Monte Carlo simulations of the Rayleigh scattering effects in symbiotic stars

Abstract

This paper investigates the Rayleigh scattering effects in symbiotic binaries using Monte Carlo simulations. The simulations calculate the scattering path of far-UV photons released from the hot component or from the emission nebula. A large fraction of these photons penetrate into the extended atmosphere and the wind of the cool giant, where Rayleigh scattering by atomic hydrogen and absorption take place. The scattering processes are thereby treated according to the Rayleigh phase matrix, which includes the light polarization effects. Light curves, spectra and polarization spectra are calculated for different model parameters and geometries. In addition, maps of the scattering geometry are presented, which allow visualization of the Rayleigh scattering problem in symbiotic systems. The calculated UV spectra and light curves from the Monte Carlo simulations are compared with IUE data of well-observed S-type symbiotic systems, such as EG And, SY Mus, BF Cyg and HBV 475. The characteristic eclipse behaviour due to Rayleigh scattering by neutral hydrogen is reproduced well by those models in which the hot component ionizes a large fraction of the cool giant's wind. The corresponding modelling of the emission-line variations shows some discrepancies between observed and simulated line light curves. However, it is expected that these differences can be removed with some adjustments in the model parameters, for example the geometry of the nebular region. The simulations for models with a very extended and dense neutral scattering region provide light curves and spectra which have not been observed in symbiotic systems. This opens the question of whether such systems are rare or whether another process (e.g. dust or Thomson scattering) must be included in the scattering models for symbiotic systems. Finally, a brief discussion is given on the potential of the Monte Carlo technique for the investigation of the geometric structure of symbiotic systems from scattering processes.