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Detailed Abundances of Two Very Metal-Poor Stars in Dwarf Galaxies

Kirby, Evan N. and Cohen, Judith G. (2012) Detailed Abundances of Two Very Metal-Poor Stars in Dwarf Galaxies. Astronomical Journal, 144 (6). Art. No. 168. ISSN 0004-6256. doi:10.1088/0004-6256/144/6/168.

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The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[α/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at –3 < [Fe/H] < –2 sample SNe with [Fe/H] Lt –3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] ~ –3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo.

Item Type:Article
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URLURL TypeDescription Paper
Kirby, Evan N.0000-0001-6196-5162
Cohen, Judith G.0000-0002-8039-4673
Additional Information:© 2012 The American Astronomical Society. Received 2011 May 18; accepted 2012 September 17; published 2012 November 8. Data herein were obtained at theW. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank the referee for a careful report that greatly improved the quality of our study. We also thank K. Lind for providing a customized table of NLTE corrections for sodium. Support for this work was provided by NASA through Hubble Fellowship grant 51256.01 awarded to E.N.K. by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. J.G.C. thanks NSF grant AST-0908139 for partial support. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. Facility: Keck:I (HIRES) - KECK I Telescope.
Funding AgencyGrant Number
NASA Hubble Fellowship51256.01
NASANAS 5-26555
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:galaxies: abundances – galaxies: dwarf – galaxies: evolution – Local Group
Issue or Number:6
Record Number:CaltechAUTHORS:20121004-112928567
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Official Citation:Evan N. Kirby and Judith G. Cohen 2012 AJ 144 168
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:34682
Deposited By: Tony Diaz
Deposited On:04 Oct 2012 21:04
Last Modified:09 Nov 2021 23:09

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