Sulphur in the Sculptor dwarf spheroidal galaxy - Including NLTE corrections

Abstract

In Galactic halo stars, sulphur has been shown to behave like other $\hat{I}\pm$-elements, but until now, no comprehensive studies have been done on this element in stars of other galaxies. Here, we use high-resolution ESO VLT/FLAMES/GIRAFFE spectra to determine sulphur abundances for 85 stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range $-2.5\le \text{[Fe/H]}\le -0.8$. The abundances are derived from the S~I triplet at 9213, 9228, and 9238~Ã . These lines have been shown to be sensitive to departure from local thermodynamic equilibrium, i.e. NLTE effects. Therefore, we present new NLTE corrections for a grid of stellar parameters covering those of the target stars. The NLTE-corrected sulphur abundances in Sculptor show the same behaviour as other $\hat{I}\pm$-elements in that galaxy (such as Mg, Si, and Ca). At lower metallicities ($\text{[Fe/H]}\lesssim -2$) the abundances are consistent with a plateau at $\text{[S/Fe]}\approx +0.16$, similar to what is observed in the Galactic halo, $\text{[S/Fe]}\approx +0.2$. With increasing [Fe/H], the [S/Fe] ratio declines, reaching negative values at $\text{[Fe/H]}\gtrsim -1.5$. The sample also shows an increase in [S/Mg] with [Fe/H], most probably because of enrichment from Type Ia supernovae.

9 pages, 11 figures, 2 tables + 3 online tables, accepted in A&A