Numerical Simulation of 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 is numerically simulated. Calculation is made for a region including the compact star and the gas-supplying companion. The equation of state is that of an ideal gas 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 from the companion via a Lagrangian L1 point towards the accretion disk, forms a fine gas beam (L1 stream), which penetrates into the disk. No hot spot therefore forms in these calculations. Another fact discovered is that the gas rotating with the disk forms, on collision with the L1 stream, a bow shock wave, which may be called an L1 shock. The disk becomes hot because the L1 shock heats the disk gas in the outer parts of the disk, so that the spiral shocks wind loosely even with $\gamma=1.01$. The L1 shock enhances the non-axisymmetry of the density distribution in the disk, and therefore the angular momentum transfer by the tidal torque works more effectively. The maximum of the effective $\alpha$ becomes $\sim 0.3$. The 'hot spot' is not formed in our simulations, but our results suggest the formation of the 'hot line', which is the L1 shock elongated along the penetrating L1 stream.