The Development of the Idea of Interacting Double Stars

Abstract

The term “interacting binaries” has become increasingly popular over the last decade, but is rarely precisely defined. Perhaps the growing popularity is a consequence of the realization that the older term “close binary systems” means rather different things to the observers of eclipsing and spectroscopic binaries on the one hand, and of visual binaries on the other, but the new term is not completely satisfactory. By definition, any binary system is composed of two stars that interact gravitationally to the extent that they are constrained to move in closed orbits about a common centre of mass. In this sense, therefore, all binaries are “interacting”. Furthermore, many if not most binaries that are observed either as eclipsing or spectroscopic binaries, show evidence of tidal distortion (another manifestation of gravitational interaction) or the mutual irradiation that we erroneously but conveniently call “reflection”. When we refer to “interacting binaries”, we usually mean to imply stronger interactions that may significantly affect the course of evolution of the two components and of the system as a whole. By “interacting binary” we mean a system in which at least one of the component stars is unstable and expelling matter, either into the gravitational field of the other, or out of the system altogether. In any given system, of course, both these things may be (and probably are) happening together, and matter expelled from the system does not necessarily leave it immediately.