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

点击翻译按钮获取中文摘要

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