ASTROPHYSICS Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters

Clumping-corrected mass-loss rates of 64 Galactic Wolf-Rayet (WR) stars are used to study the dependence of mass-loss rates, momentum transfer efficiencies and terminal velocities on the basic stellar parameters and chemical compo- sition. The luminosities of the WR stars have been determined either directly from the masses, using the dependence of L on mass predicted by stellar evolution theory, or they were deter- mined from the absolute visual magnitudes and the bolometric corrections. For this purpose we improved the relation between the bolometric correction and the spectral subclass. (1) The momentum transfer efficiencies (i.e. the ratio between the wind momentum loss and radiative momentum loss) of WR stars are found to lie in the range of 1.4 to 17.6, with the mean value of 6.2 for the 64 program stars. Such values can probably be explained by radiative driving due to multiple scattering of photons in a WR wind with an ionization stratification. However, there may be a problem in explaining the driving at low velocities. (2) We derived the linear regression relations for the depen- dence of the terminal velocity, the momentum transfer effi- ciency and the mass-loss rates on luminosity and chemical composition. We found a tight relation between the terminal velocity of the wind and the parameters of the hydrostatic core. This relation enables the determination of the mass of the WR stars from their observed terminal velocities and chemical composition with an accuracy of about 0.1 dex for WN and WC stars. Using evolutionary models of WR stars, the luminosity can then be determined with an accuracy of 0.25 dex or better. (3) We found that the mass-loss rates ( _ M ) of WR stars depend strongly on luminosity and also quite strongly on chemical composition. For the combined sample of WN and WC stars we found that _ M in M yr 1 can be expressed as _