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At a distance of 0.84 Mpc, Messier 33 is the closest galaxy harboring giant HII regions of both low and high metallicity. As such, M33 provides the ideal laboratory for investigating the degree to which metal abundance affects stellar population at the high-mass end. Moreover, by comparing the resolvable constituents of M33's giant HII regions with the resulting composite colors and emission-line properties, one can establish benchmarks for interpreting the composite light from more distant starbursting systems.
Through a coordinated combination of HST, UIT, and ground-based imaging, we have begun a program of comparing the resolved and composite properties of the giant HII regions in M33. Multiple-band images of 6 giant HII regions have been recently obtained with the HST/WFPC2. At a spatial resolution of 0.4 pc, these images show the central populations to be loosely clustered, unlike the densely packed population that is seen in 30 Doradus. Photometric measurement of the UV, U, B, and V-band images indicates the presence of some extremely hot and luminous stars in all 6 of the targeted HII regions. (Photometry and derived statistics of the stars in one such region is presented by E. Malumuth et al. as a Poster in this meeting.)
Ground-based imaging at H$\alpha$ and other visible bands provide composite spectral indices of the HII region's ionizing luminosity and effective temperature which can be compared with the stellar constituents that are resolved in the HST images. FUV and NUV imaging with the Ultraviolet Imaging Telescope reveal diffuse knots of dust-scattered emission surrounding stellar cores, whose composite (FUV - NUV) colors provide sensitive information on the reddening. Comparison of the dereddened UV and H$\alpha$ fluxes further constrain evolutionary models of the ionizing stellar populations, which in turn, can be checked against the resolved stellar properties. Progress on these investigations will be discussed.
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