Unified Model of Outbursts in Dwarf Novae

矮新星是一种喷发变星，显示出2-6级的重复爆发幅度。矮新星有三个亚类：U宝石型只显示普通爆发，Z凸轮型显示偶尔停顿，苏乌玛型显示超爆发。这项研究的主要目的是检验我们是否可以用一个统一的范式来解释矮新星中丰富多样的爆发现象：所谓的盘不稳定性模型。为此，我们利用吸积盘的热不稳定性和潮汐不稳定性的联合机制，对苏乌玛星的超爆现象进行了详细的数值模拟。结果表明，该模型可以很好地再现苏乌玛星的超爆周期。我们将吸积盘的潮汐偏心不稳定性表述为吸积盘中单臂模式振动的本征值问题，并从数值和解析两方面考察了该模式的本征值和本征函数。本文研究了SU-UMA型矮新星的盘不稳定性模型，它与WZ-SGE型矮新星有关，后者只表现出超爆发，而明显没有正常爆发。讨论了WZ SGE最极端恒星本身的一个特例，认为这是一个静止时粘度很低、次级传质速率很低的极端情况。

Dwarf novae are eruptive variables showing repetitive outbursts of amplitudes of 2-6 magnitude. There are three sub-classes in dwarf novae : the U Gem-type showing only the ordinary outbursts, the Z Cam-type showing occasional standstill, and the SU UMa-type showing superoutbursts. The main purpose of this research is to examine whether or not we can explain rich variety of outburst phenomena in dwarf novae by a unified paradigm : the so-called disk instability model. To this end, we have made detailed numerical simulations of the superoutburst phenomenon of SU UMa stars by the combined mechanism of the thermal and tidal instabilities of accretion disks. It has been found that the superoutburst cycle of SU UMa stars can be successfully reproduced by this model. We have formulated the tidal eccentric instability of an accretion disk as an eigenvalue problem of the oscillation of the one-armed mode in the accretion disk and we have examined the eigenvalue and eigenfunction of this mode both numerically and analytically. The disk instability model for SU UMa-type dwarf novae is examined in relation to the WZ Sge-type dwarf novae which exhibit only superoutbursts but apparently no normal outburst. A particular case of the most extreme star of WZ Sge itself is discussed and it is suggested as an extreme case of very low viscosity in quiescence and of rather low mass transfer rate from the secondary.