A search for the cause of cyclical wind variability in O stars -: Simultaneous UV and optical observations including magnetic field measurements of the O7.5III star ε Persei

We present the results of an extensive observing campaign on the 07.5 III star xi Persei. The UV observations were obtained with the International Ultraviolet. Explorer. xi Per was monitored continuously in October 1994 during 10 days at ultraviolet and visual wavelengths. The ground-based optical observations include magnetic field measurements, H alpha and He I lambda 6678 spectra, and were partially covered by photometry and polarimetry. We describe a method to automatically remove the variable contamination of telluric lines in the groundbased spectra. The aim of this campaign was to search for the origin of the cyclical wind variability in this star. We determined a very accurate period of 2.086(2) d in the resonance lines of Si IV and in the subordinate N IV and Ha line profiles. The epochs of maximum absorption in the UV resonance lines due to discrete absorption components (DACs) coincide in phase with the maxima in blue-shifted H alpha absorption. This implies that the periodic variability originates close to the stellar surface. The phase-velocity relation shows a maximum at -1400 km s(-1) The general trend of these observations can be well explained by the corotating interaction region (CIR) model. In this model the wind is perturbed by one or more fixed patches: on the stellar surface, which are most probably due to small magnetic field structures. Our magnetic field measurements gave however, only a null-detection with a 1 sigma errorbar of 70 G in the longitudinal component. Some observations are more difficult to fit into this picture. The 2-day period is not detected in the photospheric/transition region line He I lambda 6678. The dynamic spectrum of this line shows a pattern indicating the presence of non-radial pulsation, consistent with the previously reported period of 3.5 h. The edge variability around -2300 km s(-1) in the saturated wind lines of C IV and N V is nearly identical to the edge variability in the unsaturated Si rv Line, supporting the view that this: type of variability is also due to the moving DACs. A detailed analysis using Fourier reconstructions reveals that each DAC actually consists of 2 different components: a "fast" and a "slow" one which merge at higher velocities.