2012MNRAS.427..979S

Symbiotic binaries are comprised of nebulae, whose densest portions have electron concentrations of 10⁸–10¹²/cm3 and extend to a few astronomical units. They are optically thick enough to cause a measurable effect of the scattering of photons on free electrons. In this paper, we model the extended wings of strong emission lines by electron scattering with the aim of determining the electron optical depth, τ_e, and temperature, T_e, of symbiotic nebulae. We have applied our profile-fitting analysis to the broad wings of the OVI1032,1038Å doublet and the HeII1640Å emission line, measured in the spectra of symbiotic stars AG Dra, Z And and V1016 Cyg. The synthetic profiles fit the observed wings well. In this way, we have determined τ_e and T_eof the layer of electrons, throughout which the line photons are transferred. During quiescent phases, the mean τ_e = 0.056±0.006 and T_e= 19200±2300 K, while during active phases, the mean quantities of both parameters increase to τ_e= 0.64 ±0.11 and T_e= 32300±2000 K. During quiescent phases, the faint electron-scattering wings are caused mainly by free electrons from/around the accretion disc and the ionized wind from the hot star with the total column density, \mi N\mi e\mo <∼ \mn 10\mn 23/cm2. During active phases, the large values of τ_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 ∼ 10²⁴/cm2.