Long- and short-term variability in O-star winds. I. Time series of UV spectra for 10 bright O stars

Abstract

Abstract. -An atlas of time series of ultraviolet spectra is presented for 10 bright O stars. The spectra were obtained with the International Ultraviolet Explorer during seven observing campaigns lasting several days over 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 significantly different for each star, but remains relatively constant during the time span of our observations for a given star. A quantitative evaluation 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 velocity v∞ of the wind as measured by the asymptotic velocity reached by DACs. The amplitude of variability reaches a maximum at about 0.75 v∞ in the unsaturated resonance lines of stars showing DACs. In saturated resonance lines we find distinct changes in the steep blue edge. This edge variability is also found, although with smaller amplitude, in unsaturated resonance lines. The subordinate line of N IV at 1718Å in ξ Per shows weak absorption enhancements at low velocities in the blue-shifted absorption that are clearly associated with the DACs in the UV resonance lines. We interpret these three 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 saturated underlying wind profile; in many cases, however, at the same time two or more absorption events in different stages of their evolution can be identified in the unsaturated profiles, hampering a detailed interpretation of the edge variability. The low velocity absorption enhancements in the subordinate lines are the precursors of DACs when they are formed close to the star. The constancy of the pattern of variability over the years and the (quasi-)periodic recurrence of DACs strongly suggest that rotation of the star is an essential ingredient for controlling wind variability. The observation of low-velocity variations in subordinate lines, which are supposedly formed at the base of the stellar wind, indicate an origin of wind variability close to or at the photosphere of the star. †

Bright OB stars in the Galaxy

“…Small-scale clumping is suggested by observations of emission-line micro-variability in one of our targets (HD 66811; see Eversberg et al 1998). However, structuring of hot-star winds on large scales is indicated by the ubiquitous presence of recurrent wind profile variability in the form of discrete absorption components (DACs; see, e.g., Prinja & Howarth 1986;Kaper et al 1996). Since there is no consensus on the physical origin of DACs, the structure responsible for them is not included in the present generation of models.…”

γCassiopeiae: an X-ray Be star with personality

“…These profiles exhibit Discrete Absorption features at 1000-1100 km s −1 and edge velocites of -1800 km s −1 (Doazan 1982;Kaper et al 1996;Smith et al 1998a,b). Our analysis shows the presence of soft X-ray components whose temperatures, fluxes, and emission measures (Table 1) are consistent with those of normal massive stars reported by Walborn et al (2009), for example.…”

Wind variability of B supergiants

“…The key developmental sequence was defined in a number of time-series UV investigations (e.g. Prinja et al 1987;Prinja & Howarth 1988;Henrichs et al 1988;Kaper 1993;Kaper et al 1996;De Jong et al 2001). A relatively broad, shallow absorption feature appears at ≤0.3 of the terminal velocity (v ∞ ), which then evolves blue-wards over several hours and perhaps even days, while narrowing in velocity width.…”