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Description
As part of a larger effort to study the resolved and composite properties of the giant H II regions in Messier 33, we have analyzed multiband HST/WFPC-2 images of NGC 595 in terms of the ionizing cluster's resolved stellar population. Photometric reductions of the PC images yield 100 stars in the UV image, 272 stars on the U image, 345 stars on the B image, and 561 stars on the V image. A total of 267 stars are common to the U, B, and V images while 86 stars are detected on all 4 images. Although some clustering is evident, the degree of central concentration is less than that seen in 30 Doradus. The resulting U-B vs B-V diagram of the resolved stars is used to determine the reddening of each star. The average reddening derived from this diagram is E(B-V)=0.36+/-0.28mag. The dereddened color-M_V diagram is best fit by a model cluster having an age of 4.5+/-1.0Myr, and hence initial masses no greater than 51M_{sun}_. A total of 13 supergiant stars and 10 candidate WR stars [Drissen et al. (1993AJ....105.1400D), and references therein] are identified with M_V=-5 to -8mag. The remainder are main-sequence O-type (98) and early B-type (>145) stars with M_V=-1 to -6mag. The ratio of WR to O stars is WR/O=0.11+/-0.01, roughly the same as found in the core of 30 Doradus. The resulting luminosity function has a slope of alpha=-0.71. The derived IMF has a slope of Gamma=-1.32+/-0.02 before subtracting a background component, and Gamma=-1.00+/-0.05 after subtracting a background based on photometry of the surrounding WF images. Integration of the derived IMF down to a lower mass limit of 4M_{sun}_ yields a total mass of 7350M_{sun}_, while integration down to 0.1_{Msun}_ yields a total mass of 18000M_{sun}_. The total estimated ionizing luminosity is 5.0x10^50photon/s, roughly half that which is inferred from the Halpha luminosity in this region. This shortfall of ionizing photons can be reconciled by allowing for a spread in the stellar ages, and/or increasing the modeled EUV luminosity of the stars at the inferred cluster age.
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