Superhumps in SU UMA Dwarf Novae

Abstract

Dwarf novae belong to the class of cataclysmic variables and represent the close binary systems in the late evolutionary stages. They consist of a white dwarf and a red dwarf which fills its Roche lobe and transfers mass to the white dwarf. The gas stream flows from the secondary to the primary white dwarf forming an accretion disk around it. Dwarf novae undergo outburst in semi-periodic intervals of time, when the brightness increases by 3m to 5m. For most of them, the faint state (or quiescence) is a normal state. There are 3 types of dwarf novae: U Gem, Z Cam and SU UMa. The last ones have orbital periods about 80 −180 min, mass of the secondary 3 times less or even more than white dwarf mass. Two types of outbursts are observed in these systems: frequent normal outbursts and superoutbursts which last longer and rise to slightly higher luminosities. During superoutburst they exhibit so-called “superhumps”,—brightness increase on the small part of the light curve repeated with the period a few percents longer than the orbital one and amplitude of ∼ 0.1m −0.3m. Superhumps are the most striking phenomenon which exhibited SU UMa dwarf novae and make them interesting and promising objects for study. According to the tidal- thermal model (Osaki, 1996) superhumps are due to gravitational disturbances from the secondary. These disturbances became most effective then the matter of accretion disk reach the 3:1 resonance of the orbital motion of the secondary. The beating of the orbital and precessional periods cause periodic variations, identified as superhumps. Positive and negative superhumps are described. The evolution of superhumps is considered together with the changes of superhump period during the outburst.The observational properties of SU UMa stars are collected by photometric observations of various systems. Criteria for classification of WZ Sge dwarf novae (sub-type of SU UMa stars) are summarized. Rebrightenings or echo outbursts are one of the most striking characteristics of WZ Sge objects.