Volume 416, Number 3, March IV 2004
|Page(s)||1117 - 1138|
|Published online||09 March 2004|
GEPI, Observatoire de Paris-Meudon, 92125 Meudon Cedex, France
2 GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France
3 Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824, USA
4 European Southern Observatory (ESO), Karl Schwarschild-Str. 2, 85749 Garching b. München, Germany
5 Astronomical Observatory, NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
6 Nordic Optical Telescope Scientific Association, Apartado 474, 38 700 Santa Cruz de La Palma, Spain
7 IAG, Universidade de Sao Paulo, Departamento de Astronomia, CP 3386, 01060-970 Sao Paulo, Brazil
8 Osservatorio Astronomico di Trieste, INAF, Via G.B. Tiepolo 11, 34131 Trieste, Italy
9 Lund Observatory, Box 43, 221 00 Lund, Sweden
Corresponding author: R. Cayrel, firstname.lastname@example.org
Accepted: 29 October 2003
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. ([CITE], [CITE]), emphasizing stars of extremely low metallicity: 30 of our 35 stars are in the range , and 22 stars have . Our new VLT/UVES spectra, at a resolving power of 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. ([CITE]) and Ryan et al. ([CITE]). 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 ( nm). 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 0.05 dex. 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 100 . Moreover, the composition of the ejecta that have enriched the matter
© ESO, 2004
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