Galactic chemical evolution of sulphur

Sulphur abundances from the [S i] λ1082 nm line in giants ^{⋆,⋆⋆,⋆⋆⋆}

¹ Department of Astronomy and Theoretical PhysicsLund Observatory, Lund University, Box 43 221 00, Lund, Sweden
e-mail: ryde@astro.lu.se
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge MA 02138, USA

Received: 18 July 2013
Accepted: 29 August 2013

Abstract

Context. The Galactic chemical evolution of sulphur is still under debate. At low metallicities some studies find no correlation between [S/Fe] and [Fe/H], which is typical for α-elements, while others find [S/Fe] increasing towards lower metallicities, and still others find a combination of the two. Each scenario has different implications for the Galactic chemical evolution of sulphur.

Aims. The aim of this study is to contribute to the discussion on the Galactic chemical evolution of sulphur by deriving sulphur abundances from non-local thermodynamic equilibrium (LTE) insensitive spectral diagnostics in disk and halo stars with homogeneously determined stellar parameters.

Methods. We derived effective temperatures from photometric colours, surface gravities from stellar isochrones and Bayesian estimation, and metallicities and sulphur abundances from spectrum synthesis. We derived sulphur abundances from the [S i] λ1082 nm line in 39 mostly cool and metal-poor giants using 1D LTE MARCS model atmospheres to model our high-resolution near-infrared spectra obtained with the VLT, NOT, and Gemini South telescopes.

Results. We derive homogeneous stellar parameters for 29 of the 39 stars. Our results argue for a chemical evolution of sulphur that is typical for α-elements, contrary to some previous studies that have found high sulphur abundances ([S/Fe]  ≳ 0.6) for stars with −2.5 <  [Fe/H]  < −1. Our abundances are systematically higher by about 0.1 dex than those of other studies that arrived at similar conclusions using other sulphur diagnostics.

Conclusions. We find the [S i] line to be a valuable diagnostic of sulphur abundances in cool giants down to [Fe/H]  ≃ −2.3. We argue that a homogeneous determination of stellar parameters is necessary, since the derived abundances are sensitive to them. Our results ([S/Fe]) agree reasonably well with predictions of contemporary models of Galactic chemical evolution. In these models sulphur is predominantly created in massive stars by oxygen burning and is ejected into the interstellar medium during Type II supernovae explosions. Systematic differences with previous studies most likely fall within modelling uncertainties.