Instabilities of Accretion Disks in Cataclysmic Variable Stars

矮新星是一种喷发式变星，其爆发幅度为2-6等，爆发持续时间为几天至20天，复发时间为20-300天。它们属于一类更一般的激变变星，因为充满冷矮星的罗氏叶通过内拉格朗日点失去质量，而非磁性的白矮星通过吸积盘吸积质量。在这项研究中，我们从激变变星吸积盘的内在不稳定性出发，研究了矮新星的爆发机制。已知的矮新星吸积盘有两种不稳定性：热不稳定性(即普通的盘不稳定性)和潮汐不稳定性。热不稳定性是指当温度为…时，发生在矮新星吸积盘外部的一种不稳定性更多的圆盘变得足够冷，可以让氢重新结合。这种热不稳定性通过在吸积盘中周期性地积累物质，然后突然将其倾倒在白矮星上，导致吸积盘的热弛豫振荡，导致吸积光度的突然爆发。这一机制现在被认为是导致U型宝石型矮新星普通爆发的原因。-吸积盘中的另一种不稳定性是潮汐不稳定性，即由于次星的潮汐效应，吸积盘变形为进动偏心盘的不稳定性。在下文中，我们列举了我们在这个研究项目中所考察的问题和所取得的结果。(1)我们提出了一个新的模型来解释苏乌玛型矮新星的超暴现象。该模型基于吸积盘的热不稳定性和潮汐不稳定性的综合机制。(2)研究了矮新星爆发周期中吸积盘半径的时间演化。我们基于传质爆发模型和圆盘不稳定性模型研究了这一点。结果表明，盘不稳定模型比传质爆发模型更能解释观测到的盘半径变化。(3)对激变变星中吸积盘的潮汐不稳定性进行了二维流体动力学模拟。采用了两种不同的流体动力学模拟方法，一种是粘性质点方法，另一种是光滑质点流体力学方法(简称SPH方法)。结果表明，当次星的质量小于主星质量的约四分之一时，吸积盘形变为进动偏心形式。(4)研究偏心盘的进动速率。结果表明，进动速率可表示为吸积盘单臂振动的本征值问题。较少

Dwarf novae are eruptive variable stars showing repetitive outbursts of amplitudes of 2-6 magnitude, outburst duration of a few days to 20 days, and recurrence time of 20-300 days. They belong to a more general class of cataclysmic variable stars, in that a Roche lobe filling cool dwarf star loses mass through the inner Lagrangian point and a non-magnetic white dwarf star accretes it through an accretion disk. Various outbursting phenomena occurring in these stars are now thought to be caused by variable accretion onto the white dwarf In this research, we have investigated the outburst mechanisms of dwarf novae based on intrinsic instabilities within the accretion disks in cataclysmic variable stars.Two kinds of instabilities are known in acceretion disks of dwarf novae ; the thermal instability (i. e., the ordinary disk instability) and the tidal instability. The thermal instability is an instability which occurs in the outer-parts of accretion disk of dwarf nova when the temperature … More of the disk becomes cool enough for hydrogen to recombine. This thermal instability leads to a thermal relaxation oscillation of the accretion disk by periodically accumulating matter in the disk and then suddenly dumping it onto the white dwarf, giving rise to sudden burst of accretion luminosity. This mechanism is now believed to be responsible for the ordinary outburst of U Gem-type dwarf novae. - Another kind of instability in accretion disks is the tidal instability by which instability an accretion disk is deformed into a precessing eccentric disk due to the tidal effects of the secondary star. In what follows, we list problems that we have examined in this research project and results obtained.(1) We have proposed a new model that explains the superoutburst phenomenon in SU UMa-type dwarf novae. This model is based on the combined mechanisms of the thermal instability and the tidal instability of accretion disk.(2) We have studied time evolution of accretion disk radius in the outburst cycle of dwarf nova. We examined this based on both the mass transfer burst model and the disk instability model. It was found that the disk instability model is more favorable in explaining observed disk radius variation than the mass transfer burst model.(3) Two-dimensional hydrodynamic simulations have been performed to study the tidal instability of accretion disks in cataclysmic variable stars. Two different methods of hydrodynamic simulations are used ; one is the viscous particle method and the other is the smoothed particle hydrodynamics abbreviated as SPH method). It has been found that an accretion disk is deformed to a precessing eccentric form if the mass of the secondary star is less than about one-fourth of the mass of the primary star.(4) The precession rate of the eccentric disk is studied. It is shown that the precession rate can be formulated as an eigenvalue problem of one-armed oscillation of the accretion disk. Less