THE EFFECT OF MULTIPLE-SCATTERING ON THE POLARIZATION FROM AXISYMMETRICAL CIRCUMSTELLAR ENVELOPES .2. THOMSON SCATTERING IN THE PRESENCE OFABSORPTIVE OPACITY SOURCES

We investigate the effect on the polarization of multiple Thomson scat tered stellar radiation in axisymmetric circumstellar envelopes that c ontain sources of continuous absorptive opacity and emission. Our prev ious investigations of pure electron scattering envelopes have shown t hat multiple scattering increases the polarization level above that pr edicted by single scattering plus attenuation approximations. However, the inclusion of sources of absorptive opacity within the envelope lo wers the albedo, reducing the number of multiply scattered photons. Co nsequently, for envelopes possessing a large absorptive opacity, the n et polarization approaches the single-scattering plus attenuation leve ls (which may be positive or negative, depending on the geometry and d egree of polarimetric cancellation). Lowering the albedo further (by i ncreasing the absorptive opacity) removes photons that have, been sing ly scattered so that the polarization decreases below that predicted b y the single-scattering plus attenuation approximation. As the albedo approaches zero, few photons are scattered within the envelope (all ar e absorbed), and the only radiation reaching the observer is unscatter ed (i.e., unpolarized) stellar radiation; hence, the polarization appr oaches zero. A consequence of this behavior is that when the albedo ch anges rapidly with wavelength, as occurs across ionization edges (e.g. , across the Balmer jump), much larger changes in the polarization occ ur than predicted by single-scattering plus attenuation approximations . This occurs because just shortward of the jump, the absorptive opaci ty is large (effective albedo is small), and the polarization approach es the single-scattering plus attenuation level (since many multiply s cattered photons have been absorbed). However, just longward of the ju mp, where the absorptive opacity is small (effective albedo is close t o unity), multiple scattering is dominant, and the polarization is lar ger than the single scattering plus attenuation prediction. For this r eason we find that the combined effects of multiple scattering plus ab sorptive opacity give much larger polarization jumps than previous pre dictions; in some instances, the polarization jump is doubled. In addi tion, the slope of the polarized continuum is steeper than that derive d from single scattering plus attenuation calculations. Finally, for g eometrically thick equatorial disk-like geometries, a position angle f lip of 90 degrees occurs shortward of the Balmer jump. This is because of the large hydrogen opacity which absorbs the multiply scattered ph otons in the equatorial disk. Thus, the polarization is dominated by s ingly scattered photons from the polar regions, producing a net negati ve polarization.