Observational constraints on mass loss and evolution of massive stars

We review the empirical mass-loss properties of early-type stars in the upper part of the Hertzsprung-Russell diagram. Specifically, we focus on the relation between mass loss and chemical composition by comparing properties of massive stars in our galaxy with those in the Magellanic Clouds. The first robust empirical calibration of this relation is discussed, yielding dM/dt Z0.83±0.16, where Z is the metallicity. This compares well with theoretical expectations. It is pointed out that the largest uncertainties in the power-law index of this relation are likely connected to uncertainties in abundances, in particular of objects in the Small Magellanic Cloud. The role of clumping in stellar winds is discussed. Using Hα as the prime diagnostic, a comparison of the empirical and predicted dM/dt (Z) suggests that the outflows are only modestly clumped, with clumping factors in the Hα line forming region of about 3-5. If the properties of clumping are independent of chemical environment, the dM/dt (Z) power-law index is not affected by the actual value of the clumping factor.