Research on accretion discs by Direct Simulation Monte Carlo Method

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We have investigated gas flows in close binary systems. The targets of the present research are an accretion disc, a surface flow on a gas-loosing star, a wind accretion flow and a viscosity in a rotating gas. We studied these targets mainly by numerical techniques. The methods of calculation are both a Direct Simulation Monte Carlo Method (DSMC) and a finite volume method.In three-dimensional finite volume calculations of accretion discs, we confirmed the presence of spiral shocks, which we had discovered earlier. On the other hand, we found only traces of the spiral shocks in DSMC calculations. The reason is probably insufficient resolution at the peripheral region of the disc. In such a rarefied region, the number of particles may not be enough to guarantee occurrence of collisions between particles, which are the essential ingredient to ensure the fluidal nature of the collection of particles. This difficulty is yet to be solved.We made three-dimensional hydrodynamic simulations of flows on the surface of the mass-losing star. We discovered complex eddies produced by high/low pressure on the stellar surface. We composed Doppler maps to compare the present theoretical results with observations of super-soft X-ray sources, and we found a good agreement.Wind accretion is an important mechanism of mass exchange in close binary systems. We computed the mass accretion ratio, which is defined as a ratio of mass-accretion rate to mass-loss rate, and derived an experimental formula. We also computed specific angular momentum loss from the system by stellar wind. These parameters are important factors in order to investigate the evolution of binary systems.Transfer of angular momentum in an accretion disc is an important factor to investigate the physics of accretion discs. We pointed out that the derivation of the formula of angular momentum transfer appeared in two famous textbooks were not correct. We proposed a correct way to derive this formula.