Cataclysmic Variables: An Empirical Angular Momentum Loss Prescription from Open Cluster Data

We apply the angular momentum loss rates inferred from open cluster stars to the evolution of cataclysmic variables (CVs). We show that the angular momentum prescriptions used in earlier CV studies are inconsistent with the measured rotation data in open clusters. The timescale for angular momentum loss ( j ) above the fully convective boundary is ~2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in behavior at the fully convective boundary. This provides evidence against the hypothesis that the period gap is caused by an abrupt change in the angular momentum loss law when the secondary becomes fully convective. Furthermore, the empirical angular momentum loss law implies a timescale for CV evolution that is comparable to a Hubble time; for the same reason, it will be more difficult to produce CVs from the products of common envelope evolution, and it implies a lower space density of CVs. The predicted loss rate for short-period CVs is consistent with the observed period minimum (1.3 hr). We infer the timeaveraged mass accretion rate and derive the mass-period relation for different evolutionary states of the secondary. The steady-state accretion rates are significantly lower than the claimed observational rates; we discuss some possible explanations. The mass-period relationship is more consistent with evolved secondaries than with unevolved secondaries above the period gap. Implications for the CV period gap are discussed, including the possibility that two populations of secondaries could produce the gap.