A Catalog of New M33 Star Clusters Based on Hubble Space Telescope WFPC2 Images

Abstract

We present the results of a survey for star clusters in M33 using HSTWFPC2 archive images. We have found 104 star clusters, including 32 newones, in the images of 24 fields that were not included in previousstudies. Combining these with previous data in the literature, weincrease the number of M33 star clusters found in the HST images to 242.We have derived BVI integrated photometry of these star clusters fromthe CCD images taken with the CFH12k mosaic camera at the CFHT.Integrated color-magnitude diagrams of the M33 star clusters are foundto be similar in general to those of star clusters in the LargeMagellanic Cloud, except that M33 has a much lower fraction of blue starclusters. We find 29 red star clusters with 0.5 {\lt}= (B -V)_{0} {\lt}= 1.1 and 0.7 {\lt}= (V - I)_{0} {\lt}= 1.2,making them old globular cluster candidates. We divide the clustersample into three groups according to their (B - V)_{0} color:blue star clusters with (B - V)_{0} {\lt}= 0.3,intermediate-color star clusters with 0.3 {\lt} (B - V)_{0} {\lt}0.5, and red star clusters with (B - V)_{0} {\gt}= 0.5. Most ofthe new clusters in M33 are located along a sequence that is consistentwith the theoretical evolutionary path for Z = 0.004, Y = 0.24 in the (B- V)_{0}-(V - I)_{0} diagram, while a few of them are onthe redder side in the (V - I)_{0} color. Relatively more redclusters are found in the outer region of M33 than blue andintermediate-color clusters, and many of the blue stars are located in HII regions. The luminosity function for the blue star clusters shows apeak at M_{V} approx -7.3 mag, while that for theintermediate-color star clusters shows a peak at the fainter magnitudeM_{V} approx -6.3 mag. The luminosity function for the red starclusters also shows a peak at M_{V} approx -6.8 mag, althoughthe number of clusters is small.