1997A&A...318..157P

The stellar wind of the rapidly rotating early-B supergiant, γ Ara, is studied using time series, high-resolution IUE spectroscopy secured over ∼6 days in 1993 March.
Result. are presented based on an analysis of several line species, including N V, C IV, Si IV, Si III, C II, and Al III. Comparisons of the time-averaged wind line morphology of γ Ara to the UV spectra of other OB stars, and to profiles from spherically symmetric wind models, suggest that the wind of γ Ara is equatorially enhanced. Co-existing time variable features are identified at low-velocity (redward of ∼750km/s) and at higher-speeds extending to ~-1500km/s. The observed interface between these structures is `defined' by the appearance of a discrete absorption component which is extremely sharp (in velocity space). The central velocity of this `Super DAC' changes only gradually, over several days, between ~-400 and -750km/s in most of the ions. However, its location is shifted redward by almost 400km/s in Al III and C II, indicating that the physical structure giving rise to this feature has a substantial velocity and ionization jump. Constraints on the relative ionization properties of the wind structures are discussed. The overall wind activity in γ Ara exhibits a clear ion dependence, such that low-speed features are promoted in low-ionization species, including Al III, C II, and Si III. We also highlight that - in contrast to most OB stars - there are substantial differences in the epoch-to-epoch time-averaged wind profiles of γ Ara and that the high-speed component observed during our 1993 time series is normally not present. We discuss the potential roles of the radiative bi-stability mechanism and wind compressed regions due to rapid stellar rotation for providing gross wind distortions in γ Ara.