Age Determination of 15 Old to Intermediate-Age Small Magellanic Cloud Star Clusters

Abstract

We present color–magnitude diagrams in theVandIbands for 15 star clusters in the Small Magellanic Cloud (SMC)based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previouswork, wherein we derived cluster radial velocities and metallicities from calcium II infrared triplet (CaT) spectraalso taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimatedby previous studies, which used comparatively small telescopes, graphically illustrating the need for large aperturesto obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L4, L6,and L110, turn out to be among the oldest SMC clusters known, with ages of 7.9±1.1, 8.7±1.2, and 7.6±1.0 Gyr, respectively, helping to fill a possible “SMC cluster age gap.” Using the current ages and metallicities fromParisi et al., we analyze the age distribution, age gradient, and age–metallicity relation (AMR) of a sample of SMCclusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not showa constant slope for the first 4 Gyr, and we find no evidence for an age gradient. Due to the improved ages of ourcluster sample, we find that our AMR is now better represented in the intermediate/old period than we had derivedin Parisi et al., where we simply took ages available in the literature. Additionally, clusters younger than∼4Gyrnow show better agreement with the bursting model of Pagel & Tautvaiˇsien ̇e, but we confirm that this model isnot a good representation of the AMR during the intermediate/old period. A more complicated model is needed toexplain the SMC chemical evolution in that period.