Instabilities of Accretion Disks in Cataclysmic Variable Stars

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Dwarf novae are eruptive variable stars showing repetitive outbursts of amplitudes of 2-6 magnitude, outburst duration of a few days to 20 days, and recurrence time of 20-300 days. They belong to a more general class of cataclysmic variable stars, in that a Roche lobe filling cool dwarf star loses mass through the inner Lagrangian point and a non-magnetic white dwarf star accretes it through an accretion disk. Various outbursting phenomena occurring in these stars are now thought to be caused by variable accretion onto the white dwarf In this research, we have investigated the outburst mechanisms of dwarf novae based on intrinsic instabilities within the accretion disks in cataclysmic variable stars.Two kinds of instabilities are known in acceretion disks of dwarf novae ; the thermal instability (i. e., the ordinary disk instability) and the tidal instability. The thermal instability is an instability which occurs in the outer-parts of accretion disk of dwarf nova when the temperature … More of the disk becomes cool enough for hydrogen to recombine. This thermal instability leads to a thermal relaxation oscillation of the accretion disk by periodically accumulating matter in the disk and then suddenly dumping it onto the white dwarf, giving rise to sudden burst of accretion luminosity. This mechanism is now believed to be responsible for the ordinary outburst of U Gem-type dwarf novae. - Another kind of instability in accretion disks is the tidal instability by which instability an accretion disk is deformed into a precessing eccentric disk due to the tidal effects of the secondary star. In what follows, we list problems that we have examined in this research project and results obtained.(1) We have proposed a new model that explains the superoutburst phenomenon in SU UMa-type dwarf novae. This model is based on the combined mechanisms of the thermal instability and the tidal instability of accretion disk.(2) We have studied time evolution of accretion disk radius in the outburst cycle of dwarf nova. We examined this based on both the mass transfer burst model and the disk instability model. It was found that the disk instability model is more favorable in explaining observed disk radius variation than the mass transfer burst model.(3) Two-dimensional hydrodynamic simulations have been performed to study the tidal instability of accretion disks in cataclysmic variable stars. Two different methods of hydrodynamic simulations are used ; one is the viscous particle method and the other is the smoothed particle hydrodynamics abbreviated as SPH method). It has been found that an accretion disk is deformed to a precessing eccentric form if the mass of the secondary star is less than about one-fourth of the mass of the primary star.(4) The precession rate of the eccentric disk is studied. It is shown that the precession rate can be formulated as an eigenvalue problem of one-armed oscillation of the accretion disk. Less