Spectroscopy of giants in LMC clusters. I. Velocities, abundances, and the age-metallicity relation

We report velocities and equivalent widths derived from spectra taken at the calcium triplet (λ∼8500 Å) for more than 150 stars in ∼80 different LMC star clusters. Velocities of the individual program stars are accurate to 5 km s^-1. The sum of the pseudo-equivalent widths of the three calcium lines are compared with those of Galactic calibrators; these measures provide a sensitive abundance indicator for - 2.3 <[Fe/H] <0.0, with an accuracy of ± 0.2 dex per observation. Although the majority of the program clusters are younger than the calibrating Galactic clusters, we argue that the derived abundances are not strongly affected by ignoring differences in surface gravity. Abundances are derived for 70 clusters, most of which have no previous spectroscopic metallicity determinations. These abundances are combined with ages of 31 clusters derived from color-magnitude diagrams, and are used to construct the age-metallicity relation for the inner (radius <5°), and outer (radius >5°) parts of the LMC. This relationship can be characterized by a simple one-zone enrichment model, although much of the early enrichment history of the LMC is hidden by the lack of clusters with ages between 3 and 10 Gyr. The abundances for the inner and outer clusters at an age of 2 Gyr are nearly identical (-0.3 and - 0.42, respectively), and therefore little radial abundance gradient is evident in the cluster system.