Festering Problems in the Upper HR Diagram

Abstract

For massive stars (M_i ≳ 20M_⊙), stellar evolution is dominated by mass loss. Several problems in the evolution of massive stars have been revealed during the last decade or so. Some of the main ones are: (1) clumpy winds have reduced the estimated mass-loss rates by typically a factor of 3; (2) a discrepancy occurs between evolution-based masses and masses based on spectral analysis or Keplerian orbits for binaries; and (3) the binary frequency of Wolf-Rayet stars no longer appears to depend on ambient metallicity, as once thought. Possible solutions to these and other problems related to stellar evolution involve the effects of rotation (especially at low metallicity and when combined with high luminosity), which were not allowed for in previous evolutionary models (although still not entirely clear how to implement currently), and possibly other factors that are probably less dependent on metallicity, e.g. pulsational instabilities during the LBV stage leading to major mass-loss.