The magnetic field and confined wind of the O star  Orionis C

¹ Department of Physics, Royal Military College of Canada, PO Box 17000, Station “Forces”, Kingston, Ontario, K7K 4B4, Canada e-mail: Gregg.Wade@rmc.ca
² Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, USA
³ Dept. of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: awf@pha.jhu.edu
⁴ Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France e-mail: donati,pascal.petit@astro.obs-mip.fr
⁵ Physics & Astronomy Department, The University of Western Ontario, London, ON, N6A 3K7, Canada e-mail: jlandstr@astro.uwo.ca
⁶ Max-Planck Institut für Aeronomie Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: petit@linmpi.mpg.de
⁷ Dept. of Astronomy, University of Minnesota, 116 Church St. S.E., Minneapolis, MN 55455, USA e-mail: strasser@astro.umn.edu

Received: 20 October 2005
Accepted: 20 December 2005

Abstract

Aims.In this paper we confirm the presence of a globally-ordered, kG-strength magnetic field in the photosphere of the young O star  Orionis C, and examine the properties of its optical line profile variations.

Methods.A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the rotational cycle of  Orionis C. Using the Least-Squares Deconvolution (LSD) multiline technique, we have succeeded in detecting variable Stokes V Zeeman signatures associated with the LSD mean line profile. These signatures have been modeled to determine the magnetic field geometry. We have furthermore examined the profile variations of lines formed in both the wind and photosphere using dynamic spectra.

Results.Based on spectrum synthesis fitting of the LSD profiles, we determine that the polar strength of the magnetic dipole component is  G and that the magnetic obliquity is , assuming . The best-fit values for are and . Our data confirm the previous detection of a magnetic field in this star, and furthermore demonstrate the sinusoidal variability of the longitudinal field and accurately determine the phases and intensities of the magnetic extrema. The analysis of “photospheric” and “wind” line profile variations supports previous reports of the optical spectroscopic characteristics, and provides evidence for infall of material within the magnetic equatorial plane.