THE ROLE OF THE DWARF NOVA PERIOD DISTRIBUTION IN UNDERSTANDING THE EVOLUTION OF CATACLYSMIC VARIABLES

Within the context of disk instability theory, cataclysmic variables possessing accretion disks (i.e., nonmagnetic systems) are expected to exhibit disk instabilities that result in dwarf nova eruptions if the mass transfer rates fall below a critical level, M(crit). It is argued that the eruptive characteristics of cataclysmic variables can therefore be used to infer relative mass transfer rates among nonmagnetic cataclysmic variables. Here the dwarf nova period distribution is used to constrain the variation of mass transfer with orbital period. For orbital periods above the gap, agreement between the observed dwarf nova period distribution with those constructed from various magnetic braking models is generally disappointing. The differences arise both because the braking laws often result in M(P) relations that are unacceptably steep, and, more specifically, because the braking laws offer no explanation for the observed dearth of dwarf novae with orbital periods between 3 and approximately 4 hr. The most promising braking law is that of Mestel and Spruit. Under certain conditions this braking law is able to produce a relatively flat M(P) relation, but the braking law is not entirely satisfactory because it offers no explanation for the complete dominance of stable over unstable accretors immediately above the period gap. Speculative ideas are presented that may eventually provide a complete and satisfactory explanation for the lack of dwarf novae with periods between 3 and approximately 4 hr. As a general point, it is suggested that the dwarf nova period distribution, and not only the overall period distribution, should be considered when applying observational constraints to theories of mass transfer in cataclysmic variables.