Simulations of superhumps and superoutbursts

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Abstract
We numerically study the tidal instability of accretion discs in close binary systems using a two-dimensional SPH code. We find that the precession rate of tidally unstable, eccentric discs does not only depend upon the binary mass ratio q. Although the (prograde) disc precession rate increases with the strength of the tidal potential, it also increases with the shear viscosity. Increasing the disc temperature has a retrograde impact upon the precession rate. We find that motion relative to the binary potential results in superhump-like, periodic luminosity variations in the outer reaches of an eccentric disc. The nature and location of the luminosity modulation is a function of the shear viscosity, nu. Light curves most similar to observations are obtained for nu values appropriate for a dwarf nova in outburst. We investigate the thermal-tidal instability model for superoutburst. A dwarf nova outburst is simulated by instantaneously increasing nu, which causes a rapid radial expansion of the disc. Should the disc encounter the 3:1 eccentric inner Lindblad resonance and become tidally unstable, then tidal torques become much more efficient at removing angular momentum from the disc. The disc then shrinks and mass flux through the disc increases. The resulting increase in disc luminosity is found to be consistent with the excess luminosity of a superoutburst.

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