First Stars XIV. Sulfur abundances in extremely metal-poor (EMP) stars

TL;DR

This study accurately measures sulfur abundances in extremely metal-poor stars, revealing its behavior as an alpha-element and providing insights into early stellar nucleosynthesis and chemical evolution.

Contribution

It presents new NLTE and 3D corrected sulfur abundance measurements in EMP stars, clarifying sulfur's trend and its comparison with other elements and models.

Findings

Sulfur behaves like other alpha-elements with constant [S/Fe] below [Fe/H]=-3.

[S/Mg] decreases slightly with increasing [Mg/H].

[S/Zn] ratio is solar in EMP stars, similar to DLAs.

Abstract

Sulfur is important: the site of its formation is uncertain, and at very low metallicity the trend of [S/Fe] against [Fe/H] is controversial. Below [Fe/H]=-2.0, [S/Fe] remains constant or it decreases with [Fe/H], depending on the author and the multiplet used in the analysis. Moreover, although sulfur is not significantly bound in dust grains in the ISM, it seems to behave differently in DLAs and in old metal-poor stars. We aim to determine precise S abundance in a sample of extremely metal-poor stars taking into account NLTE and 3D effects. NLTE profiles of the lines of the multiplet 1 of SI have been computed using a new model atom for S. We find sulfur in EMP stars to behave like the other alpha-elements, with [S/Fe] remaining approximately constant for [Fe/H]<-3. However, [S/Mg] seems to decrease slightly as a function of [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S, and K are best matched by the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar, as found also in DLAs. We obtain an upper limit on the abundance of sulfur, [S/Fe] < +0.5, for the ultra metal-poor star CS 22949-037. This, along with a previous reported measurement of zinc, argues against the conjecture that the light-element abundances pattern in this star, and, by analogy, the hyper metal-poor stars HE 0107-5240 and HE 1327-2326, are due to dust depletion.