The magnetic field and wind confinement of
 Cephei:
new clues for interpreting the Be phenomenon?
J.-F. Donati1,
G.A. Wade2,
J. Babel3,
H.F. Henrichs4,
J.A. de Jong4, &
T.J. Harries5
1
Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées,
14 Av. E. Belin, F-31400 Toulouse, France
2
Dépt. de Physique, Université de Montréal,
CP 6128 succ Centre-Ville, Montréal QC, Canada H3C 3J7
3
36 rue des Battieux, 2000 Neuchatel, Switzerland
4
Astronomical Institute 'Anton Pannekoek', University of Amsterdam,
Kruislaan 403, 1098SJ Amsterdam, Netherlands
5
School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL,
UK
We use the very recent spectropolarimetric observations of
 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
Cep
in the light of the Hipparcos parallax indicates that this star
is most likely a
12 M
star with a radius of
7 R,
effective temperature of 26000 K and age of 12 Myr, viewed with an
inclination of the rotation axis of about 60°. Using two different
modelling strategies, we obtain that the magnetic field of
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°.
Although one of the weakest detected to date, this magnetic field
is strong enough to confine magnetically the wind of
Cep
up to a distance of about 8 to 9 R*. We find that
both the X-ray luminosity and variability of
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
Cep,
and finally discuss the applicability of this model to He peculiar,
classical Be and normal non-supergiant B stars.
Accepted by MNRAS
Preprints from
jean-francois.donati@obs-mip.fr
or on the web at
http://webast.ast.obs-mip.fr/people/donati
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