Ages of M33 Star Clusters Based on the HST/WFPC2 Photometry
  We present a result of age estimation for star clusters in M33. We obtain
color-magnitude diagrams (CMDs) of resolved stars in 242 star clusters from the
HST/WFPC2 images. We estimate ages of 100 star clusters among these, by fitting
the Padova theoretical isochrones to the observational CMDs. Age distribution
of the star clusters shows a dominant peak at log(t) ~ 7.8. Majority of star
clusters are younger than log(t) = 9.0, while ten star clusters are older than
log(t) ~ 9.0. There are few clusters younger than log(t) = 7 in this study,
which is in contrast with the results based on the integrated photometry of
star clusters in the previous studies. Radial distribution of the cluster ages
shows that young to intermediate-age clusters are found from the center to the
outer region, while old clusters are distributed farther from M33 center. We
discuss briefly the implication of the results with regard to the formation of
M33 cluster system.
