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Astron. Astrophys. Suppl. Ser.
Volume 128, Number 1, February_II 1998
Page(s) 187 - 196
Published online 15 February 1998
DOI: 10.1051/aas:1998134

A&A Supplement series, Vol. 128, February II 1998, 187-196

Received April 28; accepted July 10, 1997

Radiation pressure on non-spherical dust grains in envelopes of late-type giants

V.B. Il'intex2html_wrap535 and N.V. Voshchinnikovtex2html_wrap537

Send offprint request: N.V. Voshchinnikov

tex2html_wrap539  Max Planck Society, Research Unit "Dust in Star-Forming Regions'', Schillergäßchen 3, D-07745 Jena, Germany
tex2html_wrap541  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 tex2html_wrap_inline513) the radiation pressure force usually is greater than that for spheres. A very significant effect occurs for strongly absorbing particles with tex2html_wrap_inline515. 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 tex2html_wrap_inline517 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 tex2html_wrap_inline519% of the radial one for silicate grains of the size tex2html_wrap_inline521. 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

Copyright by the European Southern Observatory (ESO)

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