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

吸积盘是一种气体盘，它围绕着致密的物体形成，比如白色矮星、中子星星或黑洞。本文对密近双星系统中的吸积盘进行了三维数值模拟。所采用的方法是一个有限体积法称为SFS计划使用黎曼求解器。计算区域包括主星星吸积质量和伴星星星损失质量。作为气体的状态方程，我们采用了理想气体的状态方程。我们发现，在所考虑的参数范围内，流经L1点的气流（L1流）穿透吸积盘。我们还发现，围绕主星星旋转的盘状气体撞击L1气流，形成了一个拉长的激波。我们称之为L1休克。由于L1激波的作用，盘状气体失去了角动量并吸积到主星星上。我们已经提出了一种螺旋激波造成角动量损失的机制，但它的量级可能不足以解释观测结果。L1激波的角动量损失被认为是足够大的。1997年，英国天文学家在激变变星飞马座IP中发现了我们提出的螺旋结构。此后，螺旋激波模型引起了社会各界的广泛关注。在上述模拟中存在的问题之一是没有明确考虑辐射冷却。相反，我们将特定比率从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。我们已经得到了二维情形的初步结果。我们计划将其扩展到三维，并试图通过显式求解热传导来模拟辐射传输。

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.