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Astron. Astrophys. 327, 281-298


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Coordinated ultraviolet and H spectroscopy of bright O-type stars*

L. Kaper1,2, H.F. Henrichs2, A.W. Fullerton3, H. Ando4, K.S. Bjorkman5, D.R. Gies6, R. Hirata7, E. Kambe8, D. McDavid9, and J.S. Nichols10

1European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
2Astronomical Institute, University of Amsterdam, Netherlands
3Universitäts Sternwarte München, München, Germany
4National Astronomical Observatory, Mitaka, Tokyo 181, Japan
5The University of Toledo, Ohio, USA
6Dept. Physics and Astronomy, Georgia State University, Atlanta, USA
7Dept. of Astronomy, Fac. of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
8Dept. of Geoscience, National Defense Academy, Yokosuka, Kanagawa 239, Japan
9Limber Observatory, Texas, USA
10IPAC, California Institute of Technology, Pasadena, USA

Received 3 April 1997 / Accepted 16 June 1997

Abstract

As part of our search for the origin of stellar-wind variability, we have conducted simultaneous ultraviolet and HFORMULA spectroscopy of a number of bright O stars. The observed changes in the HFORMULA line occur at low velocity (FORMULA) on timescales that are characteristic of the development and evolution of discrete absorption components (DACs) in UV resonance lines. In some cases, a direct relationship is found between the changes occuring in the HFORMULA line and subsequent variations in the high-velocity stellar wind. On the basis of this relationship, the appearance of a DAC in the UV resonance lines can be predicted from (ground-based) HFORMULA observations.

These observations show that the stellar wind is variable down to regions close to or at the stellar surface. Since the timescales of the variations can be related to the rotation periods of the stars in our sample, we propose that a stellar magnetic field (which remains undetected) might play an important role in affecting the base of the stellar wind. The observed variations are interpreted in terms of corotating wind structures, similar to the Corotating Interaction Region (CIR) model proposed by Mullan (1986) and recently simulated by Cranmer & Owocki (1996).

Key words: stars: early-type - stars: magnetic fields - stars: mass loss - stars: rotation - ultraviolet: stars

*Based on observations collected with the International Ultraviolet Explorer from Vilspa, Madrid, Spain, and GSFC, Greenbelt, USA and the 1.52m telescope with Aurélie at O.H.P., France

Send offprint requests to: L. Kaper


© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998
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