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

In this paper, we use the very recent spectropolarimetric observations of b eta 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 fundam ental parameters of beta Cep in the light of the Hipparcos parallax indicat es 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 w ith an inclination of the rotation axis of about 60 degrees. Using two diff erent modelling strategies, we obtain that the magnetic field of beta Cep c an 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 ab out 8 to 9 R-*. We find that both the X-ray luminosity and variability of b eta Cep can be explained within the framework of the magnetically confined wind-shock model of Babel & Montmerle, in which the stellar-wind streams fr om both magnetic hemispheres collide with each other in the magnetic equato rial plane, producing a strong shock, an extended post-shock region and a h igh-density cooling disc. By studying the stability of the cooling disc, we obtain that field lines c an support the increasing disc weight for no more than a month before they become significantly elongated in an effort to equilibrate the gravitationa l 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 to wards the star. We propose that this collapse is the cause for the recurren t Be episodes of beta Cep, and finally discuss the applicability of this mo del to He peculiar, classical Be and normal non-supergiant B stars.