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

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Abstract
Numerical simulation of the hydrodynamic behavior of an accretion disk in a close binary system is reported. Calculations were carried out for a region including a compact star and its gas-supplying companion. The equation of state is that of an ideal gas characterized by a specific heat ratio γ. Two cases, with γ=1.01 and γ=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. Thus, no hot spot forms in these calculations. Another result is that the gas rotating with the disk forms —upon collision with the L1 stream— a bow shock wave, which we call 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 γ=1.01. The L1 shock enhances axial asymmetry of the density distribution in the disk, and therefore angular momentum is transferred through the tidal torque more effectively. The maximum value of the effective α becomes ∼0.3. A ‘hot spot’ is not formed in our simulations, but our results suggest the formation of a ‘hot line’, which is the L1 shock elongated along the penetrating L1 stream.
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