THE INTERMEDIATE STELLAR MASS POPULATION IN NGC-604 DETERMINED FROM HUBBLE-SPACE-TELESCOPE IMAGES

We present photometry of stars in the giant H II region NGC 604 in M33 . The photometry is measured from Hubble Space Telescope images throug h the F336W, F555W, and F814W broadband filters. Color-magnitude diagr ams of the stars in NGC 604 show a main sequence detected down to an M (F555W,0) of -1 (nearly 6 M.). In addition, there are luminous stars t hat are probably blue and red supergiants. Based on the previously kno wn presence of Wolf-Rayet stars, we take the age of the cluster to be 3-5 Myr. We measure an initial mass function for intermediate-mass sta rs (6.5-18 M.) that has a slope of -1.6 +/- 0.7. This slope is, within the uncertainties, similar to those found for OB associations in the Milky Way and Magellanic Clouds. The density of the luminous stars in NGC 604 is also comparable to that of OB associations in those galaxie s even though the total number of stars is greater in NGC 604. A few s ubclumps have densities that are higher by as much as a factor of 10. A comparison with R136 in the LMC emphasizes that the formation of a l arge number of massive stars does not necessarily entail a high concen tration of those stars.