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

吸积盘是围绕致密物体形成的气体盘，如白矮星、中子星或黑洞。在本研究中，我们对一个紧密双星系统中的吸积盘进行了三维数值模拟。所采用的方法是使用黎曼解算器的有限体积法，称为SFS方案。计算区域包括一颗吸积质量的主恒星和一颗失去质量的伴星。我们用理想气体的状态方程作为气体的状态方程。我们发现通过L1点的气流（L1流）在考虑的参数范围内穿透吸积盘。我们还发现，围绕主恒星旋转的盘状气体撞击L1流，形成拉长的激波。我们称之为L1电击。由于L1激波，盘状气体失去了角动量，向主恒星吸积。我们提出了一种螺旋激波的角动量损失机制，但它的大小可能不足以解释观测结果。发现L1激波的角动量损失足够大。1997年，英国天文学家发现了由我们提出的在天马座的大变星IP上的螺旋结构。从那时起，螺旋激波模型引起了社会的广泛关注。最近也发现了这种渗透。上述模拟中的一个问题是没有明确地考虑辐射冷却。相反，我们将比从5/3降低到1.01。在未来的研究中，我们计划采用直接模拟蒙特卡罗方法。对于二维情况，我们已经得到了初步的结果。我们计划将其扩展到三维，并尝试通过显式求解热传导来模拟辐射传递。

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.