AGES OF M33 STAR CLUSTERS BASED ON THE HUBBLE SPACE TELESCOPE/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 Hubble Space Telescope/Wide Field Planetary Camera 2 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 10 star clusters are older than log(t) ∼ 9.0. There is only one cluster 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 the M33 center. We briefly discuss the implication of the results with regard to the formation of the M33 cluster system.

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