Non-Spherical Dust Grains in the Envelopes of Late-Type Giants

Abstract

Effects of the grain shape on circumstellar dust dynamics and polarization of stellar radiation are analyzed. The grains are modeled by rotating prolate and oblate spheroids.

It is shown that an asymmetry of the geometry of light scattering by non-spherical particles results in a component of the radiation pressure force $\vec F_{pr}$ perpendicular to the wave-vector of incident light. For silicate spheroids, this component can exceed 20% of ∣$\left| {\vec F_{pr} } \right|$∣. For small metallic grains, the radiation pressure force for a spheroid can be 5–10 times greater than that for a sphere of the same volume.

A simple light scattering consideration demonstrates that the distinction in the scattering geometry of aligned non-spherical grains can explain the observed wavelength variations of the positional angle of polarization.