2004A&A...416.1117C

In the framework of the ESO Large Programme ``First Stars'', very high-quality spectra of some 70 very metal-poor dwarfs and giants were obtained with the ESO VLT and UVES spectrograph. These stars are likely to have descended from the first generation(s) of stars formed after the Big Bang, and their detailed composition provides constraints on issues such as the nature of the first supernovae, the efficiency of mixing processes in the early Galaxy, the formation and evolution of the halo of the Galaxy, and the possible sources of reionization of the Universe. This paper presents the abundance analysis of an homogeneous sample of 35 giants selected from the HK survey of Beers et al. (1992AJ....103.1987B, 1999AJ....117..981B), emphasizing stars of extremely low metallicity: 30 of our 35 stars are in the range -4.1<[Fe/H]←2.7, and 22 stars have [Fe/H]←3.0. Our new VLT/UVES spectra, at a resolving power of R∼45000 and with signal-to-noise ratios of 100-200 per pixel over the wavelength range 330-1000 nm, are greatly superior to those of the classic studies of McWilliam et al. (1995AJ....109.2757M) and Ryan et al. (1996ApJ...471..254R). The immediate objective of the work is to determine precise, comprehensive, and homogeneous element abundances for this large sample of the most metal-poor giants presently known. In the analysis we combine the spectral line modeling code ``Turbospectrum'' with OSMARCS model atmospheres, which treat continuum scattering correctly and thus allow proper interpretation of the blue regions of the spectra, where scattering becomes important relative to continuous absorption (λ<400nm). We obtain detailed information on the trends of elemental abundance ratios and the star-to-star scatter around those trends, enabling us to separate the relative contributions of cosmic scatter and observational/analysis errors. Abundances of 17 elements from C to Zn have been measured in all stars, including K and Zn, which have not previously been detected in stars with [Fe/H]←3.0. Among the key results, we discuss the oxygen abundance (from the forbidden [OI] line), the different and sometimes complex trends of the abundance ratios with metallicity, the very tight relationship between the abundances of certain elements (e.g., Fe and Cr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within the error bars, the trends of the abundance ratios with metallicity are consistent with those found in earlier literature, but in many cases the scatter around the average trends is much smaller than found in earlier studies, which were limited to lower-quality spectra. We find that the cosmic scatter in several element ratios may be as low as 0.05dex. The evolution of the abundance trends and scatter with declining metallicity provides strong constraints on the yields of the first supernovae and their mixing into the early ISM. The abundance ratios found in our sample do not match the predicted yields from pair-instability hypernovae, but are consistent with element production by supernovae with progenitor masses up to 100M_☉. Moreover, the composition of the ejecta that have enriched the matter now contained in our very metal-poor stars appears surprisingly uniform over the range 4.0≤[Fe/H]←3.0. This would indicate either that we are observing the products of very similar primordial bursts of high-mass stars, or that the mixing of matter from different bursts of early star formation was extremely rapid. In any case, it is unlikely that we observed the ejecta from individual (single) supernovae (as has often been concluded in previous work), as we do not see scatter due to different progenitor masses. The abundance ratios at the lowest metallicities (-4.0≤[Fe/H]≤-3.0) are compatible with those found by McWilliam et al. (1995AJ....109.2757M) and later studies. However, when elemental ratios are plotted with respect to Mg, we find no clear slopes below [Mg/H]=-3, but rather, a plateau-like behavior defining a set of initial yeilds.