Stellar Archaeology: A Keck Pilot Program on Extremely Metal-poor Stars from the Hamburg/ESO Survey. II. Abundance Analysis
We present a detailed abundance analysis of eight stars selected as extremely metal-poor candidates from the Hamburg/ESO Survey (HES). For comparison, we have also analyzed three extremely metal-poor candidates from the HK survey, and three additional bright metal-poor stars. With this work, we have doubled the number of extremely metal-poor stars ([Fe/H] ≤ 3.0 dex) with high-precision abundance analyses. Based on this analysis, our sample of extremely metal-poor candidates from the HES contains three stars with [Fe/H] ≤ -3.0 dex, three more with [Fe/H] ≤ -2.8 dex, and two stars that are only slightly more metal-rich. Thus, the chain of procedures that led to the selection of these stars from the HES successfully provides a high fraction of extremely metal-poor stars. We verify that our choices for stellar parameters, derived in Paper I and independently of the high-dispersion spectroscopic analysis, lead to acceptable ionization and excitation balances for Fe. Substantial non-LTE effects in Fe appear to be ruled out by the above agreement, even at these extremely low metallicities. For the α-elements Mg, Si, Ca, and Ti, the light element Al, the iron-peak elements Sc, Cr, and Mn, and the neutron-capture elements Sr and Ba, we find trends in abundance ratios [X/Fe] similar to those found by previous investigations. These trends appear to be identical for giants and for dwarfs. However, the scatter in most of these ratios, even at [Fe/H] ≤ -3.0 dex, is surprisingly small. Only Sr and Ba, among the elements we examined, show scatter larger than the expected errors. Future work (the "0Z Project") will provide much stronger constraints on the scatter (or lack thereof) in elemental abundances for a substantially greater number of stars. We discuss the implications of these results for the early chemical evolution of the Galaxy, including such issues as the number of contributing supernovae and the sizes of typical protogalactic fragments in which they were born. In addition, we have identified a very metal-poor star in our sample that appears to represent the result of the s-process chain, operating in a very metal-poor environment, and exhibits extremely enhanced C, Ba, and Pb and somewhat enhanced Sr.
Additional Information© 2002 The American Astronomical Society. Received 2001 October 1; accepted 2002 March 26. Based in large part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and NASA. 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 who continue 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. The authors wish to extend special thanks to those of Hawai'ian 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. We thank the referee, Bruce Carney, for a very careful reading of the paper and for his constructive comments. T. C. B. acknowledges partial support for this work from grants AST 00-98508 and AST 00-98549 from the National Science Foundation. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.
Published - Carretta_2002_AJ_124_481.pdf
Accepted Version - 0204083.pdf