Wolf-Rainer Hamann, Götz Gräfener, Achim Feldmeier, Lida Oskinova Andreas Barniske, Adriane Liermann
Wolf-Rayet (WR) stars are characterized by their emission-line spectra, which are formed in a fast and strong stellar wind. Adequate non-LTE model atmospheres are successfully applied for their spectral analysis.
The available models account for wind inhomogeneities ("clumping") in the approximation that clumps are optically thin. Recently we developed a statistical treatment for optically thick clumps. The porosity effect facilitates the emergence of X-rays in spite of strong continuum absorption. With porosity applied to line formation, strong spectral features become generally weaker, and the reported discrepancy between H$\alpha$ and the Pv resonance doublet in O star spectra can be reconciled without a further reduction of the mass-loss rate.
Mass-loss from hot stars is driven by radiation pressure. This has been worked out in hydrodynamical models for O stars, however on the expense of approximations in the radiative transfer. For WR stars such models used to fall short in explaining the high mechanical momentum of their strong winds. Only recently, we combined our Potsdam Wolf-Rayet (PoWR) code with the hydrodynamic equations and obtained self-consistent WR models, showing that the previous shortfalls were due to deficiencies in the radiative-transfer treatment.
The PoWR models were employed for analyzing a comprehensive sample of Galactic WN stars. The results show the separation of the WN stars in two distinct groups. The late-type WN stars (WNL) are very luminous, contain larger or smaller amounts of hydrogen in their atmosphere, and are probably hydrogen-burning stars which never crossed the HR diagram to the red side. The hydrogen-free early-type WN stars (WNE) are less luminous and stem from progenitors of lower initial mass after an excursion to the cooler part of the HRD. Hence most WNE stars have not evolved from WNL.