscopic binaries suitable for VLTI observations

  CERN Preprints

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, rh...

Guenther, E W; Mundt, R; Covino, E; Alcal, J M; Cusano, F; Stecklum, B
2007-01-01

417

A non-main-sequence secondary in SY Cancri

  CiteSeer

Summary: The cataclysmic variable (CV) SY Cancri is listed in the Ritter & Kolb (2003) catalogue of CVs as a Z Cam type dwarf nova with a period of 0.380 0.001 days (9.13 0.024 hours). In common with most Z Cam stars (Warner 1995,

Robert Connon Smith; Otto Mehes; Dave V; E Putte; Nigel A. Hawkins
2005-01-01

418

Disks and Halos in Pre-Main-Sequence Stars

  Astrophysics (arXiv)

Summary: We study the IR emission from flared disks with and without additional optically thin halos. Flux calculations of a flared disk in vacuum can be considered a special case of the more general family of models in which the disk is imbedded in an optically thin halo. In the absence of such halo, flux measurements can never rule out its existence because the disk flaring surface defines a mathematically equivalent halo that produces the exact same flux at all IR wavelengths. When a flared disk with height H at its outer radius R is imbedded in a halo whose optical depth at visual wavelengths is \\tau\\sub{halo}, the system IR flux is dominated by the halo whenever \\tau\\sub{halo} > (1/4)H/R. Even when its optical depth is much smaller, the halo can still have a significant effect on the disk temperature profile. Imaging is the only way to rule out the existence of a potential halo, and we identify a decisive test that extracts a signature unique to flared disks from imaging observations.

Dejan Vinkovic; Zeljko Ivezic; Anatoly S. Miroshnichenko; Moshe Elitzur
2003-09-01

419

Be abundances in cool main-sequence stars with exoplanets

  CERN Preprints

Summary: We present new UVES spectra of a sample of 15 cool unevolved stars with and without detected planetary companions. Together with previous determinations, we study Be depletion and possible differences in Be abundances between both groups of stars. We obtain a final sample of 89 and 40 stars with and without planets, respectively, which covers a wide range of effective temperatures, from 4700 K to 6400 K, and includes several cool dwarf stars for the first time. We determine Be abundances for these stars and find that for most of them (the coolest ones) the BeII resonance lines are often undetectable, implying significant Be depletion. While for hot stars Be abundances are aproximately constant, with a slight fall as Teff decreases and the Li-Be gap around 6300 K, we find a steep drop of Be content as Teff decreases for Teff < 5500 K, confirming the results of previous papers. Therefore, for these stars there is an unknown mechanism destroying Be that is not reflected in current models of Be depletion. More...

Mena, E Delgado; Hernndez, J I Gonzlez; Santos, N C; Rebolo, R
2011-01-01

420

Accurate fundamental parameters for Lower Main Sequence Stars

  Astrophysics (arXiv)

Summary: We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the InfraRed Flux Method to multi-band photometry. Our study is based on 104 stars for which we have excellent BVRIJHK photometry, excellent parallaxes and good metallicities. Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement. A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in literature shows that systematic effects still exist in the calibrations at the level of a few percent. Our InfraRed Flux Method temperature scale is 100K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3% smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology. Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices. We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well chosen set of G and K dwarfs would go a long way to addressing this problem.

L. Casagrande; L. Portinari; C. Flynn
2006-08-24

421

Habitable Zones Around Main-Sequence Stars: New Estimates

  CERN Preprints

Summary: Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on 1-D, cloud-free, climate model calculations by Kasting et al.(1993). The inner edge of the HZ in Kasting et al.(1993) model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our Solar system is 0.95-1.67 AU. Here, an updated 1-D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water loss (inner HZ) and maximum greenhouse (outer HZ) limits for our Solar Syste...

Kopparapu, Ravi kumar; Kasting, James F; Eymet, Vincent; Robinson, Tyler D; Mahadevan, Suvrath; Terrien, Ryan C; Domagal-Goldman, Shawn; Meadows, Victoria; Deshpande, Rohit
2013-01-01

422

Main Bearing Stiffness Investigation

  CiteSeer

Summary: AbstractSimplified coupled engine block-crankshaft models based on beam theory provide an efficient substitute to engine simulation in the design process. These models require accurate definition of the main bearing stiffness. In this paper, an investigation of this stiffness is presented. The clearance effect is studied using a smooth bearing model. It is manifested for low shaft displacement. The hydrodynamic assessment model shows that the oil film has no stiffness for low loads and it is infinitely rigid for important loads. The deformation stiffness is determined using a suitable finite elements model based on real CADs. As a result, a main bearing behaviour law is proposed. This behaviour law takes into account the clearance, the hydrodynamic sustention and the deformation stiffness. It ensures properly the transition from the configuration low rigidity to the configuration high rigidity. KeywordsClearance, deformation stiffness, main bearing behaviour law, oil film stiffness. T I.

B. Bellakhdhar; A. Dogui; J. L. Ligier

423

MAIN ENTRANCE LOADING DOCK

  Biology and Medicine Websites

Summary: MAIN ENTRANCE & LOBBY LOADING DOCK ENTER HERE BLUE WAISMAN VISITOR STALLS METERS BLUE WAISMAN VISITOR STALLS UW Hospital W DIRECTIONS: From the east--follow University Avenue through campus in stalls outlined in blue and hang a visitor's permit from rear view mirror (a parking permit

Vorperian, Houri K.

424

Oliver Kullmann Main results

  Computer Technologies and Information Sciences Websites

Summary: Oliver Kullmann Main results Complement invariance Lean clause-sets Minimal unsatisfiability SAT and Outlook SAT and the Polya Permanent Problem Oliver Kullmann Computer Science Department Swansea University SAT 2007, Lisbon, May 30, 2007 SAT: Connecting combinatorics and linear algebra #12;Oliver Kullmann

Martin, Ralph R.

425

MAIN APPLICATIONS Spot welding

  Computer Technologies and Information Sciences Websites

Summary: IRB 6400 MAIN APPLICATIONS Spot welding Press tending Material handling Machine tending Palletizing Assembly Industrial Robot High performance industrial robot The 6-axis IRB 6400 manipulator and S4Cplus precision. IRB 6400 accounts for one third of the 90,000 ABB robot installations. The family of 6400 robots

De Luca, Alessandro

426

Introduction Main Result

  Mathematics Websites

Summary: asymptotics for the Green function of stochastic volatility models in Finance Wen Cheng Nick Costanzino Anna University 2Interest Rate Quantitative Group JPMorgan Chase 3Departments of Marketing and Statistics Penn Introduction Motivation (finance) Green's Function Main Result Statement Assumptions Examples and tests Sketch

Nistor, Victor

427

Definitions Main Result

  Physics Websites

Summary: patterns in compositions S. Heubach1 S. Kitaev2 T. Mansour3 1Dept. of Mathematics, California State Univ Avoidance of partially ordered patterns in compositions #12;Background Definitions Main Result Special Types of POPs S. Heubach, S. Kitaev, T. Mansour Avoidance of partially ordered patterns in compositions #12

Heubach, Silvia

428

Introduction Main Theorem

  Mathematics Websites

Summary: for various choices of a. We use methods from rational points on curves, Falting's Theorem, height functionsIntroduction Main Theorem Abstract: We study the number of rational pre-images of a rational number over the family of maps fc(x)) on the number of rational pre-images and examine effective bounds

Silverman, Joseph H.

429

Categorical sequences

  CERN Preprints

Summary: We define and study the categorical sequence of a space, which is a new formalism that streamlines the computation of the Lusternik-Schnirelmann category of a space X by induction on its CW skeleta. The k-th term in the categorical sequence of a CW complex X, \\sigma_X(k), is the least integer n for which cat_X(X_n) >= k. We show that \\sigma_X is a well-defined homotopy invariant of X. We prove that \\sigma_X(k+l) >= \\sigma_X(k) + \\sigma_X(l), which is one of three keys to the power of categorical sequences. In addition to this formula, we provide formulas relating the categorical sequences of spaces and some of their algebraic invariants, including their cohomology algebras and their rational models; we also find relations between the categorical sequences of the spaces in a fibration sequence and give a preliminary result on the categorical sequence of a product of two spaces in the rational case. We completely characterize the sequences which can arise as categorical sequences of formal rational spaces. The ...

Nendorf, Rob; Strom, Jeffrey; 10.2140/agt.2006.6.809
2009-01-01

430

Direct sequencing

  CiteSeer

Summary: Overview of this lecture Youve seen microarrays and sequencing; here I discuss the epigenomic-?specific assays that are upstream of these readouts DNA methyla/on: enzyma/c, chemical, enrichment/affinity capture Sequencing versus microarray; high versus low resolu/on Chroma/n immunoprecipita/on, ChIP-?exo

Mark D. Robinson; Statistical Genomics; Oxford Nanopore; Unmethylated Cpg Site; Methylated Cpg Site

431

Sequence Nets

  Condensed Matter (arXiv)

Summary: We study a new class of networks, generated by sequences of letters taken from a finite alphabet consisting of $m$ letters (corresponding to $m$ types of nodes) and a fixed set of connectivity rules. Recently, it was shown how a binary alphabet might generate threshold nets in a similar fashion [Hagberg et al., Phys. Rev. E 74, 056116 (2006)]. Just like threshold nets, sequence nets in general possess a modular structure reminiscent of everyday life nets, and are easy to handle analytically (i.e., calculate degree distribution, shortest paths, betweenness centrality, etc.). Exploiting symmetry, we make a full classification of two- and three-letter sequence nets, discovering two new classes of two-letter sequence nets. The new sequence nets retain many of the desirable analytical properties of threshold nets while yielding richer possibilities for the modeling of everyday life complex networks more faithfully.

Jie Sun; Takashi Nishikawa; Daniel ben-Avraham
2008-04-23

432

Next Generation DNA sequencing

  Biology and Medicine Websites

Summary: Paradigms Real-time sequencing- by-synthesis Massively parallel sequencing Nanopore sequencing Sequencing genetics #12;Alternative Sequencing Paradigms Real-time sequencing- by-synthesis Massively parallel #12;#12;Real-time Sequencing-by-Synthesis #12;

Borenstein, Elhanan

433

Classical exponential inequalities Main results

  Mathematics Websites

Summary: Classical exponential inequalities Main results Statistical application Exponential inequalities exponential inequalities Main results Statistical application Outline 1 Classical exponential inequalities Azuma-Hoeffding's inequality Freedman's inequality De la Pe~na's inequalities 2 Main results Heavy

Bercu, Bernard

434

March 29, 2008 Operating Systems: Main Memory 1 Main Memory

  Computer Technologies and Information Sciences Websites

Summary: March 29, 2008 Operating Systems: Main Memory 1 Main Memory Chapter 8 #12;March 29, 2008 Operating Systems: Main Memory 2 Chapter Outline Background Contiguous Memory Allocation Paging Structure of the Page Table Segmentation #12;March 29, 2008 Operating Systems: Main Memory 3 Objectives To provide

Adam, Salah

435

Asuinalueiden maine kulttuurin erityispiirteiden ilmentjn.

  Open Access Theses and Dissertations

Summary: ??Asuinalueelle muodostuu tietynlainen maine, jota on vaikea esitt tilastoina, numeroina ja faktoina. Kuitenkin maine on jotain, joka on yleisesti tunnistettavissa. Ihmiset pyrkivt hakeutumaan tietyntyyppisille alueille, (more)

Pulkkila, Janne
2012-01-01

436

Learning Sequences

  CERN Preprints

Summary: We describe the algorithms used by the ALEKS computer learning system for manipulating combinatorial descriptions of human learners' states of knowledge, generating all states that are possible according to a description of a learning space in terms of a partial order, and using Bayesian statistics to determine the most likely state of a student. As we describe, a representation of a knowledge space using learning sequences (basic words of an antimatroid) allows more general learning spaces to be implemented with similar algorithmic complexity. We show how to define a learning space from a set of learning sequences, find a set of learning sequences that concisely represents a given learning space, generate all states of a learning space represented in this way, and integrate this state generation procedure into a knowledge assessment algorithm. We also describe some related theoretical results concerning projections of learning spaces, decomposition and dimension of learning spaces, and algebraic representati...

Eppstein, David
2008-01-01

437

Everywhere complex sequences and the probabilistic method

  Physics Websites

Summary: Everywhere complex sequences and the probabilistic method Andrey Yu. Rumyantsev1 1 Moscow State University, Russia Abstract The main subject of the paper is everywhere complex sequences. An everywhere that no randomized algorithm can produce an everywhere complex sequence with positive probability. On the other hand

Paris-Sud XI, Université de

438

University of Maine Cooperative Extension

  Biology and Medicine Websites

Summary: University of Maine Cooperative Extension Hand Signals Useful for Farmers Bulletin #2335 by Dawna L of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, Vaughn H. Holyoke, Director for the University of Maine Cooperative Extension, the Land Grant University of the state of Maine

Leistikow, Bruce N.

439

$?$-statistically quasi-Cauchy sequences

  Math Preprints (arXiv)

Summary: The main object of this paper is to investigate $\\lambda$-statistically quasi-Cauchy sequences. A real valued function $f$ defined on a subset $E$ of $\\textbf{R}$, the set of real numbers, is called $\\lambda$-statistically ward continuous on $E$ if it preserves $\\lambda$-statistically quasi-Cauchy sequences of points in $E$. It turns out that uniform continuity coincides with $\\lambda$-statistically ward continuity on $\\lambda$-statistically ward compact subsets.

Huseyin Cakalli; Ayse Sonmez; Cigdem Gunduz Aras
2013-04-30

440

From Guindy IIT MADRAS MAIN

  Engineering Websites

Summary: From Guindy IIT MADRAS MAIN ENTRANCE From Adyar MADHYA KAILASH SIGNAL MADHYA KAILASH TEMPLE From Technology and Business Incubator From Guindy IIT MADRAS MAIN ENTRANCE From Adyar MADHYA KAILASH SIGNAL

Kumar, M. Jagadesh

441

Calibration of White Dwarf cooling sequences: theoretical uncertainty

  Astrophysics (arXiv)

Summary: White Dwarf luminosities are powerful age indicators, whose calibration should be based on reliable models. We discuss the uncertainty of some chemical and physical parameters and their influence on the age estimated by means of white dwarf cooling sequences. Models at the beginning of the white dwarf sequence have been obtained on the base of progenitor evolutionary tracks computed starting from the zero age horizontal branch and for a typical halo chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear reaction rates, convection, mass loss and initial chemical composition are discussed. Then, various cooling sequences for a typical white dwarf mass (M=0.6 Mo) have been calculated under different assumptions on some input physics, namely: conductive opacity, contribution of the ion-electron interaction to the free energy and microscopic diffusion. Finally we present the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much effort has been spent to extend the equation of state down to the low temperature and high density regime. An analysis of the latest improvement in the physics of white dwarf interiors is presented. We conclude that at the faint end of the cooling sequence (log L/Lo=-5.5) the present overall uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr. We suggest that this uncertainty could be substantially reduced by improving our knowledge of the conductive opacity (especially in the partially degenerate regime) and by fixing the internal stratification of C and O.

P. G. Prada Moroni; O. Straniero
2002-09-03

442

Cosmological Models (mainly relativistic Cosmology)

  Physics Websites

Summary: COSMOLOGY Cosmological Models (mainly relativistic Cosmology) [References] c 2009, F. Jegerlehner R Lect. 5 R 301 #12;COSMOLOGY Cosmological Models (mainly relativistic Cosmology) [References] Objects our local experience. c 2009, F. Jegerlehner R Lect. 5 R 301 #12;COSMOLOGY Cosmological Models (mainly

Peters, Achim

443

UC Santa Cruz Main Entrance

  Environmental Sciences and Ecology Websites

Summary: UC Santa Cruz West Entrance Em pire Grade Main Entrance HagarDrive Coolid geDrive P PLAZA WEST. At the Main Entrance, proceed west on Empire Grade, then turn right on Heller (the West Entrance). Turn right-day visitor permits may be purchased at the main entrance kiosk or at the TAPS Sales Office (located in the H

Wilmers, Chris

444

Risk measures The main result

  Mathematics Websites

Summary: Outline Risk measures The main result A collection of risk measures Dual representations of risk measures Georg Ch. Pflug May 7, 2005 Georg Ch. Pflug Dual representations of risk measures #12;Outline Risk measures The main result A collection of risk measures Risk measures The main result A collection of risk

Pflug, Georg

445

Fuzzy logic recursive motion detection and denoising of video sequences

  Engineering Websites

Summary: concerning track- ing and denoising of image sequences. We process an input noisy sequence with fuzzy logic are further processed by a pro- posed spatial filter to obtain a denoised image sequence. Our mainFuzzy logic recursive motion detection and denoising of video sequences Vladimir Zlokolica

Pizurica, Aleksandra

446

Biological Sequence Analysis 1 Biological Sequence Analysis

  Mathematics Websites

Summary: Biological Sequence Analysis 1 Biological Sequence Analysis and Motif Discovery Introductory University http://www.fas.harvard.edu/~junliu jliu@stat.harvard.edu #12;Biological Sequence Analysis 2 Topics to be covered · Basic Biology: DNA, RNA, Protein; genetic code. · Biological Sequence Analysis ­ Pairwise

Liu, Jun

447

UC Santa Cruz Main Entrance

  Environmental Sciences and Ecology Websites

Summary: accept coins, bills, credit cards, and UCSC SmartCards (available at the Parking Sales Office during business hours). One-day visitor permits may be purchased at the Sales Office (H Barn) or at the main

Wilmers, Chris

448

The Binary Second Sequence in Cluster Colour--Magnitude Diagrams

  Astrophysics (arXiv)

Summary: We show how the second sequence seen lying above the main sequence in cluster colour magnitude diagrams results from binaries with a large range of mass ratios and not just from those with equal masses. We conclude that the presence of a densely populated second sequence, with only sparse filling in between it and the single star main sequence, does not necessarily imply that binary mass ratios are close to unity.

Jarrod Hurley; Christopher A. Tout
1998-07-10

449

DaimaruDaimaru Agriculture Main GateMain Gate

  Mathematics Websites

Summary: Science Bldg.#5 Kita 12-jo Sta. Kita 18-jo Sta. International Student Center MainCampusStreet SubwayNanbokuLineSubway Conference hall Visiter CenterVisiter Center D.Chuo Shokudo B.Faculty House Trillium (Enreiso) Restaurant "Elm" C.Chuo ShokudoC.Chuo Shokudo B.Faculty House Trillium (Enreiso) Restaurant "Elm" D

Tachizawa, Kazuya

450

UNIVERSITY of CALIFORNIA THE DISCOVERY OF ECLIPSING BINARIES AMONG MAIN

  Physics Websites

Summary: Stars 25 7 Short CMD of NGC 6752 27 8 Deep CMD of NGC 6752 28 9 Artificial Star 28 10 Star 5996 LightUNIVERSITY of CALIFORNIA SANTA CRUZ THE DISCOVERY OF ECLIPSING BINARIES AMONG MAIN SEQUENCE STARS. Photometry Of Stars 17 3.1. Background 17 3.2. Selecting PSF Stars 17 3.3. PSF Fitting To Measure Stellar

Belanger, David P.

451

Coherent sequences versus Radin sequences James Cummings

  Mathematics Websites

Summary: sequences of ex- tenders which are the basis of modern inner model theory. We show that in certain is a coherent non-overlapping sequence of ex- tenders, let j : V ! M be the embedding arising from ~E

Cummings, James

452

Main Title Details Author ISBN

  Computer Technologies and Information Sciences Websites

Summary: Main Title Details Author ISBN Price to Bookstore Price to Students Pub Date Silberschatz 9781118063330 $132.00 $169.95 12/11/2012 Wiley Flex OPTIONS E-Textbooks Price to Savings to Student ISBN Status Price to Price to Savings to Student ISBN Bookstore Students $$ % Binder Ready Version (BRV

Newhall, Tia

453

UNIVERSITY OF MAINE STUDENT LIFE

  Geosciences Websites

Summary: UNIVERSITY OF MAINE STUDENT LIFE CAMPUS RECREATION RELEASE AND ASSUMPTION OF RISK *** PLEASE PRINT this "Release and Assumption of Risk" on behalf of myself, my heirs and next-of-kin, my personal representatives and which could cause property damage, bodily injury and/or death. *See page 2 for specific risks

Thomas, Andrew

454

South Columbia Street (Main Hospital)

  Computer Technologies and Information Sciences Websites

Summary: Research Dental School Bridge A Bridge B East Parking Circle Deck Jackson Building Office Physicians NC South Columbia Street wood Dri Pit D rive Drive West ve (Main Hospital) Old Tarrson Brauer Dental Cancer Hospital BridgeC Em ergency R oom D rive UNC Hospitals Visitor Maps for the University of North

Whitton, Mary C.

455

South Columbia Street (Main Hospital)

  Computer Technologies and Information Sciences Websites

Summary: Dental School Bridge A Bridge B East Parking Circle Deck Jackson Building Office Physicians NC Cancer South Columbia Street wood Dri P Drive Drive West ve (Main Hospital) Old Tarrson Brauer Dental Research Hospital BridgeC Em ergency R oom D rive UNC Hospitals Visitor Maps for the University of North Carolina

Whitton, Mary C.

456

SUMMER SESSIONS 2013 MAIN CAMPUS

  Engineering Websites

Summary: , housing, and dining charges (if applicable) on-line through ECU's eBill system with the following payment for Summer Session 2. (5) The on-line payment system will not be available after 5:00 p.m. on the late feeSUMMER SESSIONS 2013 MAIN CAMPUS Fee Payment Schedule & Tuition and Fee Schedule GREENVILLE NORTH


457

EXPLORATORY FISHING FOR MAINE HERRING

  Environmental Sciences and Ecology Websites

Summary: Inshore explorations 1 Herring winter habitat 6 Coastal gill net fishing 8 Summary 9 lii #12 of Maine waters were sounded and fished with a lampara seine, gill nets, midwater trawls, and otter trawls. Inshore explorations located zero-year-class herring in the bays and inlets and traced their development


458

Library Site Finder MAIN LIBRARY

  Mathematics Websites

Summary: Library Site Finder MAIN LIBRARY Burlington Street Tel: 0161 275 3751 THE ALAN GILBERT LEARNING COMMONS Oxford Road Tel: 0161 306 4306 ART & ARCHAEOLOGY LIBRARY Mansfield Cooper Building Tel: 0161 275 3657 BRADDICK LIBRARY School of Physics & Astronomy Brunswick Street Tel: 0161 275 4078 EDDIE DAVIES

Sidorov, Nikita

459

for sequence accelerators

  Physics Websites

Summary: Wynn's -algorithm for sequence accelerators using high precision arithmetic Rachel Baumann University of Arizona Wynn's -algorithm for sequence accelerators using high precision arithmetic Rachel Baumann University of Arizona April 17, 2012 #12;Wynn's -algorithm for sequence accelerators using high

Zakharov, Vladimir

460

Accelerating genome sequence analysis.

  Open Access Theses and Dissertations

Summary: ??Genome sequence analysis is central to todays genomics research, and sequence alignment and Single-Nucleotide Polymorphism (SNP) detection are two fundamental tasks in the analysis. Sequence (more)

Lu, Mian
2012-01-01

461

Overview: Main Fundamentals for Steganography

  CERN Preprints

Summary: The rapid development of multimedia and internet allows for wide distribution of digital media data. It becomes much easier to edit, modify and duplicate digital information .Besides that, digital documents are also easy to copy and distribute, therefore it will be faced by many threats. It is a big security and privacy issue, it become necessary to find appropriate protection because of the significance, accuracy and sensitivity of the information. Steganography considers one of the techniques which used to protect the important information. The main goals for this paper, to recognize the researchers for the main fundamentals of steganography. In this paper provides a general overview of the following subject areas: Steganography types, General Steganography system, Characterization of Steganography Systems and Classification of Steganography Techniques.

AL-Ani, Zaidoon Kh; Zaidan, B B; Alanazi, Hamdan O
2010-01-01

462

Video sequence synchronization.

  Open Access Theses and Dissertations

Summary: ??[Truncated abstract] Video sequence synchronization is necessary for any computer vision application that integrates data from multiple simultaneously recorded video sequences. With the increased availability (more)

Wedge, Daniel John
2007-01-01

463

Physics with the Main Injector

  HEP - Experiment (arXiv)

Summary: The Main Injector is a new rapid cycling accelerator at Fermilab which is a source of protons to be used in antiproton production to enhance the luminosity of the Tevatron Collider and to provide extracted beams for use in a range of fixed target experiments. We discuss the current status of the accelerator and the physics which it enables. The physics ranges broadly over the standard model and beyond, from the search for neutrino mass to collider physics at the highest energy available today.

Hugh E. Montgomery
1999-04-20

464

Main Riff (continue pattern) E-|-----------------------------------------------------------|

  Geosciences Websites

Summary: -|-----------------------------------------------------------| CHORUS (guitar chords are weak/intermittent): E A E A I can't get no satisfaction I can't get no satisfaction E B7 E A Cause I try__ and I try___ and I try__ and try__ Call this part $ (start main riff) A E A D A E A D I can't get no I can't get no A E A D A E A D When I'm driving in my car and the man comes

Reiners, Peter W.

465

Sequence Conservation in Families Whose Members Have Little or No Sequence Similarity

  Biology and Medicine Websites

Summary: Sequence Conservation in Families Whose Members Have Little or No Sequence Similarity: The Four conservation beyond the simple conservation of hydrophobic and hydrophilic character at specific sites the regions that conserve the same main-chain confor- mation: the common core (CC). For members of the same

Hill, Emma

466

Dynamic Entropy-Compressed Sequences and Full-Text Indexes

  Computer Technologies and Information Sciences Websites

Summary: Outline Dynamic Entropy-Compressed Sequences and Full-Text Indexes Veli M¨akinen Gonzalo Navarro of Helsinki, Finland Universidad de Chile, Chile M¨akinen, Navarro Dynamic Compressed Sequences #12;Outline Outline Background Compressed Dynamic Binary Sequences Main Result A Succinct Version A Compressed Version

Lonardi, Stefano

467

The Parameterized Complexity of Sequence Alignment and Consensus

  Computer Technologies and Information Sciences Websites

Summary: imply lower bounds on a number of other sequence alignment and consensus problems. At issue sequence analysis. Our main results show that: 1 The Longest Common Subsequence LCS parameterized by the number of sequences to be analyzed is hard for W t for all t. 2 The LCS problem problem, parameterized

Utrecht, Universiteit

468

The Parameterized Complexity of Sequence Alignment and Consensus

  Computer Technologies and Information Sciences Websites

Summary: imply lower bounds on a number of other sequence alignment and consensus problems. At issue in biological sequence analysis. Our main results show that: (1) The Longest Common Subsequence (LCS) parameterized by the number of sequences to be analyzed is hard for W [t] for all t. (2) The LCS problem problem

Utrecht, Universiteit

469

Application Note: Sequencing Introduction

  Biology and Medicine Websites

Summary: for profiling complex microbial communities. The MiSeq system provides the easiest sequencing workflow available host-associated and free-living microbial communities and the biological conclusions drawn from sequence data should be reproducible across sequencing systems1,2 . Post-sequencing microbial community

Sliz, Piotr

470

Variational sequences Raffaele Vitolo

  Mathematics Websites

Summary: Variational sequences Raffaele Vitolo Department of Mathematics "E. De Giorgi", University of Lecce://poincare.unile.it/vitolo 1 November 2007 Abstract Variational sequences are complexes of modules or sheaf sequences in which research on the topic. Key words: Jet spaces, variational sequence, variational bicomplex, C

Vitolo, Raffaele

471

Constructing k-radius sequences

  CERN Preprints

Summary: An n-ary k-radius sequence is a finite sequence of elements taken from an alphabet of size n such that any two distinct elements of the alphabet occur within distance k of each other somewhere in the sequence. These sequences were introduced by Jaromczyk and Lonc to model a caching strategy for computing certain functions on large data sets such as medical images. Let f_k(n) be the shortest length of any k-radius sequence. We improve on earlier estimates for f_k(n) by using tilings and logarithms. The main result is that f_k(n) ~ n^2/(2k) as n tends to infinity whenever a certain tiling of Z^r exists. In particular this result holds for infinitely many k, including all k < 195 and all k such that k+1 or 2k+1 is prime. For certain k we get a sharper error term for infinitely many values of n, using the theory of logarithms.

Blackburn, Simon R
2010-01-01

472

Similarity of symbolic sequences

  CERN Preprints

Summary: A new numerical characterization of symbolic sequences is proposed. The partition of sequence based on Ke and Tong algorithm is a starting point. Algorithm decomposes original sequence into set of distinct subsequences - a patterns. The set of subsequences common for two symbolic sequences (their intersection) is proposed as a measure of similarity between them. The new similarity measure works well for short (of tens letters) sequences and the very long (of hundred thousand letters) as well. When applied to nucleotide or protein sequences may help to trace possible evolutionary of species. As an illustration, similarity of several sets of nucleotide and amino acid sequences is examined.

Kozarzewski, B
2011-01-01

473

Seven Staggering Sequences

  CERN Preprints

Summary: When my "Handbook of Integer Sequences" came out in 1973, Philip Morrison gave it an enthusiastic review in the Scientific American and Martin Gardner was kind enough to say in his Mathematical Games column that "every recreational mathematician should buy a copy forthwith." That book contained 2372 sequences. Today the "On-Line Encyclopedia of Integer Sequences" contains 117000 sequences. This paper will describe seven that I find especially interesting. These are the EKG sequence, Gijswijt's sequence, a numerical analog of Aronson's sequence, approximate squaring, the integrality of n-th roots of generating functions, dissections, and the kissing number problem. (Paper for conference in honor of Martin Gardner's 91st birthday.)

Sloane, N J A
2009-01-01

474

Towards DNA sequencing chips

  CiteSeer

Summary: Abstract. DNA sequencing is an important technology for the determina-tion of the sequences of nucleotides that make up a given DNA fragment. In view of the limitations of current sequencing technology, it would be advanta-geous to have a DNA sequencing method that provides the sequences of long DNA fragments and is amenable to automation. Sequencing by Hybridization (SBH) is a challenging alternative to the classical sequencing methods. The basic approach is to build an array (Sequencing Chip) of short DNA frag-ments of lenght I and to use biochemical methods for finding all substrings of lenght l of an unknown DNA fragment. Combinatorial algorithms are then used to reconstruct the sequence of the fragment from the l-tuple composi-tion. In this article we review biochemical, mathematical, and technological aspects of SBH and present a new sequencing chip design which might allow significant chip miniaturization without loss of the resolution of the method. 1

Pavel A. Pevzner; Robert J. Lipshutz
1994-01-01

475

Relational Sequence Alignment

  CiteSeer

Summary: Abstract. The need to measure sequence similarity arises in information extraction, music mining, biological sequence analysis, and other domains, and often coincides with sequence alignment: the more similar two sequences are, the better they can be aligned. Aligning sequences not only shows how similar sequences are, it also shows where there are differences and correspondences between the sequences. Traditionally, the alignment has been considered for sequences of flat symbols only. Many real world sequences such as protein secondary structures, however, exhibit a rich internal structures. This is akin to the problem of dealing with structured examples studied in the field of inductive logic programming (ILP). In this paper, we propose to use wellestablished ILP distance measures within alignment methods. Although straight-forward, our initial experimental results show that this approach performs well in practice and is worth to be explored. 1

Andreas Karwath; Kristian Kersting

476

Eight Hateful Sequences

  CERN Preprints

Summary: In his July 1974 Scientific American column, Martin Gardner mentioned the Handbook of Integer Sequences, which then contained 2372 sequences. Today the On-Line Encyclopedia of Integer Sequences (the OEIS) contains 140000 sequences. This paper discusses eight of them, suggested by the theme of the Eighth Gathering For Gardner: they are all infinite, and all 'ateful in one way or another. Each one is connected with an unsolved problem. The sequences are related to: hateful numbers, Angelini's 1995 puzzle, the persistence of a number, Alekseyev's 123 sequence, the curling number conjecture, Quet's prime-generating recurrence, the traveling salesman's problem, and the Riemann Hypothesis.

Sloane, N J A
2008-01-01

477

Inverse Spectral Problem Proof of Main Result

  Mathematics Websites

Summary: Inverse Spectral Problem Proof of Main Result Geodesics on Weighted Projective Spaces Zuoqin Wang of Main Result Inverse Spectral Geometry Main Result Inverse Spectral Geometry Manifold setting: (M, g Proof of Main Result Inverse Spectral Geometry Main Result Inverse Spectral Geometry Manifold setting

Stanhope, Liz

478

Perturbations Spectral sequences

  Mathematics Websites

Summary: Perturbations Spectral sequences Cascades Multicomplexes Different Approaches to Morse-Bott Homology #12;Perturbations Spectral sequences Cascades Multicomplexes Computing homology using critical points and flow lines Perturbations Generic perturbations Applications of the perturbation approach

Hurtubise, David E.

479

Parabolic geometries BGG sequences

  Mathematics Websites

Summary: Parabolic geometries are a large class of differential geometric structures, which can be described sequences Prolongation procedures Parabolic geometries are a large class of differential geometric a systematic construction of invariant differential operators for these geometries. Andreas Cap BGG sequences

Drmota, Michael

480

The LHC Sequencer

  CERN Preprints

Summary: The Large Hadron Collider (LHC) at CERN is a highly complex system made of many different sub-systems whose operation implies the execution of many tasks with stringent constraints on the order and duration of the execution. To be able to operate such a system in the most efficient and reliable way, the operators in the CERN control room use a high level control system: the LHC Sequencer. The LHC Sequencer system is composed of several components, including an Oracle database where operational sequences are configured, a core server that orchestrates the execution of the sequences, and two graphical user interfaces: one for sequence edition, and another for sequence execution. This paper describes the architecture of the LHC Sequencer system, and how the sequences are prepared and used for LHC operation.

Alemany-Fernandez, Reyes; Gorbonosov, Roman; Khasbulatov, Denis; Lamont, Mike; Le Roux, Pascal; Roderick, Chris
2011-01-01

481

Sequencing the Chimpanzee Genome

  CiteSeer

Summary: SUMMARY: Sequence of the chimpanzee (Pan troglodytes) genome should be a high priority. The chimpanzee genome sequence will have a major impact on our understanding of human disease, human evolution, and human population genetics. We propose that the first phase of a project to obtain chimpanzee sequence consist of 3-4X whole-genome shotgun coverage, with the sequence obtained from multiple individuals to provide valuable information about polymorphism rates within the species. 1.

David E. Reich; Eric S. L

482

Recycle of random sequences

  HEP - Lattice (arXiv)

Summary: The correlation between a random sequence and its transformed sequences is studied. In the case of a permutation operation or, in other word, the shuffling operation, it is shown that the correlation can be so small that the sequences can be regarded as independent random sequences. The applications to the Monte Carlo simulations are also given. This method is especially useful in the Ising Monte Carlo simulation.

Nobuyasu Ito; Macoto Kikuchi; Yutaka Okabe
1993-02-07

483

of Biological Sequence DNAGraphTool.

  CiteSeer

Summary: 1 Abstraction: DNAGraphTool was developed by Julie Bernal in 2005 in her Honour Project. It is used to display the linear structure of biological sequences and evaluate various compression models. The aims of this project is to add new functionalities into the existing platform to make it more useful and user friendly. The main works of this project relate to displaying biological sequences. During the project, Biojava which is a free opensource biological sequence analysis platform is merged to the DNAGraphTool.

Fit Advanced Projects; Minh Duc Cao; Julie Bernal; Hoang Anh Nguyen

484

Multiple Sequence Alignments of Partially Coding Nucleic Acid Sequences

  Biology and Medicine Websites

Summary: Multiple Sequence Alignments of Partially Coding Nucleic Acid Sequences Roman R. Stocsits 1 , Ivo L sequence data. Nucleic acid sequences, however, exhibit a much larger sequence heterogeneity compared use of the amino acid sequence when aligning coding nucleic acid sequences. In many cases, however

Stadler, Peter F.

485

Description of sequence files Each file comprises all partial 16s rDNA sequences (mainly of cyanobacterial origin) of

  Biology and Medicine Websites

Summary: ) are located along a precipitation gradient in the NW Negev, Israel. The southernmost study site Nizzana, with an average annual precipitation of about 130 mm and Nizzana-69 (7 km further north) with an annual precipitation of about 170 mm. The annual rainfall is characterized by a high interannual variability and occurs

Bauwe, Hermann

486

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

  Geosciences Websites

Summary: : EMPIRICAL RESULTS William Herbst1 Astronomy Department, Wesleyan University, Middletown, CT 06459; wherbst@wesleyan.edu and Reinhard Mundt Max-Planck-Institut fu¨r Astronomie, Koenigstuhl 17, D-69117 Heidelberg, Germany Received from infrared surveys of young clusters. The rapid rotators of Orion age lose less than 10

Royer, Dana

487

On Maximal Green Sequences

  CERN Preprints

Summary: Maximal green sequences are particular sequences of quiver mutations which were introduced by Keller in the context of quantum dilogarithm identities and independently by Cecotti-Cordova-Vafa in the context of supersymmetric gauge theory. Our aim is to initiate a systematic study of these sequences from a combinatorial point of view. Interpreting maximal green sequences as paths in various natural posets arising in representation theory, we prove the finiteness of the number of maximal green sequences for cluster finite quivers, affine quivers and acyclic quivers with at most three vertices. We also give results concerning the possible numbers and lengths of these maximal green sequences. Finally we describe an algorithm for computing maximal green sequences for arbitrary valued quivers which we used to obtain numerous explicit examples that we present.

Brstle, Thomas; Protin, Matthieu
2012-01-01

488

MAIN THEOREM OF COMPLEX MULTIPLICATION BRIAN CONRAD

  Mathematics Websites

Summary: MAIN THEOREM OF COMPLEX MULTIPLICATION BRIAN CONRAD In [S, Ch. IV, §18] the Main Theorem of complex and abelian varieties. The aim of the Main Theorem is as follows. Let (A, i) be an abelian variety over Q Galois automorphisms. In these notes, we give a complete proof of the Main Theorem of complex

Conrad, Brian

489

Multiple Sequence Alignments of Partially Coding Nucleic Acid Sequences

  Biology and Medicine Websites

Summary: Multiple Sequence Alignments of Partially Coding Nucleic Acid Sequences Roman R. Stocsits1 , Ivo L data. Nucleic acid sequences, however, exhibit a much larger sequence heterogeneity compared use of the amino acid sequence when aligning coding nucleic acid sequences. In many cases, however

Stadler, Peter F.

490

Astronomy & Astrophysics manuscript no. H4001 February 17, 2003 (DOI: will be inserted by hand later)

  Physics Websites

Summary: forming regions (SFR), . 1Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a #24; 1M#12; star. We consider #12;ve SFR of varying age (#26; Ophiuchi, the Orion Nebula Cluster, NGC 2264 clusters and Star Forming Regions (SFR) are ideal targets in this respect, as Send o#11;print requests to


491

A&A 402, 277292 (2003) DOI: 10.1051/0004-6361:20030203

  Physics Websites

Summary: -ray stellar activity from the age of the youngest known star forming regions (SFR), , i.e. the zero age main sequence (ZAMS) for a 1 M star. We consider five SFR of varying age parameters. Open clusters and Star Forming Regions (SFR) are ideal targets in this respect, as all stars have

Micela, Giusi

492

arXiv:0911.1483v1[astro-ph.SR]8Nov2009 Draft version November 8, 2009

  Physics Websites

Summary: enrichment which may be observed via Helium recombination lines. The excess amount of gas lost in winds may also be observed via X-ray observations as diffused X-ray sources. Subject headings: methods: N at zero age main sequence (ZAMS) at t = 0 and no gas. It is also often assumed that the IMF is a standard

Rasio, Frederic A.

493

ASTR 498N Stellar Evolution Spring 2014 Computer Exercise Due Feb 13, 2014

  Physics Websites

Summary: are to use the FORTRAN program zam.f to make a zero age main sequence solar model. Such models has a FORTRAN compiler (it may be called g77 or f77 ­ later versions like gfortran or f95 will also on your computer ­ try googling "free fortran compilers" ­ the first link is to the GNU compilers: "http

Harrington, J. Patrick

494

New games related to old and new sequences Aviezri S. Fraenkel

  Computer Technologies and Information Sciences Websites

Summary: New games related to old and new sequences Aviezri S. Fraenkel November 4, 2004 Department are related to well-known sequences, but most are new. The main result is a theorem giving necessary new family

Fraenkel, Aviezri

495

Identifying Intrinsic Disorder by Sequence Homology

  CiteSeer

Summary: Introduction Amino acid sequence determines 3D protein structure [1], with (Amino Acid Sequence) # (3D Structure) # (Function) currently being the main paradigm for the analysis of genomic protein sequences. However, there are many proteins that perform their functions and yet do not fold into specific 3D structure. The existence of such "intrinsic disorder" has led to a call for a re-assessment of the protein structure/function paradigm [4]. The focus of our work over the last few years has been the identification, analysis and prediction of intrinsic disorder [2, 3]. A major limitation of this work has been the dearth of intrinsically disordered protein examples. Here we report use of homology-based methods to find intrinsically disordered protein segments. The result is a collection currently containing 42 protein families that have 691 sequences in total, with 65,821 putative intrinsically disordered amino acids. 2 Materials and Me

Sachiko Takayama; Thomas Marshal; Christopher Oldfield; Andrew Campen; A. Keith; Dunker Celeste; J. Brown
2000-01-01

496

The Maine Coastal Current: Spring Climatological Circulation

  Engineering Websites

Summary: of Maine cyclonic circulation is persistent, with significant bimonthly modulation of key Maine Coastal­coast frontal structure. Also widely accepted is the occurrence of a large­scale cyclonic circulation


497

Intro and Main Result Outline of Proof

  Mathematics Websites

Summary: Intro and Main Result Outline of Proof The Last Two Slides Semisimple Symplectic Characters Semisimple Symplectic Characters #12;Intro and Main Result Outline of Proof The Last Two Slides Frobenius Characters #12;Intro and Main Result Outline of Proof The Last Two Slides Frobenius-Schur Indicators Let G

Argerami, Martin

498

History and previous results Main result

  Mathematics Websites

Summary: History and previous results Main result Future work Rened bounds on the number of connected;History and previous results Main result Future work Outline 1 ristory nd previous results 2 win result on a variety #12;History and previous results Main result Future work vet R e rel losed (eldF vet & R[X1

Basu, Saugata

499

Supported by: Results: Sequencing

  Computer Technologies and Information Sciences Websites

Summary: is located on the proximal long arm of the X chromosome. Middle, The eight exons are separated by introns) brings the activation domain (AD) into close proximity with the DNA-binding domain (BD), resulting the fragments. The sequences obtained were compared with the normal sequence of the gene (ENSEMBL

Groppe, Jinghua

500

Sparse Moment Sequences

  CERN Preprints

Summary: The well-known theorems of Stieltjes, Hamburger and Hausdorff establish conditions on infinite sequences of real numbers to be moment sequences. Further, works by Carath\\'{e}odory, Schur and Nevanlinna connect moment problems to problems in function theory and functions belonging to various spaces. In many problems associated with realization of a signal or an image, data may be corrupted or missing. Reconstruction of a function from moment sequences with missing terms is an interesting problem leading to advances in image and/or signal reconstruction. It is easy to show that a subsequence of a moment sequence may not be a moment sequence. Conditions are obtained to show how rigid the space of sub-moment sequences is and necessary and sufficient conditions for a sequence to be a sub-moment sequence is established. A deep connection between the sub-moment measures and the moment measures is derived and the determinacy of the moment and sub-moment problems are related. This problem is further related to complet...

Aryal, Saroj; Putinar, Mihai
2012-01-01

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