Optical emission line spectra of symbiotic binaries

Abstract

Symbiotic stars are long-period interacting binaries where the compact objects, most commonly a white dwarf, is embedded in the dense stellar wind of an evolved companion star. UV and soft X-ray emission of the accretion disk and nuclear burning white dwarf plays a major role in shaping the ionisation balance of the surrounding wind material and giving rise to the rich line emission. In this paper, we employ 2D photoionisation calculations based on C code to study the ionisation state of the circumbinary material in symbiotic systems and to predict their emission line spectra. Our simulations are parameterized via the orbital parameters of the binary and the wind mass-loss rate of the donor star, while the mass accretion rate, temperature and luminosity of the WD are computed self-consistently. We explore the parameter space of symbiotic binaries and compute luminosities of various astrophysicaly important emission lines. The line ratios are compared to the traditional diagnostic diagrams used to distinguish symbiotic binaries from other types of sources and it is shown how the binary system parameters shape these diagrams. In the significant part of the parameter space the wind material is nearly fully ionized, except for the "shadow" behind the donor star, thus the WD emission is typically freely escaping the system.