FUSE Snap-Shot Survey of O VI Variability in the Winds of 66 OB-Type Stars

We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to conduct a snap-shot survey of O VI variability in the winds of 66 OB-type stars in the Galaxy and the Magellanic Clouds. These time series consist of two or three observations separated by intervals ranging from a few days to several months. Although these time series provide the bare minimum of information required to detect variations, this survey demonstrates that the O VI doublet in the winds of OB-type stars is variable on various scales both in time and velocity. For spectral types from O3 to B1, 64% vary in time. At spectral types later than B1, no wind variability is observed. This fraction represents a lower limit on the true incidence of variability in the O VI wind lines, which is very common and probably ubiquitous. The observed variations extend over several hundreds of km/s of the wind profile and can be strong. The width over which the wind O VI profile varies is only weakly correlated with the terminal velocity (\vinf), but a significant correlation (close to a 1:1 relationship) is derived between the maximum velocity of the variation and \vinf. High velocity O VI wind absorption features (possibly related to the discrete absorption components seen in other wind lines) are also observed in 46% of the cases for spectral types from O3 to B0.5. These features are variable, but the nature of their propagation cannot be determined from this survey. If X-rays can produce sufficient O VI by Auger ionization of O VI, and the X-rays originate from strong shocks in the wind, this study suggests that stronger shocks occur more frequently near \vinf, causing an enhancement of O VI near \vinf.