Numerical Simulation of the Interaction between an L1 Stream and an Accretion Disk in a Close Binary System

The hydrodynamic behavior of an accretion disk in a close binary system isnumerically simulated. Calculation is made for a region including the compactstar and the gas-supplying companion. The equation of state is that of an idealgas characterized by the specific heat ratio $\gamma$. Two cases with $\gamma$of 1.01 and 1.2 are studied. Our calculations show that the gas, flowing fromthe companion via a Lagrangian L1 point towards the accretion disk, forms afine gas beam (L1 stream), which penetrates into the disk. No hot spottherefore forms in these calculations. Another fact discovered is that the gasrotating with the disk forms, on collision with the L1 stream, a bow shockwave, which may be called an L1 shock. The disk becomes hot because the L1shock heats the disk gas in the outer parts of the disk, so that the spiralshocks wind loosely even with $\gamma=1.01$. The L1 shock enhances thenon-axisymmetry of the density distribution in the disk, and therefore theangular momentum transfer by the tidal torque works more effectively. Themaximum of the effective $\alpha$ becomes $\sim 0.3$. The 'hot spot' is notformed in our simulations, but our results suggest the formation of the 'hotline', which is the L1 shock elongated along the penetrating L1 stream.Comment: 23 pages, 14 figures, to appear in Prog. Theor. Phys. Vol.106, No.