Sulphur in the metal poor globular cluster NGC 6397
  Sulphur (S) is a non-refractory alpha-element that is not locked into dust
grains in the interstellar medium. Thus no correction to the measured,
interstellar sulphur abundance is needed and it can be readily compared to the
S content in stellar photospheres. Here we present the first measurement of
sulphur in the metal poor globular cluster (GC) NGC 6397, as detected in a
MIKE/Magellan high signal-to-noise, high-resolution spectrum of one red giant
star. While abundance ratios of sulphur are available for a larger number of
Galactic stars down to an [Fe/H] of ~ -3.5 dex, no measurements in globular
clusters more metal poor than -1.5 dex have been reported so far. We find a
NLTE, 3-D abundance ratio of [S/Fe] = +0.52 +/- 0.20 (stat.) +/- 0.08 (sys.),
based on the S I, Multiplet 1 line at 9212.8A. This value is consistent with a
Galactic halo plateau as typical of other alpha-elements in GCs and field
stars, but we cannot rule out its membership with a second branch of increasing
[S/Fe] with decreasing [Fe/H], claimed in the literature, which leads to a
large scatter at metallicities around -2 dex. The [S/Mg] and [S/Ca] ratios in
this star are compatible with a Solar value to within the (large)
uncertainties. Despite the very large scatter in these ratios across Galactic
stars between literature samples, this indicates that sulphur traces the
chemical imprints of the other alpha-elements in metal poor GCs. Combined with
its moderate sodium abundance ([S/Na]_NLTE=0.48), the [S/Fe] ratio in this GC
extends a global, positive S-Na correlation that is not seen in field stars and
might indicate that proton-capture reactions contributed to the production of
sulphur in the (metal poor) early GC environments.
