Three-dimensional hydrodynamic simulation of an accretion flow in a close binary system

吸积是一种气体在恒星引力作用下落在恒星表面的现象。在紧密的双星系统中会出现一些有趣的现象，其中一颗恒星喷出气体，而另一颗恒星吸积气体。根据喷出气体的速度，有两种吸积模式，即风吸积和吸积盘。本研究的目的是进行三维流体力学计算机模拟，以阐明这些吸积流的行为。本文对风吸积流进行了二维数值模拟。我们发现吸积流是不稳定的，弓形激波在致密物波的上游区域形成。被吸积的气体将正号和负号的角动量传递给致密的物体。正因为如此，致密的物体可能会经历零星的上下自旋。我们把这种现象称为“触发器不稳定性”。如果这种不稳定性在自然界中发生，它可能…然而，由于计算是二维的，不稳定性可能仅仅是由于限制而产生的伪影。为了观察这种现象是否会在三维情况下发生，我们与以色列加兴和利维奥马克斯-普朗克研究所（现位于巴尔的摩）的Anzer和Borner进行了联合研究。我们发现三维情况下的流动比二维情况下的流动要弱。对于吸积盘，我们进行了二维数值模拟，发现螺旋激波是由伴星的潮汐力形成的。气体因受到冲击而失去角动量，并向致密物体吸积。由于计算的二维性质，这种现象也可能是一种人工产物。日本东北大学的Sawada和本作者对一种多向性气体进行了三维数值模拟，发现螺旋激波的形成。另一方面，计算流体动力学研究所的Nagasawa和本作者进行的三维SPH计算没有显示出明显的螺旋冲击。这一差异应在今后的研究中加以澄清。塔塔研究所的Chakrabarti和本作者研究了二维流动的时间变异性，并将结果应用于SS433。我们发现计算结果与观测结果很吻合。少

An accretion is a phenomenon by which gas falls onto a stellar surface by the stellar gravity. Interesting phenomena occur in close binary systems, in which one component star ejects gas while the other accretes it. Depending on the speed of the ejected gas, there are two modes of accretion, i.e. a wind accretion and an accretion disc. The purpose of the present research is to perform three-dimensional hydrodynamic computer simulations to clarify the behavior of these accretion flows.The present author performed two-dimensional numerical simulations of wind accretion flows. We found that the accretion flows were unstable and the bow shock formed in the upstream region of the compact object waves. The accreted gas conveys the angular momentum of both the positive and the negative sign onto the compact object. Because of it, the compact object may undergo the spin up and down sporadically. We termed this phenomenon as the flip-flop instability.If this instability occurs in nature, it may … More explain observations. However, since the computations were two-dimensional ones, the instability may be merely an artifact due to the restriction. In order to see if this phenomenon occurs in three-dimensional cases, we performed joint research with Anzer and Borner at the Max-Planck Institute, Garching and Livio in Israel(now in Baltimore). We found that the flow in three-dimensional case is less violent compared with two-dimensional counterpart.As to an accretion disc, we performed two-dimensional numerical simulations and found that spiral shocks were formed due to the tidal force by the companion star. The gas loses angular momentum by hitting the shocks and accretes onto the compact object.This phenomenon may be also an artifact due to the two-dimensional nature of the calculations. Sawada at Tohoku university and the present author performed a three-dimensional numerical simulation of an polytropic gas and found that the spiral shocks were formed. On the other hand, the three-dimensional SPH calculations performed by Nagasawa at the Institute of Computational Fluid Dynamics and the present author did not show marked spiral shocks. This discrepancy should be clarified in future researches.Chakrabarti at Tata Institute and the present author investigated the time variability of two-dimensional flows and applied the results to SS433. We found a good agreement of the computed results with observations. Less