Massive Star Formation and Evolution in Local Group Galaxies: Successes and Difficulties

Abstract

Local Group galaxies allow us to test some properties of massive star evolution which are inaccessible in our Galaxy, in particular the effects of different metallicities. Thus, after showing that we still do not know the exact process by which massive stars are formed, we examine the differences in the distributions of O-stars, blue and red supergiants and WR stars in the Local Group. The number ratios WR/O and WN/WC are well accounted for by stellar models in which the mass-loss rates depend on the metallicity, Z, as predicted by stellar wind theories. The number ratio of red supergiants to WR stars is growing for decreasing Z. Although this behaviour is qualitatively well explained, the models need some extra mixing to fit the observational data. The same is true for the explanation of the He- and N-excesses in O, B and A supergiants. Rotation and related mixing processes certainly play a major role in massive star evolution. The relative number of Be-stars is higher at lower Z, which suggests that rotation is faster, also with more mixing, in small, irregular low-metallicity galaxies.