Eccentric Orbits of close Companions to Asymptotic Giant Branch Stars

Abstract

I propose that the relatively high eccentricity found in some tidally strongly interacting binary systems, where the mass-losing star is an evolved giant star, e.g., an asymptotic giant branch star, is caused by an enhanced mass loss rate during periastron passages. Tidal interaction by itself will circularize the orbits of these systems in a relatively short time, hence a mechanism which increases the eccentricity on a shorter time scale is required. The proposed scenario predicts that the nebula formed by the mass loss process possesses a prominent departure from axisymmetrical structure. Such a departure is observed in the Red Rectangle, which has a central binary system, HD 44179, with an orbital period of 318 days, and an eccentricity of e=0.38.