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