EDP Sciences
Free access
Volume 403, Number 3, June I 2003
Page(s) 1105 - 1114
Section Formation and evolution of planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20030438

A&A 403, 1105-1114 (2003)
DOI: 10.1051/0004-6361:20030438

First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars

P. François1, E. Depagne1, V. Hill1, M. Spite1, F. Spite1, B. Plez2, T. C. Beers3, B. Barbuy4, R. Cayrel1, J. Andersen5, P. Bonifacio6, P. Molaro6, B. Nordström7, 5 and F. Primas8

1  Observatoire de Paris-Meudon, GEPI, 92195 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  IAG, Universidade de São Paulo, Departamento de Astronomia, CP 3386, 01060-970 São Paulo, Brazil
5  Astronomical Observatory, NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
6  Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
7  Lund Observatory, Box 43, 221 00 Lund, Sweden
8  European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85749 Garching b. München, Germany

(Received 27 December 2002 / Accepted 21 March 2003 )

This paper reports detailed abundance analyses for four extremely metal-poor (XMP) giant stars with $\rm [Fe/H]<-3.8$, based on high-resolution, high- S/N spectra from the ESO VLT (Kueyen/UVES) and LTE model atmosphere calculations. The derived [ $\alpha$/Fe]  ratios in our sample exhibit a small dispersion, confirming previous findings in the literature, i.e. a constant overabundance of the $\alpha$-elements with a very small (if any) dependence on [Fe/H]. In particular, the very small scatter we determine for [Si/Fe] suggests that this element shows a constant overabundance at very low metallicity, a conclusion which could not have been derived from the widely scattered [Si/Fe] values reported in the literature for less metal-poor stars. For the iron-peak elements, our precise abundances for the four XMP stars in our sample confirm the decreasing trend of Cr and Mn with decreasing [Fe/H], as well as the increasing trend for Co and the absence of any trend for Sc and Ni. In contrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], we find [Sr/Ba] in our sample to be roughly solar, with a much lower dispersion than previously found for stars in the range $\rm -3.5 < [Fe/H] <
-2.5$ .

Key words: stars: abundances -- stars: Population II -- Galaxy: abundances -- Galaxy: halo -- nuclear reactions, nucleosynthesis, abundances

Offprint request: P. François, Patrick.Francois@obspm.fr

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