Cataclysmic variables

Abstract

Over the past twenty years a significant group of all classes of eruptive variable stars have been shown to beinteracting binaries with mass transfer onto an accreting, relatively compact component. The spectral region covered bythe dominant fraction of the emitted flux depends on the nature of the accreting component, the rate of accretion, and the role of viscosity and angular momentum in generating an accretion disc. The origin of the modulation of the transfer rate causing eruptions may in principle occur either in the envelope of the mass transfering component or within the disc itself. Stability studies show that G, K, M spectral class companions should be dynamically unstable, and suffer relaxation oscillations within the envelope, with the same interval as the observed outburst interval. Time-dependent studies of disc evolution indicate that the resulting mass transfer instabilities accurately model the eruptions of dwarfnovae and certain symbiotics. For the case of white-dwarf accretion the dominant region of emission is in the far ultraviolet. The spectral evolution, eruption decay rate and stream impact behaviour observed in dwarf novae are in accord with theories of bursting mass transfer by the companion. One major area of uncertainty is the structure of the inner disc boundary-layer, at the highest accretion temperatures and possibly responsible for observed hard and soft X-ray emission. To probe this region of considerable theoretical interest and uncertainty, extended X-ray monitoring as required. © 1984 IOP publishing Ltd.