Ca II triplet spectroscopy of giants in Small Magellanic Cloud star clusters: Abundances, velocities, and the age-metallicity relation

We have obtained spectra at the Ca II triplet of individual red giants in seven Small Magellanic Cloud (SMC) star clusters whose ages range from similar to 4 to 12 Gyr. The spectra have been used to determine mean abundances for six of the star clusters to a typical precision of 0.12 dex. When combined with existing data for other objects, the resulting SMC age-metallicity relation is generally consistent with that for a simple model of chemical evolution, scaled to the present-day SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and NGC 339), however, have abundances that are similar to 0.5 dex lower than that expected from the mean age-metallicity relation. It is suggested that the formation of these clusters, which have ages of similar to 5 Gyr, may have involved the infall of unenriched gas, perhaps from the Magellanic Stream. The spectra also yield radial velocities for the seven clusters. The resulting velocity dispersion is 16 +/- 4 km s(-1), consistent with those of the SMC planetary nebula and carbon star populations.