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Session 96 - Cepheids and Supergiant Variable Stars.
Display session, Thursday, January 16
Metropolitan Ballroom,
We report the results of a 30 day IUE\/ time series (with a mean sampling of \sim 3 times a day) of wind variability in two B supergiants with typical projected rotational velocities. The implied rotation periods for the program stars are \leq 18.6 days for one and \leq 27 days for the other.
The wind variability in the more rapidly rotating supergiant clearly shows cyclical behavior with a period of \sim 7.7 days. The modulation is most clearly seen at low velocity in the low ions (C \sc ii \lambda \lambda 1335, Al \sc iii \lambda \lambda 1860, and the Si \sc iii \lambda \lambda 1300 triplets), demonstrating a photospheric origin of the disturbances. Furthermore, since the period of the variability is roughly half of the most probable rotation period of the star, we attribute the variability to rotational modulation of its wind by two distinct, equidistant surface features. We note, however, that there is also complex substructure to the modulation which is unresolved at our temporal sampling rate.
The more slowly rotating supergiant does not show distinctly repeating structures in its wind lines, but there is an indication that a single feature is repeating on the same time scale as its rotation period.
When considered in context with previous observations of a rapidly rotating supergiant, the current results indicate that wind variability in B supergiants is intimately linked to the presence of surface features on these stars.
