Wolf-Rainer Hamann

Title: Massive stars and their winds

Abstract: Massive stars play an important role in the cosmos, e.g. as source of
ionizing radiation and chemically enriched matter. Nevertheless, they
are yet poorly understood. Mass loss by stellar winds is an essential
driver of their evolution. Especially strong are the stellar winds from
Wolf-Rayet stars. The spectral analysis of their emission-line spectra
requires adequate non-LTE atmosphere models which account for the
atmospheric expansion. We have developed such models and analyzed many
stars in different environments, finding that the empirical results are
partly in odds with the predictions from stellar evolution models.

The problems become even worse with recent claims that the empirically 
mass-loss rates have been over-estimated and must be revised downwards, 
due to hitherto neglected effects of inhomogeneities in the stellar 
winds. Our studies, however, put such claims into question.  

Only recently, we succeeded in calculating hydrodynamically consistent
WR models based on radiation pressure as their driving mechanism.
However, it is not yet clear whether radiation pressure alone is
sufficient for explaining all Wolf-Rayet subtypes.