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Description
We investigate the massive star content of the three Local Group galaxies NGC 6822, M31, and M33 using crowded-field CCD UBV photometry in selected regions to identify the most luminous and massive stars. Optical spectroscopy is presented for many of these stars, allowing construction of accurate H-R diagrams and the first meaningful characterization of the massive star populations in these galaxies. The spectral types also allow investigation of the internal reddenings within these systems and provide fine candidates for stellar-wind studies in the UV. The early-type stars identified include O-type in all three systems, and we call attention to a new Luminous Blue Variable candidate in M33. Our spectroscopy of extreme B supergiants (M_V=-7.5) shows the expected changes with metallicity in comparison to similar objects in the Milky Way, LMC, and SMC, although the metal lines in the NGC 6822 stars are considerably weaker than that expected for a metallicity intermediate 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 irregular 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 of the reddening curve is normal in NGC 6822 and M33, with E(U-B)/E(B-V)~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 stars of high mass (>80M_{sun}_) and luminosity (Mbol~-11) are 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 Mbol=-10 and an inferred mass of 60M_{sun}_. Similarly, none of the OB associations in NGC 6822 are as impressive in terms of the number of massive stars as the rich associations of the Milky Way and Magellanic Clouds. However, OB78 (=NGC 206) and OB48 in M31 both contain 9-15 stars of mass >40M_{sun}_, 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 containing 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 stars may be favored. The massive star content of individual associations would not have been inferred on the basis of Halpha flux, meaning that the nebulae associated with a number of these OB associations are density 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 (>40M_{sun}_) stars to the number of WR stars is constant within all the Local Group galaxies we have studied, suggesting that (a) the effect of metallicity on the evolution of massive stars is subtle if present, and (b) that WR stars make excellent tracers of the massive star populations. The fact that this ratio is roughly 3, rather than the 10 expected given the relative H- and He-burning lifetimes, argues that either our samples are (uniformly?) incomplete or that some fraction of WR stars are H-burning objects.
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