Scattering polarization due to light source anisotropy

R. Ignace$^1$, M. Al-Malki$^2$, J. Simmons$^2$, J. Brown$^2$, D. Clarke$^2$, J. Carson$^1$

$^1$ Physics and Astronomy, East Tennessee State University
$^2$ Physics and Astronomy, University of Glasgow

We consider the polarization arising from scattering in an envelope
illuminated by a central anisotropic source. Spherical harmonics
are used to describe both the light source anisotropy and the
envelope density distribution functions of the scattering particles.
This framework demonstrates how the net resultant polarization
arises from a superposition of three basic ``shape'' functions:
the distribution of source illumination, the distribution of envelope
scatterers, and the phase function for dipole scattering. Specific
expressions for the Stokes parameters and scattered flux are derived
for the case of an ellipsoidal light source inside an ellipsoidal
envelope, with principal axes that are generally not aligned. Two
illustrative examples are considered: (a) axisymmetric mass loss
from a rapidly rotating star, such as may apply to some Luminous
Blue Variables, and (b) a Roche-lobe filling star in a binary system
with a circumstellar envelope. As a general conclusion, the
combination of source anisotropy with distorted scattering envelopes
leads to more complex polarimetric behavior such that the source
characteristics should be carefully considered when interpreting
polarimetric data.

Reference: to appear in A&A
Status: Manuscript has been accepted

Weblink: astroph/0812.2772