THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 99:551-563, 1995 August

       THE MASSIVE STAR POPULATIONS IN IONIZED GAS SHELLS IN M33 AND M31

           DEIDRE A. HUNTER, DANIELLE M. BOYD, AND WALTER N. HAWLEY
       Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001


                                   ABSTRACT

        We present BVI photometry of the massive stars in and around five
     Halpha shells in M33 and two in M31.  These data can be used to
     statistically compare the stellar population with ionization and
     formation requirements of the gas shell.  We have counted the numbers
     of massive stars encircled by the shells and embedded within the
     gaseous structures and corrected these numbers statistically for
     background and foreground interlopers.  We find that the stellar
     production exceeds the ionization requirements of the nebulae by
     factors of 4-10.  Thus, it is reasonable that the shells are
     photoionized and that a large fraction of the ionizing photons escape
     into the rest of the galaxy.  We have also compared the number of
     massive stars counted and extrapolated to 7 Msun with the number
     required to form the shells according to a simple formation model.
     With one exception, we find that the number of stars estimated to be
     present exceeds the number required by the model for our assumptions
     of age and ambient gas density.  This comparison and the morphology
     evident in Halpha images suggests that the modification of the
     interstellar medium by the massive stars in these regions has been far
     more complicated than is currently considered in formation models.
     There is evidence for multiple, and to some extent discreet, events in
     the production of the gas structures.  A more detailed modeling will
     be necessary to adequately account for the stellar populations and
     Halpha morphology.

     Subject headings: galaxies: individual (M31, M33) -- galaxies: ISM --
     galaxies: star clusters -- galaxies: stellar content -- 
     stars: early-type