  We examine the nature and role of mass loss via an equatorial decretion disk
in massive stars with near-critical rotation induced by evolution of the
stellar interior. In contrast to the usual stellar wind mass loss set by
exterior driving from the stellar luminosity, such decretion-disk mass loss
stems from the angular momentum loss needed to keep the star near and below
critical rotation, given the interior evolution and decline in the star's
moment of inertia. Because the specific angular momentum in a Keplerian disk
increases with the square root of the radius, the decretion mass loss
associated with a required level of angular momentum loss depends crucially on
the outer radius for viscous coupling of the disk, and can be significantly
less than the spherical mass loss the spherical, wind-like mass loss commonly
assumed in evolutionary calculations. We discuss the physical processes that
affect the outer disk radius, including thermal disk outflow, and ablation of
the disk material via a line-driven wind induced by the star's radiation. We
present parameterized scaling laws for taking account of decretion-disk mass
loss in stellar evolution codes, including how these are affected by
metallicity, or by presence within a close binary and/or a dense cluster.
Effects similar to those discussed here should also be present in accretion
disks during star formation, and may play an important role in shaping the
distribution of rotation speeds on the ZAMS.

Solar and Stellar Astrophysics