HST - WFPC 2 Observations of the Star Clusters in the Giant HII Regions of M 33

We present a photometric study of the stars in ionizing star clusters embedded in several giant HII regions of M33 (CC93, IC 142, NGC 595, MA2, NGC 604 and NGC 588). Our photometry are based on the HST-WFPC2 images of these clusters. Color-magnitude diagrams and color-color diagrams of these clusters are obtained and are used for estimating the reddenings and ages of the clusters. The luminosity functions (LFs) and initial mass functions (IMFs) of the massive stars in these clusters are also derived. The slopes of the IMFs range from Γ = −0.5 to −2.1. It is found interestingly that the IMFs get steeper with increasing galactocentric distance and with decreasing [O/H] abundance. M33, a spiral galaxy in the Local Group, provides an ideal laboratory to investigate the properties of ionizing star clusters and how much metal abundance affects stellar population at the high-mass end. Interestingly the giant H II regions in M33 are not powered by dense superclusters – as is the case in 30 Doradus, the Antennae and other more distant starbursting systems. Instead, the clusters underlying the giant HII regions in M33 sprawl over 100pc. Two bright clusters NGC 595 and NGC 604 in M33 were studied by Malumuth et al. (1996) and Hunter et al. (1996), respectively.