Theoretical Study of Accretion Disks in Close Binary Systems

矮新星是一种爆发变星，每隔几周到几个月就会出现2-6等的重复爆发。目前人们普遍认为，矮新星爆发可能是由灾难性变星吸积盘的不稳定性引起的，即本文作者（Osaki, 1974）在1974年首次提出的吸积盘不稳定性模型。非磁突变变星的爆发行为丰富多样，从非爆发类新星到矮星运动的各个亚类（即U gem型、Z cam型和SU uma型）。此外，最近还发现了两种极端的SU - uma型矮新星：WZ - sge型和ER - uma型。在吸积盘不稳定性模型的基本框架内，提出了一种矮新星爆发的统一模型，其中吸积盘中的两种不同的内在不稳定性（即热不稳定性和潮汐不稳定性）起着重要作用。在吸积盘中两种本征不稳定性的不同稳定行为组合图中，将非磁突变变量划分为4个区域，基本了解了它们的不同爆发行为。特别是，SU uma型矮新星的爆发可以通过热-潮不稳定性模型来理解，其中吸积盘中两种内在不稳定性的耦合起着独特的作用。热-潮不稳定性模型可以理解周期以下突变变量的丰富爆发行为（即从“永久超级驼峰”开始，到Z凸轮类苏马星，“ER乌马星”，到普通苏马星，最后到WZ Sge星）。

Dwarf novae are eruptive variables showing repetitive outbursts of amplitudes of 2-6 magnitude once every a few weeks to months. It is now widely accepted that outbursts of dwarf novae may be caused by the instability in accretion disks in cataclysmic variable stars : i.e., the disk instability model which was first proposed by the present author (Osaki, 1974) in 1974. There is a rich variety in outburst behaviors of non-magnetic cataclysmic variable stars, starting form non-outbursting nova-like stars to various sub-classes of dwarf movae (i.e., U Gem-type, Z Cam-type, and SU UMa-type) . Furthermore, two extreme cases of SU UMa-type dwarf novae have recently been recognized : WZ Sge-type and ER UMa-type.A unification model for dwarf nova outbursts is proposed within the basic framework of the disk instability model in which two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks play an essential role. Non-magnetic cataclysmic variables are classified into four regions in orbital-period versus mass-transfer rate diagram in different combination of stability behavior for the two intrinsic instabilities in accretion disks, and their different outburst behaviors are basically understood in this diagram. In particular, outbursts in SU UMa-type dwarf novae are understood by the thermal-tidal instability model in which the coupling of the two intrinsic instabilities in accretion disks plays a unique role. A rich variety of outburst behavior of cataclysmic variables below the period (i.e., starting from "permanent superhumpers", to Z Cam-like SU UMa stars, "ER UMa stars" , to ordinary SU UMa stars, finally to WZ Sge stars) is understood by the thermal-tidal instability model.