Wolf-Rayet Stars in M31's OB Associations

Extract

It is well-known that the massive star population exerts a disproportionately great influence on the environment within a galaxy, especially through the ejection of chemically enriched material and the input of energy into the interstellar medium. In order to more fully understand the structure and evolution of galaxies, it is important to determine how the massive star content of a galaxy changes with Hubble type, galaxian luminosity and metallicity. Conversely, if our goal is to understand the evolution of massive stars, it is helpful to observe them in a variety of environments, as stellar wind properties should depend on chemical abundances. We are involved in a long-term project to survey the massive star population in a diverse sample of nearby galaxies. Unfortunately, it is extremely difficult to isolate a sample of relatively unevolved massive stars in an external galaxy, since the colors and absolute visual magnitudes of a 20 M_O and 100 M_O star are nearly identical, and can be further confused by differential internal extinction. Wolf-Rayet (WR) stars, however, are relatively easy to detect due to their strong emission-line spectra and high luminosities. Since WR stars are known to have evolved from 0 stars and because their space distribution in the solar neighborhood is identical to that of the most massive 0 stars, we have decided to use WR stars as tracers of the massive star population in nearby galaxies.