April 6, 2005

Simulations Shed Light on Cataclysmic Variables

RAS -- New supercomputer simulations may help explain periodic bursts of light emitted by compact binary star systems.

Cataclysmic variables are binary systems that consist of a white dwarf and a normal star. The dense white dwarf drags material from its companion star, creating an accretion disc of matter around itself, and a hotspot develops where the gas stream from the star crashes into the accretion disc.

On Wednesday 6th April at the RAS National Astronomy Meeting in Birmingham, Dr Michael Truss will be presenting the results of three-dimensional simulations of cataclysmic variables, which show that tidal forces between the stars distort the accretion disc and cause the fluctuations in the hotspot's brightness.

For some years, the cause of these fluctuations has been attributed to instabilities in the gas circulating within the accretion disc. However, in 2001, observations of a cataclysmic variable, called WZ Sagittae, reopened debate as to the origin of the fluctuations.

The observations appeared to show that the brightening of the hot-spot was due to an increased rate of mass transferred from the companion star to the accretion disc around the white dwarf, something that had never been seen before.

The new simulations show that tidal forces from the lower-mass companion star distort the accretion disc, just like the tides of the moon distort the Earth. Simulations of the intensity of emissions show clear wave-like features, known as "superhumps", caused by tides stretching the gas in the accretion disc. The new simulations show that there is no need for a change in mass-transfer rate and, instead, support an alternative theory that attributes the brightening of the hotspot to heating caused by tides.

Dr Truss ran three simulations at the UK Astrophysical Fluids Facility, a supercomputer at the University of Leicester dedicated to theoretical astrophysics. In the first simulation the mass-transfer rate stayed the same, in the second it was decreased to one fifth and in the third simulation, mass-transfer rate was increased by a factor of five.

In the simulations where the mass-transfer rate stayed the same or decreased, superhumps formed and the subsequent tidal heating could account for the energy emitted by WZ Sagittae in observations. In the simulation with enhanced mass-transfer, the disc became more circular in shape, the superhumps did not form and no brightening occurred.

Dr Truss will also be presenting simulations of the disc in a microquasar, where an accretion disc surrounds a black hole. Here the processes are complicated because, in addition to the tidal forces, the disc is bathed in X-rays produced near the black hole. The high radiation-pressure blows gas out of the disc, resulting in a complex flow structure.


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