Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. III. Abundances and Velocities for a Sample of 14 Clusters

TL;DR

This study presents homogeneous metallicity and velocity measurements for 36 SMC clusters using CaII triplet spectroscopy, revealing a bimodal metallicity distribution and complex chemical evolution.

Contribution

It provides the largest, most accurate, and homogeneous metallicity dataset for SMC clusters, highlighting bimodality and challenging existing chemical evolution models.

Findings

Bimodal metallicity distribution with peaks at -1.1 and -0.8 dex.

No strong metallicity gradient detected across clusters.

Significant metallicity dispersion indicating complex chemical history.

Abstract

We obtained spectra of red giants in 15 Small Magellanic Cloud (SMC) clusters in the region of the CaII lines with FORS2 on the Very Large Telescope (VLT). We determined the mean metallicity and radial velocity with mean errors of 0.05 dex and 2.6 km/s, respectively, from a mean of 6.5 members per cluster. One cluster (B113) was too young for a reliable metallicity determination and was excluded from the sample. We combined the sample studied here with 15 clusters previously studied by us using the same technique, and with 7 clusters whose metallicities determined by other authors are on a scale similar to ours. This compilation of 36 clusters is the largest SMC cluster sample currently available with accurate and homogeneously determined metallicities. We found a high probability that the metallicity distribution is bimodal, with potential peaks at -1.1 and -0.8 dex. Our data show no strong evidence of a metallicity gradient in the SMC clusters, somewhat at odds with recent evidence from CaT spectra of a large sample of field stars Dobbie et al. (2014). This may be revealing possible differences in the chemical history of clusters and field stars. Our clusters show a significant dispersion of metallicities, whatever age is considered, which could be reflecting the lack of a unique AMR in this galaxy. None of the chemical evolution models currently available in the literature satisfactorily represents the global chemical enrichment processes of SMC clusters.