Wind Variability of B Supergiants. I. The Rapid Rotator HD 64760 (B0.5 Ib)

We present the results of a 6 day time series of observations of the rapidly rotating B0.5 Ib star HD 64760, We point out several reasons why such intermediate luminosity B supergiants are ideal targets for wind variability studies and then present our results that show the following: (1) continuous wind activity throughout the 6 day run with the wind never in steady state for more than a few hr; (2) wind variability very near nu = 0 km s(-1) in the resonance lines from the lower ionization stages (Al III and C II); (3) a distinct correlation between variability in the Si III lambda lambda 1300 triplets, the strong C III lambda 1247 singlet, and the onset of extremely strong wind activity, suggesting a connection between photospheric and wind activity; (4) long temporal coherence in the behavior of the strong absorption events; (5) evidence for large-scale spatial coherence, implied by a whole scale, simultaneous weakening in the wind absorption over a wide range in velocities; (6) ionization variability in the wind accompanying the largest changes in the absorption strengths of the wind lines.In addition, modeling of the wind lines provides the following information about the state the wind in HD 64760. (1) The number of structures on the portion of a constant velocity surface occulting the stellar disk at a particular time must be quite small, while the number on the entire constant velocity surface throughout the wind must be large. (2) The escape probability at low velocity is overestimated by a normal beta similar to 1 velocity law, perhaps due to the presence of low-velocity shocks deep in the wind or a shallow velocity gradient at low velocity. (3) Estimates of the ionization structure in the wind indicate that the ionization ratios are not those expected from thermal equilibrium wind models or from an extrapolation of previous O star results. The large observed q(N v)/q(Si IV) ratio is almost certainly due to distributed X-rays, but the level of ionization predicted by distributed X-ray wind models is inconsistent with the predicted mass-loss rate. Thus, it is impossible to reconcile the observed ionization ratios and the predicted mass-loss rate within the framework of the available models,