A Model for the Galactic Population of Symbiotic Stars with White Dwarf Accretors

Abstract

By means of a population synthesis code, we investigate the formation of symbiotic systems in which the hot components are assumed to be white dwarfs which are either burning hydrogen steadily or are in a post-nova plateau" phase, in the evolution of exploding white dwarfs. Our estimate for the total number of symbiotic systems in the Galaxy, ˜3000-30,000 (depending on different model assumptions), is compatible with observational estimates. The crucial parameter for the determination of the birthrate and number of symbiotic stars is the mass of the hydrogen layer which the white dwarf can accumulate prior to hydrogen ignition. We model the distributions of symbiotic stars over orbital periods, masses of the components, mass-loss rates by the cool components, and brightness of components, and we obtain a reasonable agreement with observations. We show that in systems which are the most efficient in producing the symbiotic phenomenon, the accretors have to capture up to ˜30% of the matter lost by the cool component via a stellar wind. If the fraction of captured matter is significantly lower, it becomes impossible to explain even the lowest observational estimates of the number of symbiotic stars. The theoretical estimate of the average rate of symbiotic novae is ˜0.1 yr-1, compatible with the observed one. The apparent normal chemical composition of symbiotic novae can be explained if the white dwarfs in these systems, which have systematically lower masses than in cataclysmic binaries, manage to preserve "buffer" helium layers between their CO cores and the accreted hydrogen envelopes. Mass exchange in symbiotic systems does not lead to SN Ia's via the accumulation of a Chandrasekhar mass. However, if sub-Chandrasekhar-mass, double-detonation models indeed produce SN Ia's, then symbiotic systems can be the progenitors of ≲⅓ of the events. According to the model, SN Ia's in symbiotic binaries belong to young and intermediate-age populations (t ≲ 6 × 109 yr).