White dwarf binaries

Abstract

The white dwarf binaries considered in this chapter are better known as cataclysmic variables (CVs) and are interacting binaries in that the white dwarf is accreting material from its (usually) cool, late-type companion star in a short (of the order of hours) orbital period. They are one of the few classes of object considered in this book that were actually known and observed prior to the twentieth century. The novae become naked-eye objeets (e.g. Nova Cyg 1975 which, at a peak of 2nd magnitude, completely transformed the appearance of Cygnus for a few weeks in the summer of 1975) and hence have been observed throughout human his-lory, and dwarf novae were first reported in the middle of Ihe nineteenth Century (Hind 1856). It was the non-periodic but continuous eruptions displayed by dwarf novae (such as U Gern and SS Cyg) that led to their cataelysmic designation, a term now widely applied to all interacting binaries where white dwarfs are accreting. However, their physical nature was not understood until the pioneering spectroscopic studies of Kraft in the late 1950s which revealed their binary signature. CVs are of considerable importance for astronomy in general because of the significance of aecretion processes on virtually all scales, from star and planetary formation (the proto-star aecretes material from its surrounding molecular cloud) to aecretion onto supermassive black holes in the cen-tres of active galactic nuclei.