Coordinated ultraviolet and H alpha spectroscopy of bright O-type stars

As part of our search for the origin of stellar-wind variability, we have conducted simultaneous ultraviolet and Her spectroscopy of a number 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 (DACs) in UV resonance lines. In some cases, a direct relationship is found between the changes occuring in the H alpha line and subsequent variations in the high-velocity stellar wind. On the basis of this relationship, the appearance of a DAC in the UV resonance lines can be predicted from (ground-based) H alpha observations. These observations show that the stellar wind is variable down to regions close to or at the stellar surface. Since the timescales of the variations can be related to the rotation periods of the stars in our sample, we propose that a stellar magnetic field (which remains undetected) might play an important role in affecting the base of the stellar wind. The observed variations are interpreted in terms of corotating wind structures, similar to the Corotating Interaction Region (CIR) model proposed by Mullan (1986) and recently simulated by Cranmer & Owocki (1996).