What the galaxies of the Local Group tell us about massive star evolution

Abstract

We consider what we've learned about massive star evolution from observations of the resolved stellar content of Local Group galaxies. Studies of mixed-age (galaxy-wide) and coeval (single associations) populations reveal much about massive star evolution, and how it is controlled by metallicity, demonstrating the ‘Conti scenario’ in action! The number of WC stars to WN stars increases with increasing metallicity, as expected: in regions of higher metallicity stars of somewhat lower luminosity can evolve all the way to the WC stage. The exception is the starburst galaxy IC 10, for which I speculate that the IMF may be weighted towards high mass stars. The highest luminosity red supergiants are lacking in galaxies of higher metallicity, suggesting that the stars that would have become these RSGs are spending more of their time as WRs. The presence of luminous RSGs is highly correlated with the presence of WC and WN stars in OB associations, suggesting that many massive stars evolve through both a RSG and WR stage. The relative number of RSGs and WRs does decrease strongly with increasing metallicity, again consistent with higher metallicity systems leading to increased time in the WR phase. The various WC subclasses appear to be the result of the influence of metallicity on stellar wind structure in these stars, and are not due to to differences in mass or luminosity. Data on the field population in the Magellanic Clouds suggest that stars more massive than 30 become WRs in the LMC, while the limit may be more like 50 in the SMC, again as expected. Studies of the turn-off masses in clusters and associations in the MCs and Milky Way are nearing completion, while investigations in the more distant galaxies of the Local Group are just getting underway. For the LMC we find the following: WNE stars come from a large mass range of progenitor (30–100 ), and have very large (negative) bolometric corrections (−6 to −8 mag). The Ofpe/WN9 stars seem to come from lower mass progenitor (20–30 ), and have more modest BCs (−1 to −3 mag). WC stars come from stars with masses > 60–70 , and have BCs of −3 to −4 mag. Both ‘B2I+WN3’ systems and LBV stars like S Doradus are found only in clusters containing very high turn off masses (>70–90 ).