The geometry and ionisation structure of circumstellar envelopes of hot stars from analysis of spectropolarimetric absorption line profiles
Session 31 -- Massive Hot Stars
Display presentation, Tuesday, 31, 1994, 9:20-6:30

## [31.10] The geometry and ionisation structure of circumstellar envelopes of hot stars from analysis of spectropolarimetric absorption line profiles

Kenneth Wood and Jon Bjorkman (University of Wisconsin)

We present the wavelength dependent linear polarisation and position angle variations arising from Thomson scattering of continuum stellar radiation in an axisymmetric, hot, planar disc which is in bulk motion. The wavelength dependence arises through the combination of Doppler shifted line attenuation, prior to scattering, and the Doppler redistribution of the scattered radiation due to the bulk and thermal motions of the disc electrons. We find that when the scattering electrons are cold there are large polarisation and position angle variations across the resulting spectropolarimetric absorption line profile -- agreeing with previous investigations. However, when the thermal Doppler velocities of the scattering electrons are comparable to or greater than the electron bulk velocity (as is often the case) then the scattered line profile is considerably broadened and the amplitude of the polarimetric variations are reduced. The properties of the electron thermal smearing function are such that the equivalent width of any scattered spectropolarimetric feature is independent of the electron temperature -- as expected from the conservation of areas under convolution. An important result of this is that the position angle variations across any line formed through the above process in an axisymmetric circumstellar envelope will average to zero. The implications of this result are that the large position angle variations observed in the UV spectra of $\pi$ Aquarii, $\zeta$ Tauri and PP Carinae by the Wisconsin Ultraviolet Photo Polarimeter (WUPPE), whose wavelength averages are non--zero, cannot be explained by pre--scattering attenuation of stellar flux by the multitude of iron lines present in that part of the spectrum. This implies that the polarisation features arise from Fe {\sc ii} and Fe {\sc iii} line attenuation after the scattering occurs. Combining our model with the WUPPE data yields information on not only the geometry of the circumstellar envelope, but also on the spatial distribution of the line forming regions, in particular the Fe {\sc ii} and Fe {\sc iii} must occur outwith the scattering region.