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Astron. Astrophys. 327, 281-298 (1997)

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

L. Kaper 1, 2, H.F. Henrichs 2, A.W. Fullerton 3, H. Ando 4, K.S. Bjorkman 5, D.R. Gies 6, R. Hirata 7, E. Kambe 8, D. McDavid 9 and J.S. Nichols 10

1 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany
2 Astronomical Institute, University of Amsterdam, Netherlands
3 Universitäts Sternwarte München, München, Germany
4 National Astronomical Observatory, Mitaka, Tokyo 181, Japan
5 The University of Toledo, Ohio, USA
6 Dept. Physics and Astronomy, Georgia State University, Atlanta, USA
7 Dept. of Astronomy, Fac. of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
8 Dept. of Geoscience, National Defense Academy, Yokosuka, Kanagawa 239, Japan
9 Limber Observatory, Texas, USA
10 IPAC, 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 H [FORMULA] spectroscopy of a number of bright O stars. The observed changes in the H [FORMULA] 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 H [FORMULA] 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) H [FORMULA] 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

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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