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 S i line and the stronger λλ9212.9, 9237.5 pair of S i lines. For one star, the S abundance is also derived from the S i 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. Conclusions. The trend of S/Fe vs. Fe/H corresponds to the trends of Mg/Fe, Si/Fe, and Ca/Fe and indicates that sulphur in Galactic halo stars has been made by α-capture processes in massive SNe. The observed scatter in S/Fe is much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.Key words. stars: abundances -stars: atmospheres -Galaxy: halo -galaxies: abundances -galaxies: high-redshift IntroductionDespite several recent papers on sulphur abundances in Galactic stars, there is still no agreement on the trend of [S/Fe] Additional problems have been revealed by Takeda et al. (2005), who find that S abundances derived from the weak λ8694.6 S i line are systematically higher than S abundances derived from the stronger λλ9212.9, 9237.5 pair of lines when their new non-LTE corrections are applied.Article published by EDP Sciences and available at

The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars

“…The Hβ-based temperatures are higher by 50-100 K when [Fe/H] > −2, but lower by about 100 K for [Fe/H] < −2.5. In the transition region, −2.5 ≤ [Fe/H] ≤ −2, there is a large scatter (Nissen et al 2007). This is essentially the same behaviour as just discussed when comparing with the estimates by …”

“…This UVES sample is the same as that used by Nissen et al (2007); therefore we direct the reader to the description of the observations therein. Here we just mention that the UVES spectra cover the blue region 3800−5000 Å and the near-IR region 6700−10500 Å, both with a resolving power R ≡ λ/Δλ 60 000.…”

“…For a detailed discussion of the procedure and the improved accuracy of the T eff determinations, see Nissen et al (2007). As discussed therein, differential values are determined with a precision of ∼30 K for metal-poor turnoff stars.…”

Sulfur in the globular clusters 47 Tucanae and NGC 6752

“…Sulfur abundances in Milky Way (MW) stars have been obtained by several authors in the last years (Nissen et al 2007; Based on observations made with the ESO VLT-Kueyen telescope at the Paranal Observatory, Chile, in the course of the ESO-Large Programme 165.L-0263. Caffau et al 2007a;Ryde 2006;Caffau et al 2005b;Korn & Ryde 2005;Nissen et al 2004;Ryde & Lambert 2004, among the most recent), but agreement on its behavior at low metallicities has not been reached yet.…”

“…While some of the cited authors (e.g. Nissen et al 2007) find that S follows the same trend as other α-elements ( [S/Fe] showing a plateau at low metallicities), others point towards a steady growth in [S/Fe] with decreasing metallicity (Israelian & Rebolo 2001;Takada-Hidai et al 2002), or an increase in the scatter of [S/Fe] at low [Fe/H] (Caffau et al 2005b). The cited debate can in fact be extended to the overall behavior of α-elements.…”