The effect of multiple scattering on the polarization from axisymmetric circumstellar envelopes .2. Thomson scattering in the presence of absorptive opacity sources

We investigate the effect on the polarization of multiple Thomson scattered stellar radiation in axisymmetric circumstellar envelopes that contain sources of continuous absorptive opacity and emission. Our previous investigations of pure electron scattering envelopes have shown that multiple scattering increases the polarization level above that predicted by single scattering plus attenuation approximations. However, the inclusion of sources of absorptive opacity within the envelope lowers the albedo, reducing the number of multiply scattered photons. Consequently, for envelopes possessing a large absorptive opacity, the net polarization approaches the single-scattering plus attenuation levels (which may be positive or negative, depending on the geometry and degree of polarimetric cancellation). Lowering the albedo further (by increasing the absorptive opacity) removes photons that have, been singly scattered so that the polarization decreases below that predicted by the single-scattering plus attenuation approximation. As the albedo approaches zero, few photons are scattered within the envelope (all are absorbed), and the only radiation reaching the observer is unscattered (i.e., unpolarized) stellar radiation; hence, the polarization approaches zero. A consequence of this behavior is that when the albedo changes rapidly with wavelength, as occurs across ionization edges (e.g., across the Balmer jump), much larger changes in the polarization occur than predicted by single-scattering plus attenuation approximations. This occurs because just shortward of the jump, the absorptive opacity is large (effective albedo is small), and the polarization approaches the single-scattering plus attenuation level (since many multiply scattered photons have been absorbed). However, just longward of the jump, where the absorptive opacity is small (effective albedo is close to unity), multiple scattering is dominant, and the polarization is larger than the single scattering plus attenuation prediction. For this reason we find that the combined effects of multiple scattering plus absorptive opacity give much larger polarization jumps than previous predictions; in some instances, the polarization jump is doubled. In addition, the slope of the polarized continuum is steeper than that derived from single scattering plus attenuation calculations. Finally, for geometrically thick equatorial disk-like geometries, a position angle flip of 90 degrees occurs shortward of the Balmer jump. This is because of the large hydrogen opacity which absorbs the multiply scattered photons in the equatorial disk. Thus, the polarization is dominated by singly scattered photons from the polar regions, producing a net negative polarization.