Tidal Interactions of Close Binary Systems

Abstract

Observations show that close binary systems (in the sense that the orbital separation D is not much larger than the stellar dimensions) tend to have small orbital eccentricities (e.g. Young & Koniges 1977, Koch & Hrivnak 1981, Middelkoop & Zwaan 1982). There appears to be no such correlation for wide binary systems, which show a wide scatter of orbital eccentricities. This phenomenon is usually explained in terms of tidal interaction between the two stars, which is only effective in close binary systems. The mutual gravitational attraction between the two stars in a binary system induces a “tidal” deformation. In the case of an eccentric orbit, or when the rotation of the binary stars is not synchronized with the orbital revolution, each star in the binary feels a varying potential as its companion revolves around in its orbit. The varying potential excites tidal oscillations in the stars. Inevitably, part of the orbital energy fed into the tidal oscillations is dissipated, causing a secular variation of the orbital parameters.