Sulphur abundances in halo stars from Multiplet 3 at 1045 nm
  Sulphur is a volatile alpha-element which is not locked into dust grains in
the interstellar medium (ISM). Hence, its abundance does not need to be
corrected for dust depletion when comparing the ISM to the stellar atmospheres.
The abundance of sulphur in the photosphere of metal-poor stars is a matter of
debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low
metallicity, while, according to other studies, there is a large scatter or
perhaps a bimodal distribution. In metal-poor stars sulphur is detectable by
its lines of Mult.1 at 920 nm, but this range is heavily contaminated by
telluric absorptions, and one line of the multiplet is blended by the hydrogen
Paschen zeta line. We study the possibility of using Mult. 3 (at 1045 nm) for
deriving the sulphur abundance because this range, now observable at the VLT
with the infra-red spectrograph CRIRES, is little contaminated by telluric
absorption and not affected by blends at least in metal-poor stars. We compare
the abundances derived from Multiplets 1 and 3, taking into account NLTE
corrections and 3D effects. Here we present the results for a sample of four
stars, although the scatter is less pronounced than in previous analysis, we
cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur
abundance at low metallicity.
