Mass loss as a function of stellar parameters

Henny J.G.L.M. Lamers
Astronomical Institute and SRON Laboratory for Space Research, Utrecht

Mass loss from hot stars can be observed and studied in various ways:

The UV lines give the best indication of the wind velocities, but the mass loss estimates are hampered by uncertainties in the ionization. The emission lines and radio flux are almost independent of ionization and could give reliable mass loss rates but they are sensitive to clumping in the wind. The mass loss rates derived from the IR excess is very sensitive to the windmodel in the accelerating part just above the photosphere.

I will discuss the determinations of mass loss rates and wind velocities from the observations of O-star, WR stars and Central Stars of PN, and describe the dependence of the mass loss on the stellar parameters. I will compare it with predictions of the radiation driven wind models and I will show where the main discrepancies are and what the physiscal reason might be. This suggests improvements to the stellar wind theories, including the bi-stability jumps in mass loss rates and terminal velocity.

I derive expressions for the mass loss rates and for the wind velocity, which might be usefull for calculating stellar evolutions.