Discovery of the magnetic field in the pulsating B star β Cephei ^⋆,⋆⋆

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: h.f.henrichs@uva.nl
² European Space Agency (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁴ Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France
⁵ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁶ Physics and Astronomy Department, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
⁷ Department of Physics, Royal Military College of Canada, PO Box 17000, Kingston, Ontario, K7K 7B4, Canada
⁸ Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße, 85748 Garching bei München, Germany
⁹ Harvard–Smithsonian Center for Astrophysics, 60 Garden Str., Cambridge, MA 02138, USA
¹⁰ European Space Astronomy Center, Villanueva de la Cañada, 28691 Madrid, Spain
¹¹ W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA

Received: 27 March 2013
Accepted: 22 April 2013

Abstract

Context. Although the star itself is not helium enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star β Cephei are similar to what is observed in magnetic helium-peculiar B stars, suggesting that β Cep is magnetic.

Aims. We searched for a magnetic field using high-resolution spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data.

Methods. We used 130 time-resolved circular polarisation spectra that were obtained from 1998 (when β Cep was discovered to be magnetic) to 2005, with the MuSiCoS échelle spectropolarimeter at the 2 m Télescope Bernard Lyot. We applied the least-square deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on equivalent-width measurements of UV wind lines obtained over 17 years. We also analysed the short- and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion, respectively.

Results. β Cep hosts a sinusoidally varying magnetic field with an amplitude 97 ± 4 G and an average value − 6 ± 3 G. From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability.

Conclusions. This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with the ~90-year orbit around its Be-star binary companion.