THE ASTRONOMICAL JOURNAL    VOLUME 110, NUMBER 6, PAGE 2715       DECEMBER 1995

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

              PHILIP MASSEY, TAFT E. ARMANDROFF, AND RANDALL PYKE

   Kitt Peak National Observatory, National Optical Astronomy Observatories,
                  P.O. Box 26732, Tucson, Arizona 85726-6732

                      KANAN PATEL AND CHRISTINE D. WILSON

           Department of Physics and Astronomy, McMaster University,
                       Hamilton, Ontario L8S 4M1, Canada


                                   ABSTRACT

    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 (>80 Msun) 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 60 Msun.  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 > 40 Msun, 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 (>40 Msun) 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.