Sulphur and zinc abundances in Galactic halo stars revisited

Abstract

Aims. Based on a new set of sulphur abundances in very metal-poor stars and an improved analysis of previous data, we aim at resolving current discrepancies on the trend of S/Fe vs. Fe/H and thereby gain better insight into the nucleosynthesis of sulphur. The trends of Zn/Fe and S/Zn will also be studied. Methods. High resolution VLT/UVES spectra of 40 main-sequence stars with −3.3 < [Fe/H] < −1.0 are used to derive S abundances from the weak λ8694.6 Si line and the stronger λλ9212.9, 9237.5 pair of Si lines. For one star, the S abundance is also derived from the Si triplet at 1.046 µm recently observed with the VLT infrared echelle spectrograph CRIRES. Fe and Zn abundances are derived from lines in the blue part of the UVES spectra, and effective temperatures are obtained from the profile of the Hβ line. Results. Comparison of sulphur abundances from the weak and strong Si lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE corrections into account, the Galactic halo stars distribute around a plateau at [S/Fe] ∼ +0.2 dex with a scatter of 0.07 dex only. [Zn/Fe] is close to zero for metallicities in the range −2.0 < [Fe/H] < −1.0 but increases to a level of [Zn/Fe] ∼ +0.1 to +0.2 dex in the range −2.7 < [Fe/H] < −2.0. At still lower metallicities [Zn/Fe] rises steeply to a value of [Zn/Fe] ∼ +0.5dex at [Fe/H] = −3.2.