Volume 368, Number 2, March III 2001
|Page(s)||601 - 621|
|Section||Stellar structure and evolution|
|Published online||15 March 2001|
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 ξ Persei
Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands; e-mail: firstname.lastname@example.org; email@example.com
2 Leiden Observatory, University of Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands e-mail: firstname.lastname@example.org
3 Harvard/Smithsonian Center for Astrophysics, 60 Garden Str., Cambridge, MA 02138, USA e-mail: email@example.com
4 Ritter Astrophysical Research Center, The University of Toledo, Toledo, OH 43606, USA e-mail: firstname.lastname@example.org; email@example.com
5 National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 W. Saanich Road, Victoria BC, Canada V9E 2E7; e-mail: firstname.lastname@example.org
6 Beijing Astronomical Observatory, Beijing 100012, PR China; e-mail: email@example.com
7 Steward Observatory, University of Arizona, Tuczon, AZ 85712, USA; e-mail: firstname.lastname@example.org
8 Physics Dept. University of Montreal, PQ H3C 3J7 Montreal, Canada; e-mail: email@example.com
9 Tartu Observatory, 61602 Tõravere, Estonia; e-mail: firstname.lastname@example.org
10 Dept. of Physics and Astronomy, College of Charleston, Charleston, SC 29424, USA; e-mail: email@example.com
11 Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA e-mail: firstname.lastname@example.org
12 Isaac Newton Group of Telescopes, NWO (Netherlands Organisation for Scientific Research), Apartado 321, 38700 Santa Cruz de La Palma, Spain; e-mail: email@example.com
Corresponding author: J. A. de Jong, firstname.lastname@example.org
Accepted: 11 December 2000
We present the results of an extensive observing campaign on the O7.5 III star ξ Persei. The UV observations were obtained with the International Ultraviolet Explorer. ξ Per was monitored continuously in October 1994 during 10 days at ultraviolet and visual wavelengths. The ground-based optical observations include magnetic field measurements, Hα and Hei λ6678 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 Siiv and in the subordinate Niv and Hα 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 Hα 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 1σ 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 Hei λ6678. 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 Civ and Nv is nearly identical to the edge variability in the unsaturated Siiv 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.
Key words: stars / early-type / atmospheres / winds, outflows / magnetic fields
© ESO, 2001
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