On the evolution of accretion disc flow in cataclysmic variables - III. Outburst properties of constant and uniform- model discs

We continue our investigation of accretion disc models relevant to cataclysmic-variable systems. In this paper, we examine the stability and evolution of some simple accretion disc models in which the viscosity is prescribed by an ad hoc uniform-α model. We are primarily concerned with systems in which the mass-input rate from the secondary to the disc around the primary, $${\dot M}_\ast$$, is assumed to be constant. However, initial calculations with variable mass-input rates are also performed. The time-dependent visual magnitude light-curves are constructed for cataclysmic binaries with a range of disc size, primary mass, mass-input rate, and magnitude of viscosity (i.e. value of α). Comparisons are made between these theoretical results and the observed properties of various subclasses of cataclysmic variables. These results indicate that the observational differences between novae and dwarf novae may be due to differences in the mass-input rate. In nova systems, the mass-input rate is sufficiently large for most of the disc to be highly ionized always, leading to stable disc flow. In dwarf-nova systems, the mass-input rate may be sufficiently low for incomplete hydrogen ionization. The opacity of the disc material is then strongly temperature-dependent. The hydrogen-ionization zones are thermally unstable and a thermal relaxation limit cycle is induced by these thermal instabilities. Our models can reproduce the gross observational features of U Gem-type dwarf-nova outbursts: the outburst cycles, their relative strength, and their absolute magnitude. We show that in order to reproduce the detailed light-curve and the duty cycles of U Gem systems, a non-uniform α prescription will be necessary. For SS Cyg and Z Cam systems, modestly variable mass-input rates may also be required.