OB-type lose mass via stellar winds, at a rate of a few millionths of a solar mass per year (about an earth mass per year). Over their lifetimes (several million years) this has a dramatic effect on their evolution. We are studying the variability in their outflows. The denser winds of WR stars are well known to be variable on timescales of hours based on previous ultraviolet and optical investigations.

Our ideas about WR and O star winds have changed dramatically over the last ten years; the picture that is now emerging is one in which the winds are time-dependent and permeated by shocks driven by radiative instabilities. We have continued extensive studies of stellar wind variability in WR stars, with particular attention to extensive IUE monitoring (over continuous periods of 6-7 days). We have been involved with the IUE MEGA campaign in which a Wolf-Rayet star, an O-supergiant and a B supergiant have been continuously monitored with the IUE satellite over 15 days. This will allow us to investigate the time-dependent UV profile variability over several consecutive stellar rotation periods, to probe the connection between rotation and the occurrence of wind structure/variability linked to radiatively-induced wind instabilities. An intensive optical variability study of an Of star, HD151804, has also recently been performed ( Prinja, Fullerton and Crowther 1996 A&A 311 264).

From our previous UV studies of WR6 the P Cygni absorption profiles are seen to vary in the sense that extra absorption, at velocities exceeding the wind terminal velocity, increases and decays with a recurrence timescale of 1 day. The much smaller variations detected in the P Cygni emission components suggest that the material giving rise to the variability has a characteristic linear scale of the order of the core radius. The nature of these variations is most easily explained by radiative instabilities driving high speed, rarefied shocked gas through the stellar wind of WR6 (Willis & Stevens 1996 A&A 310 577). In a parallel study we have secured Guest Observer PSPC ROSAT observations of WR6 at two epochs. Our results show no phase--dependent modulation in the X-ray flux, no evidence for variations on timescales of less than 1 hour (previously suggested from EINSTEIN data), and evidence for significant variations (at the 30% level) on a daily timescale. These results suggest a pattern of X-ray variability consistent with our IUE-based conclusions above.


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02-Dec-99 zuserver2.star.ucl.ac.uk