Observational constraints on the efficiency of acceleration in the optically thin parts of Wolf-Rayet winds

Wolf-Rayet stars have such strong winds that their inner regions are optica lly thick, preventing us from seeing the hydrostatic stellar cores. One mig ht expect considerable acceleration of the wind to occur in the optically t hick part. However, we show empirically that at least 50%, and in some case s up to 100%, of the wind's acceleration occurs in the optically thin part of the WR wind.