On the Origin of Wind Line Variability in O Stars

Abstract

We analyze 10 UV time series for five stars that fulfill specific sampling and spectral criteria to constrain the origin of large-scale wind structure in O stars. We argue that excited state lines must arise close to the stellar surface and are an excellent diagnostic complement to resonance lines which, due to radiative transfer effects, rarely show variability at low velocity. Consequently, we splice dynamic spectra of the excited state line N iv λ1718 at low velocity with those of Si iv λ λ 1400 at high velocity in order to examine the temporal evolution of wind line features. These spliced time series reveal that nearly all of the features observed in the time series originate at or very near the stellar surface. Furthermore, we positively identify the observational signature of equatorial corotating interaction regions in two of the five stars and possibly two others. In addition, we see no evidence of features originating further out in the wind. We use our results to confirm the fact that the features seen in dynamic spectra must be huge in order to remain in the line of sight for days, persisting to very large velocity, and that the photospheric footprint of the features must also be quite large, ∼15%-20% of the stellar diameter.