Late Evolution of Cataclysmic Variables

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Abstract
We study the evolution of hydrogen‐rich cataclysmic variables (CVs) near minimum orbital period at ~78 minutes. As has been known for many years, these are among the most intrinsically common CVs, but they hide fairly well because of their faintness and low incidence of eruptions. We discuss their number and observational signatures, paying special attention to those that may have passed minimum orbital period—the "period bouncers." The status of binaries near minimum period is best determined by the mass ratio, and this is best constrained by measuring the accretion disk precession frequency, because that frequency is readily accessible to observation and proportional to the secondary star's mass. This method reveals four stars that are good candidates to have survived period bounce; two appear to have secondaries as puny as 0.02 M⊙. But each star can have bounced only recently if at all. There is still no strong evidence of any long era of evolution in a state of increasing period. This conflicts sharply with discussions of observational data that have identified dozens of known CVs with this state. The total space density of cataclysmic variables is ~10−5 pc−3, with short‐period systems constituting ~75% of the total. Both estimates are far less than predicted by simple theories of evolution. It is probably necessary to have some means of destroying CVs before they reach the predicted very high space densities. This can be done by invoking an angular momentum loss mechanism that does not quickly subside as the mass ratio becomes very low.