Long - and short - term variability in O - star winds ⋆

A quantitative analysis of time series of ultraviolet spectra from a sample of 10 bright O-type stars (cf. Kaper et al. 1996, Paper I) is presented. Migrating discrete absorption components (DACs), responsible for the observed variability in the UV resonance doublets, are modeled. To isolate the DACs from the underlying P Cygni lines, a method is developed to construct a template (" least-absorption ") spectrum for each star. The central velocity, central optical depth, width, and column density of each pair of DACs is measured and studied as a function of time. It turns out that the column density of a DAC first increases and subsequently decreases with time when the component is approaching its asymptotic velocity. Sometimes a DAC vanishes before this velocity is reached. In some cases the asymp-totic DAC velocity systematically differs from event to event. In order to determine the characteristic timescale(s) of DAC variability, Fourier (CLEAN) analyses have been performed on the time series. The recurrence timescale of DACs is derived for most targets, and consistent results are obtained for different spectral lines. The DAC recurrence timescale is interpreted as an integer fraction of the stellar rotation period. In some datasets the variability in the blue edge of the P Cygni lines exhibits a longer period than the DAC variability. This might be related to the systematic difference in asymptotic velocity of successive DACs. The phase information provided by the Fourier analysis confirms the expected change in phase with increasing velocity. This supports the interpretation that the DACs are responsible for the detected periodicity. The phase diagram for the O giant ξ Per shows clear evidence for so-called " phase bowing " , which is an observational indication for the presence of curved wind structures like corotating interaction regions in the stellar wind. An important difference with the results obtained for the B supergiant HD 64760 (Fullerton et al. 1997) is that in this O star the phase bowing can be associated with the DACs. No other O stars in our sample convincingly show phase bowing, but this could be simply due to the absence of periodic signal and hence coherent phase behaviour at low wind velocities.