Clues on the Galactic evolution of sulphur from star clusters

Abstract

Context. 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 H ii 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. Aims. 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. Methods. 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 M 4, and the open clusters Trumpler 5, NGC 2477, and NGC 5822. Results. We find A (S) NLTE = 6.11 ± 0.04 for M 4, A (S) NLTE = 7.17 ± 0.02 for NGC 2477, and A (S) NLTE = 7.13 ± 0.06 for NGC 5822. 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. Conclusions. 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 0.4 dex 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 S i lines in this star and in other stars of the same cluster, before one can conclude that the sulphur abundance in Trumpler 5 is indeed 0.4 dex 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 0.05 dex.

Galactic evolution of sulphur as traced by globular clusters

“…There are S abundances derived within the entire span of metallicities covered by stars in Galactic clusters, although the sample is still limited and further studies are desirable. In all analysed clusters, sulphur seems to behave as a typical α element, meaning that the stars in metal poor Galactic GCs Messier 4 is the only GC analysed by two teams independently (Caffau et al 2014, and this work) and a more careful comparison between the results is possible. Caffau et al (2014) derived S abundances for ten stars using line profile fitting of multiplet 1 at 920 nm taking into account the telluric absorption lines as well.…”

“…In all analysed clusters, sulphur seems to behave as a typical α element, meaning that the stars in metal poor Galactic GCs Messier 4 is the only GC analysed by two teams independently (Caffau et al 2014, and this work) and a more careful comparison between the results is possible. Caffau et al (2014) derived S abundances for ten stars using line profile fitting of multiplet 1 at 920 nm taking into account the telluric absorption lines as well. Although both studies seem to agree on consistent [S/Fe] abundance ratios for this GC (see Fig.…”

“…Koch & Caffau (2011) analysed a single star in the metal poor GC NGC 6397 ([Fe/H] = −2.0 dex), determining [S/Fe] = 0.52 ± 0.2 dex, consistent with the halo field population. We also note the recent work by Caffau et al (2014), which provides [S/Fe] measurements for several metal-rich open clusters and includes the GC M 4 that is analysed in the present work.…”

“…6), Caffau et al (2014) assumed a lower metallicity for their stars and adopted [Fe/H] = −1.36 dex for M 4 (Monaco et al 2012), while the mean [Fe/H] for our stars is = −1.08 dex (Marino et al 2008). This discrepancy results in a much lower value of A(S) = 6.11 ± 0.04 dex estimated by Caffau et al (2014), compared to our estimate of A(S) = 6.46 ± 0.02 dex in NLTE. The NLTE corrections for both multiplets are similar and in the order of 0.2 dex (Takeda et al 2005).…”

“…At the same time, these brighter stars are often the only accessible tracers in remote and faint GCs. To date, the strong multiplet 1 (920 nm) has been used to obtain sulphur abundances in GCs (Caffau et al 2005b(Caffau et al , 2014Sbordone et al 2009;Koch & Caffau 2011), but these lines are often blended with telluric absorption lines, and one of them is positioned in the blue wing of the Paschen ζ line and thus cannot always be reliably measured. This makes the Mult.…”