The Hamburg/ESO R-process enhanced star survey (HERES) - II. Spectroscopic analysis of the survey sample

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 similar to 54 per pixel (ranging from 17 to 308), R similar to 20 000, lambda = 3760 - 4980 angstrom. 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 3 sigma 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] less than or similar to - 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.