O-star winds in the Magellanic Clouds and the Milky Way

UV P-Cygni profiles of OB-stars in the Magellanic Clouds (observed with HST), and the galaxy (observed with IUE) are analyzed empirically using a line formation procedure similar to the one described by Lamers et al. (1987). The assumption of a constant microturbulencev_turb throughout the wind is dropped and replaced by a radially increasing turbulence parameterv_turb(v), thus improving the fit for the emission peaks substantially, and at the same time avoiding the need for a justification of extremely supersonic turbulence in the vicinity of the wind's sonic point. The Sobolev optical depth is determined interatively at fixed velocities in the wind, which removes the bias introduced by the choice of a specific parameterization function. Where it was possible and necessary a full photospheric spectrum was used to illuminate the wind line. The terminal velocitiesv are are found to be largest in the Galaxy, smallest in the Small Magellanic Cloud, and intermediate or similar to galactic in the Large Cloud.