2016ApJ...833...75L

Symbiotic stars are regarded as wide binary systems consisting of a hot white dwarf and a mass losing giant. They exhibit unique spectral features at 6825 and 7082 Å, which are formed via Raman scattering of O VIλλ 1032 and 1038 with atomic hydrogen. We adopt a Monte Carlo technique to generate the same number of O VIλ1032 and λ1038 line photons and compute the flux ratio F(6825)/F(7082) of these Raman scattered O VI features formed in neutral regions with a simple geometric shape as a function of H I column density N_H I_. In cylindrical and spherical neutral regions with the O VI source embedded inside, the flux ratio F(6825)/F(7082) shows an overall decrease from 3 to 1 as N_H I_ increases in the range 10^22–24 cm^–2. In cases of slab geometry and other geometries with the O VI source outside the H I region, Rayleigh escape operates to lower the flux ratio considerably. For moderate values of N_HI∼10²³ cm^–2 the flux ratio behaves in a complicated way to exhibit a broad bump with a peak value of 3.5 in the case of a sphere geometry. We find that the ratio of Raman conversion efficiencies of O VIλλ 1032, 1038 ranges from 0.8 to 3.5. Our high resolution spectra of "D" type HM Sge and "S" type AG Dra obtained with the Canada-France-Hawaii Telescope show that the flux ratio F(6825)/F(7082) of AG Dra is significantly smaller than that of HM Sge, implying that "S" type symbiotics are characterized by higher N_H i_ than "D" type symbiotics.