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Current Status of Modeling Wolf-Rayet Atmospheres

D. John Hillier
University of Pittsburgh

While major advances in our understanding of Wolf-Rayet stars have been made, there are still significant uncertainties. To reduce these uncertainties, and to help clarify the role of Wolf-Rayet stars in massive star evolution, accurate stellar parameters and abundances need to be deduced from a detailed analysis of Wolf-Rayet spectra. Spectral calculations are feasible, and these generally provide a satisfactory fit to the observations. However, there are two major limitations with the current modeling: clumping and the determination of the mass-loss rate and velocity law from first principles. Clumping is generally treated via a volume-filling-factor approach; the technique, as demonstrated by spectral fitting, works surprisingly well. Unfortunately it is difficult to constrain the radial dependence of the volume-filling factor, which necessarily introduces freedom into the modeling. Recently, the Potsdam group (Graefener & Hamann 2005, 2008) has made significant strides towards understanding Wolf-Rayet winds from first principles. These calculations illustrate the importance of the iron opacity bumps, and demonstrate the strong dependence of the mass-loss rate on the Eddington parameter. Unfortunately not all Wolf-Rayet spectral classes have been successfully modeled, and clumping must still be treated as a free parameter.