Raman scattering in symbiotic stars. II. Numerical models

We present numerical models of Raman-line formation in symbiotic systems, based on Monte-Carlo simulations of scattering of O vi resonance-line photons from a hot source in the extended, expanding atmosphere of a cool companion, and we explore the sensitivity of the model results to the input parameters. The model successfully reproduces the observed line ratios, velocities, and polarizations, and the viewing-angle (orbital) dependence of the line strengths and polarized intensities. In principle, the position-angle dependence of the polarization in resolved Raman lines provides a diagnostic of the extent of the ionized region in symbiotic systems; however, axisymmetric geometries with `up-down' symmetry, such as we use, fail to reproduce the most commonly observed dependence of PA on velocity. Raman-line polarization morphology is shown to be a powerful probe of the conditions in the red-giant wind; this is particularly true if observations are available at different orbital phases, so that geometrical and physical variables may more easily be isolated.