Discovery of the magnetic field in the pulsating B starβCephei

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. Using 130 time-resolved circular polarisation spectra that were obtained with the MuSiCoS échelle spectropolarimeter at the 2m Telescope Bernard Lyot from 1998, when β Cep was discovered to be magnetic, until 2005, we applied the least-squares 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.

Fundamental properties of nearby single early B-type stars

“…It may be speculated that initially this was a faster rotator, thus explaining the mixing of the surface with CN-processed material, while magnetic breaking due to angular momentum losses by a magnetically confined line-driven stellar wind has lead to the spin-down (ud-Doula et al 2009). Overall, there is excellent agreement of β Cep's fundamental parameters derived by Henrichs et al (2013) and our values, despite somewhat different atmospheric parameters. Note that the M 0 V = −5.8 ± 0.2 given by Henrichs et al (2013) is probably a typo.…”

“…Overall, there is excellent agreement of β Cep's fundamental parameters derived by Henrichs et al (2013) and our values, despite somewhat different atmospheric parameters. Note that the M 0 V = −5.8 ± 0.2 given by Henrichs et al (2013) is probably a typo.…”

Period change and stellar evolution ofβCephei stars

“…Briquet et al (2012) measured rotation rates of about 70−75 km s −1 and smaller amounts of convective core overshooting than for other β Cephei variable stars. The prototype β Cep is similarly a magnetic He-strong star with a varying magnetic field of about 100 G (Henrichs et al 2013). The three β Cep stars that cannot be constrained, i.e., that are consistent with all of the computed models, appear to have little in common.…”

Magnetic fields in early-type stars

“…The first genuinely massive (> 10 M ⊙ ) magnetic stars were only detected about 15 years ago (e.g. the classical B1III pulsator β Cep (Henrichs et al 2000(Henrichs et al , 2013Donati et al 2001); the young massive O7V star θ 1 Ori C (Donati et al 2002)). Prior to 2009, only 3 magnetic O-type stars were even known (θ 1 Ori C; the Of?p star HD 191612 (Donati et al 2006); the evolved O9.7Ib binary star ζ Ori A (Bouret et al 2008)), and 9 of the 13 of the most massive stars were discovered between 2005 and 2009.…”