COORDINATED ULTRAVIOLET AND H-ALPHA SPECTROSCOPY OF BRIGHT O-TYPE STARS

As part of our search for the origin of stellar-wind variability, we h ave conducted simultaneous ultraviolet and Her spectroscopy of a numbe r of bright O stars. The observed changes in the H alpha line occur at low velocity (0-0.2v(infinity)) on timescales that are characteristic of the development and evolution of discrete absorption components (D ACs) in UV resonance lines. In some cases, a direct relationship is fo und between the changes occuring in the H alpha line and subsequent va riations in the high-velocity stellar wind. On the basis of this relat ionship, the appearance of a DAC in the UV resonance lines can be pred icted from (ground-based) H alpha observations. These observations sho w that the stellar wind is variable down to regions close to or at the stellar surface. Since the timescales of the variations can be relate d to the rotation periods of the stars in our sample, we propose that a stellar magnetic field (which remains undetected) might play an impo rtant role in affecting the base of the stellar wind. The observed var iations are interpreted in terms of corotating wind structures, simila r to the Corotating Interaction Region (CIR) model proposed by Mullan (1986) and recently simulated by Cranmer & Owocki (1996).