Polarization of light scattered from the winds of early-type stars

There is evidence from visible wavelength polarimetry that the matter in the winds of early-type stars is not distributed uniformly about the stars. In this paper equations for the observable polarization from an axisymmetric distribution of electrons are derived first from the basic point source model of Brown and McLean (1977) and then from extended atmosphere radiative transfer theory. A factor is derived which corrects the Brown and McLean model for the finite size of stars. This finite disk factor is found to reduce the magnitude of the predicted polarization to less than half of the point source prediction. The effects of having the mass outflow concentrated in a plume are considered. The effects of absorption in an unocculted plume are shown to give rise to a wavelength-dependent polarization that is described using a 'polarization color' parameter. A continuum opacity index is defined that allows for a straightforward interpretation of the polarization of stars of differing effective temperatures.