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Title: THE C+N+O ABUNDANCE OF {omega} CENTAURI GIANT STARS: IMPLICATIONS FOR THE CHEMICAL-ENRICHMENT SCENARIO AND THE RELATIVE AGES OF DIFFERENT STELLAR POPULATIONS

Abstract

We present a chemical-composition analysis of 77 red-giant stars in Omega Centauri. We have measured abundances for carbon and nitrogen, and combined our results with abundances of O, Na, La, and Fe that we determined in our previous work. Our aim is to better understand the peculiar chemical-enrichment history of this cluster by studying how the total C+N+O content varies among the different metallicity stellar groups, and among stars at different places along the Na-O anticorrelation. We find that the (anti)correlations among the light elements that would be expected on theoretical grounds for matter that has been nuclearly processed via high-temperature proton captures. The overall [(C+N+O)/Fe] increases by {approx}0.5 dex from [Fe/H] {approx}-2.0 to [Fe/H] {approx}-0.9. Our results provide insight into the chemical-enrichment history of the cluster, and the measured CNO variations provide important corrections for estimating the relative ages of the different stellar populations.

Authors:
 [1]; ;  [2];  [3];  [4];  [5];  [6]; ;  [7];  [8]
  1. Max-Planck-Institut fuer Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei Muenchen (Germany)
  2. Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands (Spain)
  3. Dipartimento di Astronomia, Universita di Padova, Vicolo dell'Osservatorio 3, Padova, I-35122 (Italy)
  4. INAF-Osservatorio Astronomico di Teramo, Via M. Maggini, 64100 Teramo (Italy)
  5. INAF-Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio (Italy)
  6. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  7. INAF-Osservatorio Astronomico di Padova, Vicolo dellOsservatorio 5, 35122 Padova (Italy)
  8. Departamento de Astronomia, Universidad de Concepcion, Casilla 160-C, Concepcion (Chile)
Publication Date:
OSTI Identifier:
22011852
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 746; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; CARBON; CHEMICAL COMPOSITION; NITROGEN; RED GIANT STARS

Citation Formats

Marino, A. F., Milone, A. P., Aparicio, A., Piotto, G., Cassisi, S., D'Antona, F., Anderson, J., Bedin, L. R., Renzini, A., and Villanova, S., E-mail: amarino@MPA-Garching.MPG.DE, E-mail: milone@iac.es, E-mail: aparicio@iac.es, E-mail: giampaolo.piotto@unipd.it, E-mail: cassisi@oa-teramo.inaf.it, E-mail: dantona@oa-roma.inaf.it, E-mail: jayander@stsci.edu, E-mail: luigi.bedin@oapd.inaf.it, E-mail: alvio.renzini@oapd.inaf.it, E-mail: svillanova@astro-udec.cl. THE C+N+O ABUNDANCE OF {omega} CENTAURI GIANT STARS: IMPLICATIONS FOR THE CHEMICAL-ENRICHMENT SCENARIO AND THE RELATIVE AGES OF DIFFERENT STELLAR POPULATIONS. United States: N. p., 2012. Web. doi:10.1088/0004-637X/746/1/14.
Marino, A. F., Milone, A. P., Aparicio, A., Piotto, G., Cassisi, S., D'Antona, F., Anderson, J., Bedin, L. R., Renzini, A., & Villanova, S., E-mail: amarino@MPA-Garching.MPG.DE, E-mail: milone@iac.es, E-mail: aparicio@iac.es, E-mail: giampaolo.piotto@unipd.it, E-mail: cassisi@oa-teramo.inaf.it, E-mail: dantona@oa-roma.inaf.it, E-mail: jayander@stsci.edu, E-mail: luigi.bedin@oapd.inaf.it, E-mail: alvio.renzini@oapd.inaf.it, E-mail: svillanova@astro-udec.cl. THE C+N+O ABUNDANCE OF {omega} CENTAURI GIANT STARS: IMPLICATIONS FOR THE CHEMICAL-ENRICHMENT SCENARIO AND THE RELATIVE AGES OF DIFFERENT STELLAR POPULATIONS. United States. doi:10.1088/0004-637X/746/1/14.
Marino, A. F., Milone, A. P., Aparicio, A., Piotto, G., Cassisi, S., D'Antona, F., Anderson, J., Bedin, L. R., Renzini, A., and Villanova, S., E-mail: amarino@MPA-Garching.MPG.DE, E-mail: milone@iac.es, E-mail: aparicio@iac.es, E-mail: giampaolo.piotto@unipd.it, E-mail: cassisi@oa-teramo.inaf.it, E-mail: dantona@oa-roma.inaf.it, E-mail: jayander@stsci.edu, E-mail: luigi.bedin@oapd.inaf.it, E-mail: alvio.renzini@oapd.inaf.it, E-mail: svillanova@astro-udec.cl. Fri . "THE C+N+O ABUNDANCE OF {omega} CENTAURI GIANT STARS: IMPLICATIONS FOR THE CHEMICAL-ENRICHMENT SCENARIO AND THE RELATIVE AGES OF DIFFERENT STELLAR POPULATIONS". United States. doi:10.1088/0004-637X/746/1/14.
@article{osti_22011852,
title = {THE C+N+O ABUNDANCE OF {omega} CENTAURI GIANT STARS: IMPLICATIONS FOR THE CHEMICAL-ENRICHMENT SCENARIO AND THE RELATIVE AGES OF DIFFERENT STELLAR POPULATIONS},
author = {Marino, A. F. and Milone, A. P. and Aparicio, A. and Piotto, G. and Cassisi, S. and D'Antona, F. and Anderson, J. and Bedin, L. R. and Renzini, A. and Villanova, S., E-mail: amarino@MPA-Garching.MPG.DE, E-mail: milone@iac.es, E-mail: aparicio@iac.es, E-mail: giampaolo.piotto@unipd.it, E-mail: cassisi@oa-teramo.inaf.it, E-mail: dantona@oa-roma.inaf.it, E-mail: jayander@stsci.edu, E-mail: luigi.bedin@oapd.inaf.it, E-mail: alvio.renzini@oapd.inaf.it, E-mail: svillanova@astro-udec.cl},
abstractNote = {We present a chemical-composition analysis of 77 red-giant stars in Omega Centauri. We have measured abundances for carbon and nitrogen, and combined our results with abundances of O, Na, La, and Fe that we determined in our previous work. Our aim is to better understand the peculiar chemical-enrichment history of this cluster by studying how the total C+N+O content varies among the different metallicity stellar groups, and among stars at different places along the Na-O anticorrelation. We find that the (anti)correlations among the light elements that would be expected on theoretical grounds for matter that has been nuclearly processed via high-temperature proton captures. The overall [(C+N+O)/Fe] increases by {approx}0.5 dex from [Fe/H] {approx}-2.0 to [Fe/H] {approx}-0.9. Our results provide insight into the chemical-enrichment history of the cluster, and the measured CNO variations provide important corrections for estimating the relative ages of the different stellar populations.},
doi = {10.1088/0004-637X/746/1/14},
journal = {Astrophysical Journal},
number = 1,
volume = 746,
place = {United States},
year = {Fri Feb 10 00:00:00 EST 2012},
month = {Fri Feb 10 00:00:00 EST 2012}
}
  • We have investigated the color-magnitude diagram of {omega} Centauri and find that the blue main sequence (bMS) can be reproduced only by models that have a helium abundance in the range Y = 0.35-0.40. To explain the faint subgiant branch of the reddest stars ('MS-a/RG-a' sequence), isochrones for the observed metallicity ([Fe/H] Almost-Equal-To -0.7) appear to require both a high age ({approx}13 Gyr) and enhanced CNO abundances ([CNO/Fe] Almost-Equal-To 0.9). Y Almost-Equal-To 0.35 must also be assumed in order to counteract the effects of high CNO on turnoff colors and thereby to obtain a good fit to the relatively bluemore » turnoff of this stellar population. This suggests a short chemical evolution period of time (<1 Gyr) for {omega} Cen. Our intermediate-mass (super-)asymptotic giant branch (AGB) models are able to reproduce the high helium abundances, along with [N/Fe] {approx}2 and substantial O depletions if uncertainties in the treatment of convection are fully taken into account. These abundance features distinguish the bMS stars from the dominant [Fe/H] Almost-Equal-To -1.7 population. The most massive super-AGB stellar models (M{sub ZAMS} {>=} 6.8 M{sub Sun }, M{sub He,core} {>=} 1.245 M{sub Sun }) predict too large N enhancements, which limit their role in contributing to the extreme populations. In order to address the observed central concentration of stars with He-rich abundance, we show here quantitatively that highly He- and N-enriched AGB ejecta have particularly efficient cooling properties. Based on these results and on the reconstruction of the orbit of {omega} Cen with respect to the Milky Way, we propose the Galactic plane passage gas purging scenario for the chemical evolution of this cluster. The bMS population formed shortly after the purging of most of the cluster gas as a result of the passage of {omega} Cen through the Galactic disk (which occurs today every {approx}40 Myr for {omega} Cen) when the initial mass function of the dominant population had 'burned' through most of the Type II supernova mass range. AGB stars would eject most of their masses into the gas-depleted cluster through low-velocity winds that sink to the cluster core due to their favorable cooling properties and form the bMS population. In our discussion we follow our model through four passage events, which could explain some key properties not only of the bMS but also of the MS-a/RGB-a and the s-enriched stars.« less
  • Abundances of C, N, and O are determined in four bright red giants that span the known abundance range for light (Na and Al) and s-process (Zr and La) elements in the globular cluster NGC 1851. The abundance sum C+N+O exhibits a range of 0.6 dex, a factor of 4, in contrast to other clusters in which no significant C+N+O spread is found. Such an abundance range offers support for the Cassisi et al. scenario in which the double subgiant branch populations are coeval but with different mixtures of C+N+O abundances. Further, the Na, Al, Zr, and La abundances aremore » correlated with C+N+O, and therefore NGC 1851 is the first cluster to provide strong support for the scenario in which asymptotic giant branch stars are responsible for the globular cluster light element abundance variations.« less
  • Abundances relative to iron for carbon, nitrogen, strontium, and barium are presented for 33 stars on the red giant branch (RGB) of the globular cluster {omega} Centauri. They are based on intermediate-resolution spectroscopic data covering the blue spectral region analyzed using spectrum synthesis techniques. The data reveal the existence of a broad range in the abundances of these elements, and a comparison with similar data for main-sequence stars enables insight into the evolutionary history of the cluster. The majority of the RGB stars were found to be depleted in carbon, i.e., [C/Fe] < 0, while [N/Fe] for the same starsmore » shows a range of {approx}1 dex, from [N/Fe] {approx} 0.7 to 1.7 dex. The strontium-to-iron abundance ratios varied from solar to mildly enhanced (0.0 {<=} [Sr/Fe] {<=} 0.8), with [Ba/Fe] generally equal to or greater than [Sr/Fe]. The carbon and nitrogen abundance ratios for the one known CH star in the sample, ROA 279, are [C/Fe] = 0.6 and [N/Fe] = 0.5 dex. Evidence for evolutionary mixing on the RGB is found from the fact that the relative carbon abundances on the main sequence are generally higher than those on the RGB. However, comparison of the RGB and main-sequence samples shows that the upper level of nitrogen enhancement is similar in both sets at [N/Fe] {approx} 2.0 dex. This is most likely the result of primordial rather than evolutionary mixing processes. One RGB star, ROA 276, was found to have Sr and Ba abundance ratios similar to the anomalous Sr-rich main-sequence star S2015448. High-resolution spectra of ROA 276 were obtained with the Magellan Telescope/MIKE spectrograph combination to confirm this result, revealing that ROA 276 is indeed an unusual star. For this star, calculations of the depletion effect, the potential change in surface abundance that results from the increased depth of the convective envelope as a star moves from the main sequence to the RGB, strongly suggest that the observed Sr enhancement in ROA 276 is of primordial origin, rather than originating from a surface accretion event.« less
  • Oxygen and carbon are important elements in stellar populations. Their behavior refers to the formation history of the stellar populations. C and O abundances would also obviously influence stellar opacities and the overall metal abundance Z . With observed high-quality spectroscopic properties, we construct stellar models with C and O elements to give more accurate ages for 70 metal-poor dwarfs, which have been determined to be high- α halo, low- α halo, and thick-disk stars. Our results show that high- α halo stars are somewhat older than low- α halo stars by around 2.0 Gyr. The thick-disk population has anmore » age range in between the two halo populations. The age distribution profiles indicate that high- α halo and low- α halo stars match the in situ accretion simulation by Zolotov et al., and the thick-disk stars might be formed in a relatively quiescent and long-lasting process. We also note that stellar ages are very sensitive to O abundance, since the ages clearly increase with increasing [O/Fe] values. Additionally, we obtain several stars with peculiar ages, including 2 young thick-disk stars and 12 stars older than the universe age.« less
  • We present the latitude-normalized radial velocity (v{sub b} ) distribution of 3318 subsolar metallicity, V {approx}< 13.5 stars from the Grid Giant Star Survey (GGSS) in southern hemisphere fields. The sample includes giants mostly within {approx}5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde 'cloud' of stars at l {approx} 285 Degree-Sign and extended, retrograde 'streams' visible as relatively tight l-v{sub b} sequences. One sequence in the fourth Galactic quadrant lies within the l-v{sub b} spacemore » expected to contain tidal debris from the 'star cluster' {omega}Centauri. Not only does {omega}Cen lie precisely in this l-v{sub b} sequence, but the positions and v{sub b} of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with {omega}Cen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the {omega}Cen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in {omega}Cen. The newly discovered {omega}Cen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests {omega}Cen is a dominant contributor of retrograde giant stars in the inner Galaxy.« less