EDP Sciences
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Volume 415, Number 3, March I 2004
Page(s) 993 - 1007
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20034063

A&A 415, 993-1007 (2004)
DOI: 10.1051/0004-6361:20034063

Sulphur and zinc abundances in Galactic stars and damped Ly $\alpha$ systems

P. E. Nissen1, Y. Q. Chen2, M. Asplund3 and M. Pettini4

1  Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
2  National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
    e-mail: cyq@bao.ac.cn
3  Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia
    e-mail: martin@mso.anu.edu.au
4  Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
    e-mail: pettini@ast.cam.ac.uk

(Received 9 July 2003 / Accepted 19 November 2003 )

High resolution spectra of 34 halo population dwarf and subgiant stars have been obtained with VLT/UVES and used to derive sulphur abundances from the $\lambda \lambda 8694.0, 8694.6$ and $\lambda \lambda 9212.9, 9237.5$ $\ion{S}{i}$ lines. In addition, iron abundances have been determined from 19  $\ion{Fe}{ii}$ lines and zinc abundances from the $\lambda \lambda 4722.2, 4810.5$ lines. The abundances are based on a classical 1D, LTE model atmosphere analysis, but effects of 3D hydrodynamical modelling on the [S/Fe], [Zn/Fe] and [S/Zn] ratios are shown to be small. We find that most halo stars with metallicities in the range $-3.2 < \mbox{\rm [Fe/H]}< -0.8$ have a near-constant $\mbox{\rm [S/Fe]}\simeq +0.3$; a least square fit to [S/Fe] vs. [Fe/H] shows a slope of only $-0.04 \pm 0.01$. Among halo stars with $-1.2 < \mbox{\rm [Fe/H]}< -0.8$ the majority have $\mbox{\rm [S/Fe]}\simeq +0.3$, but two stars (previously shown to have low $\alpha$/Fe ratios) have $\mbox{\rm [S/Fe]}\simeq 0.0$. For disk stars with $\mbox{\rm [Fe/H]}> -1$, [S/Fe] decreases with increasing [Fe/H]. Hence, sulphur behaves like other typical $\alpha$-capture elements, Mg, Si and Ca. Zinc, on the other hand, traces iron over three orders of magnitude in [Fe/H], although there is some evidence for a small systematic Zn overabundance ( $\mbox{\rm [Zn/Fe]}\simeq +0.1$) among metal-poor disk stars and for halo stars with $\mbox{\rm [Fe/H]}< -2.0$. Recent measurements of S and Zn in ten damped Ly $\alpha$ systems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances in the range $-2.1 < \mbox{\rm [Zn/H]}< -0.15$ show an offset relative to the [S/Zn] - [Zn/H] relation in Galactic stars. Possible reasons for this offset are discussed, including low and intermittent star formation rates in DLAs.

Key words: stars: abundances -- stars: atmospheres -- galaxy: evolution -- galaxies: high-redshift -- quasars: absorption lines

Offprint request: P. E. Nissen, pen@phys.au.dk

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