Spectropolarimetry as a probe of stellar winds

The use of spectropolarimetry as a diagnostic probe of stellar-wind structure is investigated by using high-quality observations and state-of-the-art analytical and numerical models. The winds of late-type giant stars are studied through the highly polarized 6830A and 7088A Raman-scattered emission lines that are observed in many symbiotic systems. A spectropolarimetric survey of 28 symbiotic stars is presented. A Monte-Carlo code is developed in order to aid interpretation of the lines and the parameter sensitivity of the Raman line polarization spectrum is investigated. It is demonstrated that the observed line polarization morphologies can be reproduced by using realistic physical parameters and that the line polarization structure is a powerful diagnostic of the cool stellar wind. The binary phase dependence of the line structure is studied with the aim of using multi-epoch observations to derive orbital parameters. The polarization spectrum of the symbiotic star B1 Crucis is analyzed and discussed in relation to the geometry of its extended bipolar nebula. The first detection of emission-line (Ha) polarization structure in an O supergiant (? Puppis) is presented. Model polarization spectra are computed using statistical equilibrium calculations and Monte-Carlo radiative transfer. It is demonstrated that the latitudinal wind density structure predicted by radiation-driven wind theory is incapable of producing the observed polarization signature. Multi-epoch observations of the pathological WN star EZ Canis Majoris are presented. These observations enable the accurate determination of the interstellar polarization (ISP) vector. The reliability of techniques used to estimate the ISP is assessed using tests performed on numerical models. The observed variability of the continuum polarization is explained in terms of scattering off density inhomogeneities propagating through the stellar wind. A spectropolarimetric survey of 15 galactic Wolf-Rayet stars is presented. Emission- line polarization structure is observed in four of the survey stars. These data are combined with results of similar surveys in order to determine the frequency of line polarization structure in galactic WR stars. Detailed analyses are performed on the polarization spectra of the dust-producing WC star WR 137 and the WN6 star WR 134.