EFFECTS OF PRESCATTERING ATTENUATION ON SPECTROPOLARIMETRIC LINE-PROFILES

We present the wavelength dependent linear polarization and position a ngle variations arising from single Thomson scattering of continuum st ellar radiation in an axisymmetric, hot, planar disk, which is in bulk motion. The wavelength dependence arises through the combination of D oppler shifted line attenuation, prior to scattering, and the Doppler redistribution of the scattered radiation due to the bulk and thermal motions of the disk electrons. We find that when the scattering electr ons are cold, there are large polarization and position angle variatio ns across the resulting spectropolarimetric absorption line profile-ag reeing with previous investigations. However, when the thermal Doppler velocities of the scattering electrons are comparable to or greater t han the electron bulk velocity (as is often the case), then the scatte red line profile is broadened and the amplitude of the polarimetric va riations are considerably reduced. The properties of the electron ther mal smearing function are such that the equivalent width of any scatte red spectropolarimetric feature is independent of the electron tempera ture-a consequence of the conservation of areas under convolution. An important result of this is that for an axisymmetric circumstellar env elope the position angle variations will average to zero across any li ne formed through the above process. We expect that the effect of mult iple line blending will yield a depolarization equivalent width that i s given by the sum of the individual equivalent widths and the positio n angle variations will average to zero. The implications of this are that the large depolarizations observed in the UV spectra of pi Aquari i, zeta Tauri and PP Carinae by the Wisconsin Ultraviolet Photo Polari meter Experiment (WUPPE) can only be explained with prescattering atte nuation of stellar flux if there are a sufficient number of multiple o verlapping lines in that part of the spectrum.