The influence of stellar wind variability on measurements of interstellar OVI along sight lines to early-type stars

A primary goal of the Far Ultraviolet Spectroscopic Explorer mission is to understand the origin of the O VI ion in the interstellar medium of the Gal axy and the Magellanic Clouds. Along sight lines to OB-type stars, these in terstellar components are usually blended with O VI stellar wind profiles, which frequently vary in shape. In order to assess the effects of this time -dependent blending on measurements of the interstellar O VI lines, we have undertaken a minisurvey of repeated observations toward OB-type stars in t he Galaxy and the Large Magellanic Cloud. These sparse time series, which c onsist of two to three observations separated by intervals ranging from a f ew days to several months, show that wind variability occurs commonly in O VI (similar to 60% of a sample of 50 stars), as indeed it does in other res onance lines. However, in the interstellar O VI lambda 1032 region, the O V I lambda 1038 wind varies only in similar to 30% of the cases. By examining cases exhibiting large amplitude variations, we conclude that stellar wind variability generally introduces negligible uncertainty for single interst ellar O VI components along Galactic lines of sight but can result in subst antial errors in measurements of broader components or blends of components like those typically observed toward stars in the Large Magellanic Cloud. Due to possible contamination by discrete absorption components in the stel lar O VI line, stars with terminal velocities greater than or equal to the doublet separation (1654 km s(-1)) should be treated with care.