In this paper, we use the very recent spectropolarimetric observations of beta Cep
collected by Henrichs et al. (2001) 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 re-examination of the fundamental parameters of beta Cep in the
light of the Hipparcos parallax indicates that this star
is most likely a 12 Msun star with a radius of 7 Rsun,
effective temperature of 26000 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, whose axis of symmetry is tilted with respect to
the rotation axis by about 85+-10 degrees.
Although one of the weakest detected to date, this magnetic field is strong enough to confine magnetically the wind of beta Cep up to a distance of about 8 to 9 Rstar. 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 (1997a), 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 postshock 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 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.