A Catalog of New M33 Star Clusters Based on the HST/WFPC2 Images

Abstract

We present the result of a survey for star clusters in M33 using the HST/WFPC2 archive images. We have found 104 star clusters, including 32 new ones, in the images of 24 fields that were not included in the previous studies. Combining these with previous data in the literature, we increase the number of M33 star clusters found in the HST images to 242. We have derived BVI integrated photometry of these star clusters from the CCD images taken with CFH12k mosaic camera at CFHT. Integrated color-magnitude diagrams of the M33 star clusters are found to be similar in general to those of star clusters in the Large Magellanic Cloud, except that M33 has a much lower fraction of blue star clusters. We find 29 red star clusters with 0.5 <= (B-V)_0 <= 1.1 and 0.7 <=(V-I)_0 <= 1.2, which are old globular cluster candidates. We divide the cluster sample into three groups according to their (B-V)_0 color: blue star clusters with (B-V)_0 <= 0.3, intermediate color star clusters with 0.3 < (B-V)_0 <0.5, and red star clusters with (B-V)_0 >= 0.5. Most of the new clusters in M33 are located along the sequence that is consistent with 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 in the redder side in the (V-I)_0 color. The red clusters are found relatively more in the outer region of M33 than the blue and intermediate color clusters, and that many of the blue stars are located in the HII regions. The luminosity function for the blue star clusters shows a peak at Mv ~ -7.3 mag, while that for the intermediate color star cluster shows a peak at the fainter magnitude Mv ~ -6.3 mag. The luminosity function for the red star clusters shows also a peak at Mv ~ -6.8 mag, although the number of the clusters is small.