Abstract. We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars −3.8 ≤ [Fe/H] ≤ −1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically S /N ∼ 54 per pixel (ranging from 17 to 308), R ∼ 20 000, λ = 3760-4980 Å. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3σ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S /N, or when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of [Fe/H] = −2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with [Fe/H] < −3 and 181 stars with −3 < [Fe/H] < −2. We find one star with [Fe/H] < −3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at [Fe/H] < ∼ −2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.

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We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R ∼ 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. Although currently about ten stars at [Fe/H] < −3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] ∼ −5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < −3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.

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We demonstrate that there are systematic scale errors in the [Fe/H] values determined by the Hamburg/ESO Survey (HES; and by inference by the HK survey in the past) for certain extremely metal-poor, highly C-enhanced giants. The consequences of these scale errors are that (1) the fraction of carbon stars at extremely low metallicities has been overestimated in several papers in the recent literature, (2) the number of extremely metal-poor stars known is somewhat lower than has been quoted in the recent literature, and (3) the yield for extremely metalpoor stars by the HES is somewhat lower than is stated in the recent literature. A preliminary estimate for the frequency of carbon stars among the giants in the HES sample with Ϫ4 ! [Fe/H] ! Ϫ2.0 dex is 7.4% ‫ע‬ 2.9%; adding an estimate for the C-enhanced giants with [C/Fe] 1 1.0 dex without detectable C 2 bands raises the fraction to 14% ‫ע‬ 4%. We rely on the results of an extensive set of homogeneous, detailed abundance analyses of stars expected to have [Fe/H] ≤ Ϫ3.0 dex selected from the HES to establish these claims. We have found that the Fe metallicity of the cooler ( K) C stars as derived from spectra taken with HIRES at Keck are a T Շ 5200 eff factor of ∼10 higher than those obtained via the algorithm used by the HES project to analyze the moderateresolution follow-up spectra, and this algorithm is identical to that used until very recently by the HK survey. This error in the Fe abundance estimate for C stars arises from a lowering of the emitted flux in the continuum bandpasses of the KP index (Ca ii l3933) and, particularly, the HP2 (Hd) index used to estimate [Fe/H] due to absorption from strong molecular bands.