Unified Model of Outbursts in Dwarf Novae

矮新星是一种不断爆发的变星，其亮度在2-6等之间。矮新星有三个亚类：U gem型只显示普通的爆发，Z cam型偶尔显示静止，SU uma型显示超爆发。本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，即所谓的圆盘不稳定模型。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SU - UMa恒星的超爆发现象进行了详细的数值模拟。研究发现，该模型可以很好地再现SU - UMa恒星的超爆发周期。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态振荡的本征值问题，并对该模态的本征值和本征函数进行了数值和解析研究。通过与只有超爆发而没有正常爆发的WZ sge型矮新星的比较，验证了SU uma型矮新星的盘不稳定性模型。讨论了WZ Sge最极端恒星本身的一种特殊情况，认为它是静止状态下粘度极低、从次级恒星传质速率极低的一种极端情况。

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.