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Abundances In Very Metal-Poor Dwarf Stars

Cohen, Judith G. and Christlieb, Norbert and McWilliam, Andrew and Shectman, Steve and Thompson, Ian and Wasserburg, G. J. and Ivans, Inese and Dehn, Matthias and Karlsson, Torgny and Meléndez, J. (2004) Abundances In Very Metal-Poor Dwarf Stars. Astrophysical Journal, 612 (2). pp. 1107-1135. ISSN 0004-637X. doi:10.1086/422576. https://resolver.caltech.edu/CaltechAUTHORS:20190522-083404445

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Abstract

We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far no additional turnoff stars with [Fe/H] < -3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the α-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other α-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by ~0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that α-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with ^(12)C/^(13)C ~7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1086/422576DOIArticle
https://arxiv.org/abs/astro-ph/0405286arXivDiscussion Paper
ORCID:
AuthorORCID
Cohen, Judith G.0000-0002-8039-4673
Christlieb, Norbert0000-0002-4043-2727
Wasserburg, G. J.0000-0002-7957-8029
Meléndez, J.0000-0002-4933-2239
Additional Information:© 2004 The American Astronomical Society. Received 2004 May 5; accepted 2004 May 13. Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The entire Keck/HIRES user community owes a huge debt to Jerry Nelson, Gerry Smith, Steve Vogt, and many other people who have worked to make the Keck Telescope and HIRES a reality and to operate and maintain the Keck Observatory. We are grateful to the W. M. Keck Foundation for the vision to fund the construction of the W. M. Keck Observatory. We thank Steve Shectman and the many people who worked to design and build the Magellan Telescopes. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. J. G. C. and J. M. are grateful for partial support from NSF grant AST 02-05951. N. C. acknowledges financial support through a Henri Chretien International Research Grant administered by the American Astronomical Society, and he is very grateful to Gaston Araya and J. G. C. for their hospitality in Altadena. I. I. I. acknowledges research funding from NASA through Hubble Fellowship grant HST-HF-01151.01-A from the Space Telescope Science Inst., operated by AURA, under NASA contract NAS5-26555.
Funders:
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
NSFAST 02-05951
American Astronomical SocietyUNSPECIFIED
NASA Hubble FellowshipHST-HF-01151.01-A
NASANAS5-26555
Subject Keywords:Galaxy: abundances — Galaxy: halo — stars: abundances — stars: chemically peculiar — stars: Population II
Issue or Number:2
DOI:10.1086/422576
Record Number:CaltechAUTHORS:20190522-083404445
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190522-083404445
Official Citation:Judith G. Cohen et al 2004 ApJ 612 1107
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95686
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:22 May 2019 22:30
Last Modified:16 Nov 2021 17:14

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