Far Ultraviolet Spectroscopic Explorer snapshot survey of O VI variability in the winds of 66 OB-type stars

We have used the Far Ultraviolet Spectroscopic Explorer to conduct a snapshot 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 in both 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. In view of the limitations of this survey, 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. In contrast, for S IV and P V, only a small percentage of the whole sample shows wind variations, although this may be principally due to selection effects. The observed variations extend over several hundreds of kilometers per second 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 (v _∞), but a significant correlation (close to a 1 : 1 relationship) is derived between the maximum velocity of the variation and v_∞. 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 IV and the X-rays originate from strong shocks in the wind, this study suggests that stronger shocks occur more frequently near v_∞, causing an enhancement of O VI near V_∞.