A Pulsational Mechanism for Producing Keplerian Disks around Be Stars

Steven R. Cranmer$^1$

$^1$ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

Classical Be stars are an enigmatic subclass of rapidly rotating
hot stars characterized by dense equatorial disks of gas that have
been inferred to orbit with Keplerian velocities.
Although these disks seem to be ejected from the star and not
accreted, there is substantial observational evidence to show that
the stars rotate more slowly than required for centrifugally
driven mass loss.
This paper develops an idea (proposed originally by Hiroyasu
Ando and colleagues) that nonradial stellar pulsations inject
enough angular momentum into the upper atmosphere to spin up a
Keplerian disk.
The pulsations themselves are evanescent in the stellar photosphere,
but they may be unstable to the generation of resonant oscillations
at the acoustic cutoff frequency.
A detailed theory of the conversion from pulsations to resonant
waves does not yet exist for realistic hot-star atmospheres, so
the current models depend on a parameterized approximation for
the efficiency of wave excitation.
Once resonant waves have been formed, however, they grow in
amplitude with increasing height, steepen into shocks, and exert
radial and azimuthal Reynolds stresses on the mean fluid.
Using reasonable assumptions for the stellar parameters, these
processes were found to naturally create the inner boundary
conditions required for dense Keplerian disks, even when the
underlying B-star photosphere is rotating as slowly as 60 percent
of its critical rotation speed.
Because there is evidence for long-term changes in Be-star
pulsational properties, this model may also account for the
long-term variability of Be stars, including transitions between
normal, Be, and shell phases.

Reference: ApJ, 701, in press (August 20, 2009).
Status: Manuscript has been accepted

Weblink: http://arxiv.org/abs/0906.2772


Email: scranmer@cfa.harvard.edu