Disc accretion onto white dwarfs
von Matthias Schreiber
Datum der mündl. Prüfung:2001-01-29
Betreuer:PD Dr. Karl Mannheim
Gutachter:PD Dr. Karl Mannheim
Gutachter:Prof. Dr. Ulrich Christensen
EnglischIn non-magnetic cataclysmic variables (CVs) a white dwarf accretes matter from a main-sequence secondary star via an accretion disc. The dynamical behaviour of the accretion disc determines the accretion rate onto the white dwarf. Thermal instabilities in the accretion disc associated with the ionisation of hydrogen can lead to a limit-cycle behaviour in which the disc switches quasi-periodically between high and low accretion states. This thermal limit-cycle model is the generally accepted explanation for dwarf nova outbursts observed in many CVs. The process of disc accretion in non-magnetic CVs is subject to a number of external conditions, namely mass transfer variations of the secondary star, stream overflow and irradiation by the white dwarf. In this thesis I develop a model for time-dependent disc accretion onto white dwarfs and analyse the influence of these external conditions on the accretion process.I examine the effects of mass transfer variations by deriving real mass transfer variations from light curve monitoring of the disc-less CV AM Her. These mass transfer variations I include in simulations of disc accretion onto white dwarfs in non-magnetic systems and find that the mass accretion rate of the disc relaxes to an equilibrium with the prevailing mass transfer rate on a rather short timescale. I conclude that the observed changes in outburst duration and outburst magnitude are caused by nearly simultaneous variations of the mass loss rate from the secondary.I also present a new model for the stripping of the stream by the accretion disc, and find that stream overflow can have subtle effects on the evolution of the accretion disc only if the amount of overflowing stream material exceeds 25% of the mass transfer rate. I conclude that solely very large stream overflow fractions can change the outbursts of dwarf novae. For realistic amounts of stream overflow the overall outburst behaviour is marginally changed.The accretion disc is mostly influenced by the white dwarf irradiation. I present a self-consistent model for irradiated accretion discs and find that efficient irradiation in dwarf nova systems causes small "echo" outbursts following the larger ones immediately. This result contrasts with the observations of dwarf nova outbursts. As an explanation for this discrepancy I suggest that the reprocessing efficiency of disc irradiation is rather small. This is in agreement with results I obtain from detailed simulations of irradiated discs in post novae. These systems are excellent laboratories for studying the effects of disc irradiation because the white dwarf heated during the nova eruption provides a much stronger irradiation of the disc than in normal dwarf novae. I derive time-limits for the occurrence of dwarf nova outbursts in post novae and present detailed simulations of the evolution of irradiated discs in post novae.In addition to the developed theory of irradiated discs around white dwarfs, I show preliminary results of an intensive observing campaign on the post nova system V446 Her. Finally, discussing the influence of disc irradiation on the post nova evolution in the light of the current working hypothesis leads me to put forward a new scenario.
Keywords: accretion discs; white dwarfs; post novae; irradiation; mass transfer; stream overflow