  Gravitational contraction always generates a radially directed momentum flux.
A particularly simple example occurs in the electron-degenerate cores of AGB
stars, which contract steadily under the addition of helium ashes from shell
hydrogen burning. The resulting momentum flux is quantified here. And since the
cores of AGB stars lack efficient momentum cancellation mechanisms, they can
maintain equilibrium by exporting their excess momentum flux to the stellar
envelope, which disposes of much of it in a low velocity wind. Gravitational
contraction easily accounts for the momentum flux in the solar wind, as well as
the flux required to lift mass into the dust formation zone of every AGB star,
whereon radiation pressure continues its ejection as a low velocity wind. This
mechanism explains the dependence of the AGB mass-loss rate on core mass; its
generalization to objects with angular momentum and/or strong magnetic fields
suggests a novel explanation of why most planetary nebulae and proto planetary
nebulae exhibit axial symmetry.
  Quasistatic contraction is inherently biased to the generation of the maximum
possible momentum flux. Its formalism is therefore readily adapted to providing
an upper limit to the momentum flux needed to sustain mass loss when this
begins from a semi-continuous rather than impulsive process.

Astrophysics