A Model for WZ Sagittae-Type Dwarf Novae: SU UMa/WZ Sge Connection

Abstract Based on the disk instability model (or the thermal-tidal instability model) for SU UMa stars proposed by Osaki (1989, AAA 50.064.103), we examine the connection between ordinary SU UMa stars exhibiting both normal outbursts and superoutbursts, and “WZ Sge” stars exhibiting only large-amplitude superoutbursts with a very long interval. It is argued that the continuous sequence of outburst activity among these stars may be understood as a variation in the input parameters, particularly as a variation in the mass-transfer rate, decreasing with the activity sequence. This may in turn be understood as an evolutionary sequence of cataclysmic variable stars as activity groups, where these stars evolve from very active SU UMa stars to stars having intermediate activity, and finally to the least-active “WZ Sge” stars. A particular case of the most extreme star of WZ Sge itself is discussed; it may be understood as an extreme case of very low viscosity in quiescence and having a rather low mass-transfer rate from the secondary.