SIMULATIONS OF THE RAMAN-SCATTERED O-VI EMISSION-LINES SYMBIOTIC STARS

This paper investigates the Raman-scattered O VI emission lines lambda lambda 6825, 7082 in symbiotic binaries using Monte Carlo simulations . The simulations calculate the scattering path of O VI lambda lambda 1032, 1038 line photons released from an extended emission nebula. A l arge fraction of these photons penetrate into the extended atmosphere and the wind of the cool giant, where Raman scattering, Rayleigh scatt ering and absorption take place. The employed formalism includes the p olarization effects and the Doppler shifts introduced by the scatterin g processes. As a result, the simulations provide the flux, the integr ated polarization and the spectropolarimetric structure of the Raman-s cattered O VI lines. Additional results are the flux properties of the O VI lines in the far UV, and maps which visualize the scattering geo metry. Results are presented in detail for four model geometries. They are supplemented by many additional computations in order to explore the model parameter space. The calculations are compared with the exis ting observational data of the Raman-scattered emission lines in symbi otic systems. The simulations are in general agreement with observatio ns of the line strength, of the integrated polarization and of the pha se-locked polarization variability. The simulations also reproduce the main features of the observed spectropolarimetric structure in the Ra man-scattered lines. Further, the model results support the observed c orrelation of so-called type III profiles with systems having a red gi ant with strong mass-loss. The comparison between observations and sim ulations also reveals the limitations of the adopted scattering model. A major shortcoming of the present models is the two-dimensional (rot ationally symmetric) geometry, which was introduced to limit the compu tational effort. Therefore the obtained results cannot account for the three-dimensional polarization structure observed in the Raman-scatte red lines of many symbiotic systems. The computations neglect gas flow s and radiative transfer effects in the nebular O VI emission tone. As a result, the computations give much less spectroscopically structure d Raman lines than are observed. Nevertheless, it is found that symbio tic systems have preferentially an ionization geometry with an X-param eter X(H+) approximate to 2. Roughly speaking, this 'average' shape of the ionization front does not differ strongly from a plane between th e two stellar components.