A&A Supplement series, Vol. 128, February II 1998, 187-196
Received April 28; accepted July 10, 1997
V.B. Il'in
and N.V. Voshchinnikov
Send offprint request: N.V. Voshchinnikov
Max Planck Society, Research Unit "Dust in Star-Forming Regions'',
Schillergäßchen 3, D-07745 Jena, Germany
Astronomical Institute, St. Petersburg University,
St. Petersburg 198904, Russia
Stellar winds are typical features of the AGB evolution. They are formed as the result of the radiation pressure on dust grains.
We calculate the radiation pressure force of a red giant acting on prolate and oblate spheroidal grains of different size, aspect ratio and chemical composition. The exact solution to the light scattering problem for spheroids is used. The force and the grain drift velocity (relative to gas) are compared for spheroids and spheres of the same volume.
It is found that for small spheroids
(radii of equivolume sphere 
)
the radiation pressure force usually is greater than that for spheres.
A very significant effect occurs
for strongly absorbing particles with 
.
It is caused by the resonance absorption of incident radiation
whose electric vector is parallel to the major axis of a particle.
As a result, the velocity of a sphere and equivolume spheroid of iron
can differ in 
times or more.
Another effect is the deviations of
the radiation pressure force from the direction of the
wave-vector of incident radiation.
This is due to an azimuthal asymmetry of geometry of
light scattering by non-spherical particles.
The transversal component of the force
is more important for dielectric particles and can reach
up to 
% of the radial one for
silicate grains of the size 
.
It should increase the path of non-spherical grains in stellar envelopes
and the number of dust-gas collisions in the comparison with spherical
grains.
The tendency of a strengthening of the radial component of the radiation pressure force and a weakness of the transversal one with a decrease of the stellar effective temperature is noted.
keywords: circumstellar matter -- stars: AGB, post-AGB