LONG-TERM AND SHORT-TERM VARIABILITY IN O-STAR WINDS .1. TIME-SERIES OF UV SPECTRA FOR 10 BRIGHT O-STAR

An atlas of time series of ultraviolet spectra is presented for 10 bri ght O stars. The spectra were obtained with the International Ultravio let Explorer during seven observing campaigns lasting several days ove r a period of 6 years. The UV P Cygni lines in 9 out of the 10 studied stars exhibit a characteristic pattern of variability in the form of discrete absorption components (DACs) migrating through the absorption troughs on a timescale of a day to a week. This pattern is significan tly different for each star, but remains relatively constant during th e time span of our observations for a given star. A quantitative evalu ation of the statistical significance of the variability is given. The winds of a number of stars appear to vary over the full range of wind velocities: from 0 km s(-1) up to velocities exceeding the terminal v elocity v(infinity) of the wind as measured by the asymptotic velocity reached by DACs. The amplitude of variability reaches a maximum at ab out 0.75 v(infinity) in the unsaturated resonance lines of stars showi ng DACs. In saturated resonance lines we find distinct changes in the steep blue edge. This edge variability is also found, although with sm aller amplitude, in unsaturated resonance lines. The subordinate Line of N IV at 1718 Angstrom in xi Per shows weak absorption enhancements at low velocities in the blue-shifted absorption that are clearly asso ciated with the DACs in the UV resonance Lines. We interpret these thr ee manifestations of variation as reflecting a single phenomenon. The DACs are the most conspicuous form of the variability. The changes at the edge can often be interpreted as DACs, but superposed on a saturat ed underlying wind profile; in many cases, however, at the same time t wo or more absorption events in different stages of their evolution ca n be identified in the unsaturated profiles, hampering a detailed inte rpretation of the edge variability. The low velocity absorption enhanc ements in the subordinate lines are the precursors of DACs when they a re formed close to the star. The constancy of the pattern of variabili ty over the years and the (quasi-)periodic recurrence of DACs strongly suggest that rotation of the star is an essential ingredient for cont rolling wind variability. The observation of low-velocity variations i n subordinate lines, which are supposedly formed at the base of the st ellar wind, indicate an origin of wind variability close to or at the photosphere of the star.(dagger)