  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% 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.
