Long- and short-term variability in O-star winds II. Quantitative analysisof DAC behaviour

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. M igrating discrete absorption components (DACs), responsible for the observe d variability in the UV resonance doublets, are modeled. To isolate the DAC s from the underlying P Cygni lines, a method is developed to construct a t emplate ("least-absorption'') spectrum for each star. The central velocity, central optical depth, width, and column density of each pair of DACs is m easured and studied as a function of time. It turns out that the column density of a DAC first increases and subsequen tly decreases with time when the component is approaching its asymptotic ve locity. Sometimes a DAC vanishes before this velocity is reached. In some c ases the asymptotic DAC velocity systematically differs from event to event In order to determine the characteristic timescale(s) of DAC variability, F ourier (CLEAN) analyses have been performed on the time series. The recurre nce timescale of DACs is derived for most targets, and consistent results a re obtained for different spectral lines. The DAC recurrence timescale is i nterpreted as an integer fraction of the stellar rotation period. In some d atasets the variability in the blue edge of the P Cygni lines exhibits a lo nger period than the DAC variability. This might be related to the systemat ic difference in asymptotic velocity of successive DACs. The phase information provided by the Fourier analysis confirms the expecte d change in phase with increasing velocity. This supports the interpretatio n that the DACs are responsible for the detected periodicity. The phase dia gram for the O giant xi Per shows clear evidence for so-called "phase bowin g", which is an observational indication for the presence of curved wind st ructures like corotating interaction regions in the stellar wind. An import ant difference with the results obtained for the B supergiant HD 64760 (Ful lerton et al. 1997) is that in this O star the phase bowing can be associat ed with the DACs. No other O stars in our sample convincingly show phase bo wing, but this could be simply due to the absence of periodic signal and he nce coherent phase behaviour at low wind velocities.