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 8 stars selected as extremelymetal poor candidates from the Hamburg/ESO Survey (HES). For comparison, wehave also analysed 3 extremely metal-poor candidates from the HK survey, and 3additional bright metal-poor stars. With this work, we have doubled the numberof extremely metal-poor stars ([Fe/H]$\le 3.0$) with high-precision abundanceanalyses. Our sample of extremely metal-poor candidates from the HES contains 3stars with [Fe/H] $\le -3.0$, 3 more with [Fe/H]$\le -2.8$, and 2 stars thatare only slightly more metal rich. Thus, the chain of procedures that led tothe selection of these stars from the HES successfully provides a high fractionof extremely metal-poor stars. We verify that our stellar parameters, derivedin Paper I, lead to acceptable ionization and excitation balances for Fe,ruling out substantial non-LTE effects in Fe. For the $\alpha-$elements Mg, Si,Ca, Ti, the light element Al, the iron-peak elements Sc, Cr, Mn, and theneutron capture elements Sr and Ba, we find trends in abundance ratios [X/Fe]similar to those found by previous studies. However,the scatter in most ofthese ratios, even at [Fe/H]$\le -3.0$ dex, is surprisingly small. Only Sr andBa show scatter larger than the expected errors. Future work (the 0Z project)will provide much stronger constraints on the scatter (or lack thereof) inabundances for a greater number of stars. We discuss the implications of theseresults for the early chemical evolution of the Galaxy, including such issuesas the number of contributing SN, and the sizes of typical fragments in whichthey were born. In addition, we have identified a very metal poor star thatappears to be the result of the s-process chain, operating in a very metal-poorenvironment, with extremely enhanced C, Ba, and Pb, and somewhat enhanced Sr.Comment: 36 pages, 9 tables, 14 figures included; accepted for publication in the July 2002 issue of The Astronomical Journa