Numerical Simulation of spiral shocks in accretion discs and comparison with observations

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An accretion disc is a gas disc formed around a compact object such as a white dwarf, a neutron star or a black hole. In the present research we performed three-dimensional numerical simulation of an accretion disc in a close binary system. The method employed was a finite volume method called SFS scheme using a Riemann solver. The computational region includes a main star accreting mass and a companion star losing mass. As an equation of state of gas we employed that of the perfect gas. We found that the gas stream (L1 stream) flowing through the L1 point penetrates into the accretion disc in the parameter range considered. We also found that the disc gas rotating about the main star hits the L1 stream and forms an elongated shock. We termed it as the L1 shock. Because of the L1 shock, the disc gas loses its angular momentum and accretes to the main star. We have proposed a mechanism of angular momentum loss by spiral shocks, but its magnitudes may not be large enough to explain observations. The angular momentum loss by the L1 shock was found to be large enough. In 1997 British astronomers discovered spiral structure proposed by us in the cataclysmic variable IP Pegasi. Since then the spiral shock model attracted much attention in the community. The penetration was also discovered recently.One of problems in the above simulation is that radiation cooling was not taken into account explicitly. Instead we reduce the specific ratio from 5/3 to 1.01. In the future research we plan to employ Direct Simulation Monte Carlo method. We already obtained a preliminary result for a two-dimensional case. We plan to extend it to three-dimension, and try to simulate a raditive transfer by solving thermal conduction explicitly.