The IUE MEGA Campaign: Modulated Structure in the Wind of HD 64760 (B0.5 Ib)

We highlight systematic variability in the stellar wind of the early B type supergiant, HD 64760, whose UV line profiles were monitored for almost 16 days in 1995 January as part of the IUE "MEGA Campaign." The extensive coverage reveals a pattern of rapidly evolving discrete optical depth changes which typically migrate from ~-200 km s^-1 to ~-1500 km s^-1 in less than 12 hr. These features coexist with more slowly evolving structures lasting several days. Time-series analysis of the Si IV, Si III, and N V profile variations presents a clear 1.2 day periodicity, which is a quarter of the estimated maximum rotation period of HD 64760. The line profile changes are consistent with an interpretation in terms of a set of corotating wind features which occult the stellar disk at least 3 times during the observing run. These data are combined with UV observations collected in 1993 March to argue in favor of rotationally modulated wind variations in HD 64760.

The basic result of very regular, large-scale optical depth variations points to a "clock" whose origin is on the stellar surface, rather than a mechanism that is entirely intrinsic to the stellar wind.