Festering Problems in the Upper HR Diagram

Abstract

For massive stars (M_{i} {\gt}~ 20M_{solar}), stellarevolution is dominated by mass loss. Several problems in the evolutionof massive stars have been revealed during the last decade or so. Someof the main ones are: (1) clumpy winds have reduced the estimatedmass-loss rates by typically a factor of 3; (2) a discrepancy occursbetween evolution-based masses and masses based on spectral analysis orKeplerian orbits for binaries; and (3) the binary frequency ofWolf-Rayet stars no longer appears to depend on ambient metallicity, asonce thought. Possible solutions to these and other problems related tostellar evolution involve the effects of rotation (especially at lowmetallicity and when combined with high luminosity), which were notallowed for in previous evolutionary models (although still not entirelyclear how to implement currently), and possibly other factors that areprobably less dependent on metallicity, e.g. pulsational instabilitiesduring the LBV stage leading to major mass-loss.