HOT, LUMINOUS STARS IN SELECTED REGIONS OF NGC-6822, M31, AND M33

We investigate the massive star content of the three Local Group galax ies NGC 6822, M31, and M33 using crowded-field CCD UBV photometry in s elected regions to identify the most luminous and massive stars. Optic al spectroscopy is presented for many of these stars, allowing constru ction of accurate H-R diagrams and the first meaningful characterizati on of the massive star populations in these galaxies. The spectral typ es also allow investigation of the internal reddenings within these sy stems and provide fine candidates for stellar-wind studies in the UV. The early-type stars identified include O-type in all three systems, a nd we call attention to a new Luminous Blue Variable candidate in M33. Our spectroscopy of extreme B supergiants (M(V) = -7.5) shows the exp ected changes with metallicity in comparison to similar objects in the Milky Way, LMC, and SMC, although the metal lines in the NGC 6822 sta rs are considerably weaker than that expected for a metallicity interm ediate between that of the LMC and SMC, suggesting that this galaxy is more metal poor than usually supposed. There is considerable internal reddening within all three galaxies, including even the dwarf irregul ar NGC 6822, where the color excesses show a systematic spatial trend from E(B - V) = 0.26 near the edges to 0.45 in the middle. The slope o f the reddening curve is normal in NGC 6822 and M33, with E(U - B)/E(B - V) approximate to 0.72, but in M31 we find that this ratio is 0.4 - 0.5 in all three of our fields. We spectroscopically confirm that sta rs of high mass (> 80 M.) and luminosity (M(bol) approximate to -11) a re found in M31 and M33. We have not found stars of similar high mass or luminosity in NGC 6822, where the most luminous star present has M( bol) = -10 and an inferred mass of 60 M.. Similarly, none of the OB as sociations in NGC 6822 are as impressive in terms of the number of mas sive stars as the rich associations of the Milky Way and Magellanic Cl ouds. However, OB78 (= NGC 206) and OB48 in M31 both contain 9-15 star s of mass > 40 M., making them comparable to impressive sites of star formation in the Milky Way and Magellanic Clouds. M33 contains regions that are even more extreme, with M33 - OB127 and M33 - OB21 containin g 20 - 30 such stars. The low number of very massive stars in NGC 6822 is consistent with the overall star-formation rate inferred by other means, but the M33 results suggest that the formation of high-mass sta rs may be favored. The massive star content of individual associations would not have been inferred on the basis of Her flux, meaning that t he nebulae associated with a number of these OB associations are densi ty rather than radiation bounded, probably due to holes blown in the H I. We have also found that the ratio of the number of very massive (> 40 M.) stars to the number of WR stars is constant within all the Loc al Group galaxies we have studied, suggesting that (a) the effect of m etallicity on the evolution of massive stars is subtle if present, and (b) that WR stars make excellent tracers of the massive star populati ons. The fact that this ratio is roughly 3, rather than the 10 expecte d given the relative H- and He-burning lifetimes, argues that either o ur samples are (uniformly?) incomplete or that some fraction of WR sta rs are H-burning objects. (C) 1995 American Astronomical Society.