Transfer Instabilities and Accretion Disc Structure in Dwarf Novae

Abstract

In the past four years the importance of accretion in dwarf nova binaries has been increasingly appreciated. It is now widely accepted that their eruptive activity is most probably due to bursts of accretion flow through an accretion disc onto the blue component. Two possible mechanisms for modulating the accretion process into quasi-periodic bursts have been suggested. Either the rate of mass transfer by the red component is modulated by enhanced outflow during each eruption (Bath 1973, 1975; Gorbatskii 1975; Papaloizou and Bath 1975), or alternatively, the disc itself (continuously accreting from the red component in a steady way) undergoes some form of intrinsic instability which results in the infall of material normally stored in the outer disc regions (Osaki 1974). In this paper some of the general arguments which support these accretion models will be outlined, and the present status of the two accretion theories reviewed. A fuller discussion will be found in Bath (1976). The main purpose of this paper is, firstly, to emphasise the importance of the observational relationship found by Bailey (1975) between the outburst decline rate and the binary period, and, secondly, to note the uncertainty in the present estimates of the masses of accretion discs in dwarf novae.