Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud

An empirical relation is presented for estimating the ages of rich star clusters in the Large Magellanic Cloud (LMC), to within a factor of about 2, from their integrated UBV colors. The calibration is based on published ages for 58 LMC clusters derived from main-sequence photometry, integrated spectra, or the extent of the asymptotic giant branches. Using stellar population models, a sample of LMC clusters more massive than about 10,000 solar masses is isolated, which is corrected for incompleteness as a function of magnitude. An unbiased age distribution for three clusters is then determined. The number of clusters decreases with increasing age in a manner that is qualitatively similar to the age distribution for the open clusters in our Galaxy. The LMC age distribution is, however, flatter, and the median age of the clusters is greater. If the formation rate has been approximately constant over the history of the two galaxies, then the age distribution obtained here implies that clusters are disrupted more slowly in the LMC. The results contain no evidence for bursts in the formation of clusters, although fluctuations on small time scales and slow variations over the lifetime of the LMC cannot be ruled out.