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

We present BVI photometry of the massive stars in and around five Ha 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 ofthe galaxy. We have also compared the number of massive stars counted and extrapolated to 7 M. 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 Her images suggests that the modification ofthe 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 ofthe gas structures. A more detailed modeling will be necessary to adequately account for the stellar populations and H-alpha morphology.