Spectroscopy and hydrodynamics of dense stellar winds

Analyzing the spectra from Wolf‐Rayet stars requires adequate non‐LTE modeling of their expanding atmosphere. The numerical schemes for solving the radiative transfer in the co‐moving frame of reference have been developed by Mihalas and co‐workers 30 years ago. The most elaborate codes can cope today with many hundred explicit non‐LTE levels or super‐levels and account for metal‐line blanketing.The limited agreement with observed spectra indicates that the model simplifications are still severe. One approximation that has to be blamed is homogeneity. Stellar‐wind clumping on small scales was easily implemented, while “macro‐clumping” is still a big challenge. First studies showed that macro‐clumping can reduce the strength of predicted P‐Cygni line profiles in O‐star spectra, and largely affects the X‐ray line spectra from stellar winds.The classical model for radiation‐driven winds by Castor, Abbot and Klein fails to explain the very dense winds from Wolf‐Rayet stars. Only when we solved the detailed non‐LTE radiative transfer consistently with the hydrodynamic equations, mass‐loss rates above the single‐scattering limit have been obtained.