Theoretical Study of Accretion Disks in Close Binary Systems

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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.