Figures of Equilibrium

Abstract

The aim of the present chapter will be to provide a systematic introduction to the study of phenomena exhibited by close binary systems — with special regard to such effects as may become photometrically or spectroscopically observable. If their components were — as in visual binaries — sufficiently far apart to attract each other as as pair of mass-points, their orbits would be Keplerian ellipses — perturbed only very seldom by chance encounters with neighbouring stars or interstellar clouds. In a field of star density comparable with that obtaining in our galactic neighbourhood, cumulative effects of chance encounters may be safely discounted even on the nuclear time-scale (cf. Ambartsumian, 1937; Chandrasekhar, 1944; Yabushita, 1966; or — for encounters with gas clouds — Takase, 1953); and the dimensions of the components will in no way affect their motion or other observable manifestations. If, however, consistent with our definition of close binaries already set forth in Chapter I, the proximity phenomena arising from mutual interaction of the components are to be regarded as the essential cause of the problems we wish hereafter to consider, the consequences of chance encounters with external celestial objects can utterly be ignored (except, possibly, their effects on the apparent (observed) orbital periods in very dense star fields — existing, for example, in the interiors of globular clusters or elliptical galaxies). On the other hand, the proximity phenomena — observable both photometrically and spectroscopically — arising from the deformation of both components by their axial rotation and mutual tidal action may become noticeable, or even conspicuous.