Research on accretion discs by Direct Simulation Monte Carlo Method

我们研究了密近双星系统中的气体流动。本研究的目标是吸积盘、气体释放星星表面流、风吸积流和旋转气体中的粘性。我们主要通过数值方法研究这些目标。计算方法有直接模拟蒙特卡罗方法（DSMC）和有限体积方法，在三维有限体积计算中，我们证实了早先发现的螺旋激波的存在。另一方面，在DSMC计算中，我们只发现了螺旋激波的痕迹。其原因可能是光盘周边区域的分辨率不足。在这样的稀薄区域中，颗粒的数量可能不足以保证颗粒之间发生碰撞，而颗粒之间的碰撞是确保颗粒集合的流体性质的基本成分。我们对质量损失的星星表面的流动进行了三维流体动力学模拟。我们发现了由恒星表面的高/低压产生的复杂漩涡。我们制作了多普勒图，将理论结果与超软X射线源的观测结果进行了比较，发现两者吻合很好。风吸积是密近双星系统中质量交换的一个重要机制。我们计算了质量吸积比，定义为质量吸积率与质量损失率的比值，并导出了一个实验公式。我们还计算了恒星风对系统角动量的影响。这些参数是研究双星系统演化的重要因素，吸积盘中的角动量转移是研究吸积盘物理的重要因素。指出了两本著名教科书中关于角动量转移公式的推导是不正确的。我们提出了一个正确的推导公式的方法。

We have investigated gas flows in close binary systems. The targets of the present research are an accretion disc, a surface flow on a gas-loosing star, a wind accretion flow and a viscosity in a rotating gas. We studied these targets mainly by numerical techniques. The methods of calculation are both a Direct Simulation Monte Carlo Method (DSMC) and a finite volume method.In three-dimensional finite volume calculations of accretion discs, we confirmed the presence of spiral shocks, which we had discovered earlier. On the other hand, we found only traces of the spiral shocks in DSMC calculations. The reason is probably insufficient resolution at the peripheral region of the disc. In such a rarefied region, the number of particles may not be enough to guarantee occurrence of collisions between particles, which are the essential ingredient to ensure the fluidal nature of the collection of particles. This difficulty is yet to be solved.We made three-dimensional hydrodynamic simulations of flows on the surface of the mass-losing star. We discovered complex eddies produced by high/low pressure on the stellar surface. We composed Doppler maps to compare the present theoretical results with observations of super-soft X-ray sources, and we found a good agreement.Wind accretion is an important mechanism of mass exchange in close binary systems. We computed the mass accretion ratio, which is defined as a ratio of mass-accretion rate to mass-loss rate, and derived an experimental formula. We also computed specific angular momentum loss from the system by stellar wind. These parameters are important factors in order to investigate the evolution of binary systems.Transfer of angular momentum in an accretion disc is an important factor to investigate the physics of accretion discs. We pointed out that the derivation of the formula of angular momentum transfer appeared in two famous textbooks were not correct. We proposed a correct way to derive this formula.