A search for O VI in the winds of B-type stars

We have conducted a survey of FUSE spectra of 235 Galactic B-type stars in order to determine the boundaries in the H-R diagram for the production of the superion $\ion{O}{vi}$ in their winds. By comparing the locations and morphology of otherwise unidentified absorption features in the vicinity of the $\ion{O}{vi}$ resonance doublet with the bona fide wind profiles seen in archival IUE spectra of the resonance lines of $\ion{N}{v}$, $\ion{Si}{iv}$ and $\ion{C}{iv}$, we were able to detect blueshifted $\ion{O}{vi}$ lines in the spectra of giant and supergiant stars with temperature classes as late as B1. No features attributable to $\ion{O}{vi}$ were detected in dwarfs later than B0, or in stars of any luminosity class later than B1, although our ability to recognize weak absorption features in these stars is severely restricted by blending with photospheric and interstellar features. We discuss evidence that the ratio of the ion fractions of $\ion{O}{vi}$ and $\ion{N}{v}$ is substantially different in the winds of early B-type stars than O-type stars.