2014A&A...568A..29C

The abundances of α-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-α galaxies and extragalactic HII regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. In this context measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur. The aim of this paper is to determine sulphur abundances in several Galactic clusters that span a metallicity range -1.5<[Fe/H]<0.0. We use a standard abundance analysis, based on 1D model atmospheres in local thermodynamical equilibrium (LTE) and literature corrections for non-LTE (NLTE), as well as 3D corrections based on hydrodynamical model atmospheres, to derive sulphur abundances in a sample of stars in the globular cluster M4, and the open clusters Trumpler5, NGC2477, and NGC5822. We find <A(S)>_NLTE=6.11±0.04 for M4, <A(S)>_NLTE=7.17±0.02 for NGC2477, and <A(S)>_NLTE=7.13±0.06 for NGC5822. For the only star studied in Trumpler 5 we find A(S)_NLTE=6.43±0.03 and A(S)_LTE=6.94±0.05. Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another α-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. We believe that further investigation of departures from LTE is necessary, as well as observation of other SI lines in this star and in other stars of the same cluster, before one can conclude that the sulphur abundance in Trumpler5 is indeed 0.4dex lower than in field stars of comparable metallicity. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05dex.