Sulphur and zinc abundances in Galactic stars and damped Lyα systems

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 lambdalambda8694.0, 8694.6 and lambdalambda9212.9, 9237.5 S I lines. In addition, iron abundances have been determined from 19 Fell lines and zinc abundances from the lambdalambda4722.2,4810.5 lines. The abundances are based on a classical ID, 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 < [Fe/H] < -0.8 have a near-constant [S/Fe] similar or equal to +0.3; a least square fit to [S/Fe] vs. [Fe/H] shows a slope of only -0.04 +/- 0.01. Among halo stars with -1.2 < [Fe/H] < -0.8 the majority have [S/Fe] = +0.3, but two stars (previously shown to have low alpha/Fe ratios) have [S/Fe] similar or equal to 0.0. For disk stars with [Fe/H] > - 1, [S/Fe] decreases with increasing [Fe/H]. Hence, sulphur behaves like other typical a-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 ([Zn/Fe] similar or equal to +0.1) among metal-poor disk stars and for halo stars with [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 < [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.