Novae and Accretion Disc Evolution

Abstract

AbstractAt outburst the classical nova generates an extended optically-thick wind driven by radiation pressure in the continuum. At maximum light the optical luminosity is close to the Eddington-limit. The subsequent decline illustrates the interaction between radiation and matter in a wind which gradually thins as the mass loss rate falls at an approximately constant Eddington-limit luminosity. As the wind thins so the effective photosphere shrinks back into the underlying binary, and an increasing fraction of the radiation is emitted at ultraviolet wavelengths. Model atmosphere computations show how the increasing flux of ultraviolet photons is associated with the shell becoming more and more ionized through radiative ionization. Attempts to study the internal structure of the wind confirm that the luminosity must be close to the Eddington limit and must be expelled from close to the white dwarf surface. It is generally agreed that the outbursts are caused by runaway nuclear burning of accreted material at the white dwarf surface, but it is possible that some events of the classical nova type may be caused by runaway accretion at a super-critical rate.In dwarf novae very different behaviour is evident. The outbursts are located within the accretion disc and are generated either by mass-transfer bursts due to dynamical instability of the Roche-lobe filling star, or by an instability within the disc itself. In either case the eruption behaviour is due to an enhanced accretion flux through the accretion disc. One important aspect of the radiation hydrodynamics is the luminosity generated by impact of the mass-transfer stream with the accretion disc and penetration by the stream within the disc. Attempts at examining this penetration region are described and results compared with observed behaviour of disc evolution through the course of an outburst. The possibility that disc instabilities will not propogate in realistic discs which deviate from axial symmetry is considered.