The Galactic evolution of sulphur as traced by globular clusters
  Sulphur is an important, volatile alpha element but its role in the Galactic
chemical evolution is still uncertain. We derive the S abundances in RGB stars
in three Galactic globular clusters (GC) that cover a wide metallicity range
(-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large
scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent
with canonical chemical evolution models. To date, very few measurements of
[S/Fe] exist for stars in GCs, which are good tracers of the chemical
enrichment of their environment. However, some light and alpha elements show
star-to-star variations within individual GCs and it is yet unclear whether
sulphur also varies between GC stars. We used the the infrared spectrograph
CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per
px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars
selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in
M30). Multiplet 3 is better suited for S abundance derivation than the more
commonly used lines of multiplet 1 at 920 nm, since its lines are not blended
by telluric absorption or other stellar features at low metallicity. We used
spectral synthesis to derive the [S/Fe] ratio of the stars assuming local
thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20
dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex
for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE
corrections are estimated to be in the order of the systematic uncertainties.
With the tentative exception of two stars with measured high S abundances, we
conclude that sulphur behaves like a typical alpha element in the studied
Galactic GCs, showing enhanced abundances with respect to the solar value at
metallicities below [Fe/H] = -1.0 dex without a considerable spread.
