The magnetic field and wind confinement of β Cephei:: new clues for interpreting the Be phenomenon?

In this paper, we use the very recent spectropolarimetric observations of beta Cep collected by Henrichs et al. and propose for this star a consistent model of the large-scale magnetic field and of the associated magnetically confined wind and circumstellar environment. A reexamination of the fundamental parameters of beta Cep in the light of the Hipparcos parallax indicates that this star is most likely a 12-M-circle dot star with a radius of 7 R-circle dot, effective temperature of 26 000 K and age of 12 Myr, viewed with an inclination of the rotation axis of about 60 degrees. Using two different modelling strategies, we obtain that the magnetic field of beta Cep can be approximately described as a dipole with a polar strength of 360 +/- 30 G, the axis of symmetry of which is tilted with respect to the rotation axis by about 85 degrees +/- 10 degrees. Although one of the weakest detected to date, this magnetic field is strong enough to magnetically confine the wind of beta Cep up to a distance of about 8 to 9 R-*. We find that both the X-ray luminosity and variability of beta Cep can be explained within the framework of the magnetically confined wind-shock model of Babel & Montmerle, in which the stellar-wind streams from both magnetic hemispheres collide with each other in the magnetic equatorial plane, producing a strong shock, an extended post-shock region and a high-density cooling disc. By studying the stability of the cooling disc, we obtain that field lines can support the increasing disc weight for no more than a month before they become significantly elongated in an effort to equilibrate the gravitational plus centrifugal force, thereby generating strong field gradients across the disc. The associated current sheet eventually tears, forcing the field to reconnect through resistive diffusion and the disc plasma to collapse towards the star. We propose that this collapse is the cause for the recurrent Be episodes of beta Cep, and finally discuss the applicability of this model to He peculiar, classical Be and normal non-supergiant B stars.