Dwarf-nova outbursts

Outburst mechanisms of dwarf novae are discussed. There is a rich variety in outburst behaviors of nonmagnetic cataclysmic variable stars, starting from nonoutbursting nova-like stars to various sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type, and SU 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. Nonmagnetic cataclysmic variables are classified into four regions in the orbital-period versus mass-transfer-rate diagram, showing different combinations of stability behavior for the two intrinsic instabilities in accretion disks, and their different outburst behaviors are basically understood in this diagram. We discuss several problems concerning the thermal limit-cycle instability model for dwarf novae above the period gap. We then discuss the thermal-tidal instability model for SU UMa stars, dwarf novae below the period gap, in which the coupling of the two intrinsic instabilities in the accretion disk plays a unique role. In particular, a rich variety of outburst behaviors of cataclysmic variables below the period gap (i.e., starting from ''permanent superhumpers,'' to Z Cam-like SU UMa stars, ''ER UMa stars,'' to ordinary SU UMa stars, and finally to WZ Sge stars) is understood by the thermal-tidal instability model.