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Electron optical depths and temperatures of symbiotic nebulae from Thomson scattering

Symbiotic binaries are comprised of nebulae, whose densest portions have electron concentrations of 1E+8 - 1E+12 cm-3 and extend to a few AU. They are optically thick enough to cause a measurable effect of the scattering of photons on free electrons. In this paper we introduce modelling the extended wings of strong emission lines by the electron scattering with the aim to determine the electron optical depth, tau(e) and temperature, T(e), of symbiotic nebulae. We applied our profile-fitting analysis to the broad wings of the OVI 1032, 1038 A doublet and HeII 1640 A emission line, measured in the spectra of symbiotic stars AG Dra, Z And and V1016 Cyg. Synthetic profiles fit well the observed wings. By this way we determined tau(e) and T(e) of the layer of electrons, throughout which the line photons are transferred. During quiescent phases, the mean tau(e) = 0.056 +/- 0.006 and T(e) = 19 200 +/- 2 300 K, while during active phases, mean quantities of both parameters increased to tau(e) = 0.64 +/- 0.11 and T(e) = 32 300 +/- 2 000 K. During quiescent phases, the faint electron-scattering wings are caused mainly by free electrons from/around the accretion disk and the ionized wind from the hot star with the total column density, N(e) <~ 1E+23 cm-2. During active phases, the large values of tau(e) are caused by a supplement of free electrons into the binary environment as a result of the enhanced wind from the hot star, which increases N(e) to ~1E+24 cm-2.