THE WIND GEOMETRY OF THE WOLF-RAYET STAR HD-191765

We present a time-dependent spectropolarimetric data set of the single WN6 star HD 191765 in the wavelength range 3159 angstrom less-than-or-equal-to lambda-less-than-or-equal-to 7593 angstrom. At all epochs our observations display a large and strongly wavelength-dependent continuum polarization and reduced levels of polarization across the emission lines. The data imply a significant intrinsic continuum polarization which requires a general deviation of the electron distribution from spherical symmetry. This global shape is quite stable as a function of time; small fluctuations may arise from localized density/temperature changes. The line polarizations are consistent with an axisymmetric wind geometry and ionization stratification. The polarized line profile of the He II line at 4686 angstrom is redshifted and of a different shape than the flux line profile, due to electron scattering of line photons in the expanding envelope. The intrinsic continuum polarization rises into the ultraviolet and cannot be understood by electron scattering alone. We develop a qualitative model for polarization in a Wolf-Rayet atmosphere. We argue that the blueward rise of the continuum polarization in HD 191765 can be explained if the density in the wind is high, resulting in a competition of thermal and electron-scattering continuum opacity in the optical. EZ CMa, on the other hand, displays a flat polarization spectrum at most epochs; we find that electron scattering is the main continuum opacity source at optical wavelengths.