Most massive stars and their clumped winds

We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities and clumping characteristics and compare these to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities.

We confirm a cluster age of 1.0-2.5Myr and derive an initial stellar mass of >=250M_{sun}_ for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of fcl=29+/-15. We find tentative trends in the wind-structure parameters as a function of mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped.

We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduces both quantities. The prescription of Krticka & Kubat matches best the observed mass-loss rates.