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] less than or equal to 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] less than or equal to -3.0 dex, three more with [Fe/H] less than or equal to -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 alpha-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] less than or equal to 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.