2007AJ....134.2168P

We present the results of a survey for star clusters in M33 using 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 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 the CFH12k mosaic camera at the 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, making them 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 a 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 on the redder side in the (V-I)₀color. Relatively more red clusters are found in the outer region of M33 than blue and intermediate-color clusters, and many of the blue stars are located in H II regions. The luminosity function for the blue star clusters shows a peak at M_V~-7.3 mag, while that for the intermediate-color star clusters shows a peak at the fainter magnitude M_V~-6.3 mag. The luminosity function for the red star clusters also shows a peak at M_V~-6.8 mag, although the number of clusters is small.