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

吸积是气体在恒星引力作用下落到恒星表面的现象。在紧密的双星系统中会出现有趣的现象，在这个系统中，一颗组成恒星喷出气体，而另一颗恒星吸走气体。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水动力计算机模拟，以阐明这些吸积流的行为。我们发现，吸积流是不稳定的，在致密目标波的上游区域形成了弓激波。吸积的气体将正号和负号的角动量传递到致密物体上。正因为如此，致密物体可能会零星地经历自转。我们将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，它可能会…更多解释观察结果。然而，由于计算是二维的，由于限制，不稳定可能仅仅是一种伪影。为了了解这种现象是否发生在三维情况下，我们与马克斯·普朗克研究所的安泽尔和博纳、以色列的加钦和利维奥(现位于巴尔的摩)进行了联合研究。对于吸积盘，我们进行了二维数值模拟，发现伴星的潮汐作用力形成了螺旋激波。气体通过撞击激波而失去角动量，并附着在致密物体上。由于计算的二维性，这种现象也可能是一种人工制品。东北大学的泽田和作者对多变气体进行了三维数值模拟，发现螺旋激波已经形成。另一方面，长泽和本作者在计算流体动力学研究所进行的三维SPH计算没有显示出明显的螺旋激波。塔塔研究所的查克拉巴蒂和本文作者研究了二维流的时间变异性，并将结果应用于SS433。我们发现计算结果与观测结果有很好的一致性。较少

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