The extended envelopes around cool giant stars are believed to be a place of dust formation and important sources of recycled interstellar matter. Though the single mechanism initiating the wind from evolved stars is not acknowledged yet, it is generally accepted that the mass loss is controlled by radiation pressure on dust grains (Lafon & Berruyer 1991; Winters 1994). Momentum of stellar radiation is transferred to the circumstellar gas through radiation pressure on the grains and gas-grain collisions. Recently, Netzer & Elitzur (1993) and Habing et al. (1994) have connected the rate of mass loss from cool giants with the observed gas outflow velocity.
The shape of particles should be an important factor that affects the momentum transfer. However, this effect has been considered only in a few cases. Voshchinnikov & Il'in (1983b,c) investigated the pressure of radiation of different stars on infinite circular cylinders. Il'in (1994) analyzed the motion of very small spheroidal grains in the envelopes of M giants.
The fact that starlight is polarized in many lines of sight indicates that interstellar grains are non-spherical and aligned. As neither theoretical nor observational arguments exist in favor of changes of particles shapes in the diffuse interstellar medium, we can propose that the shape of grains originating in the envelopes of evolved stars differ from the spherical one.
An indirect indication that the non-spherical grains are present
in the envelopes comes from the observed time and wavelength variations of
the positional angle of polarization of the red giants.
The difference of its values in the blue and red bands
reaches 20
even after the subtraction of interstellar
polarization (Dyck & Jennings 1971; Shawl 1975).
This behaviour is hard to explain within the model of a single star with
the envelope populated by spherical grains only.
On the other hand, variations of the degree and the direction of
orientation of non-spherical circumstellar grains could naturally
produce the effect.
In this paper, we consider how the particle's shape can affect the grain motion in the vicinities of late-type stars. The radiation pressure force is calculated for prolate and oblate spheroids of different semiaxes ratio, size and composition. The force and the drift velocity obtained from the solution of a simple equation of dust motion are compared with those for spheres of the same volume. The model and equations used are presented in Sects. 2 and 3, the results of calculations are discussed in Sect. 4, and the conclusions are given in Sect. 5.