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

Abstract

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 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 a 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. It also implies that the timescale for CV evolution is much longer than it was than previously thought, 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 implies a lower space density of CVs. The empirical loss law is consistent with the observed period minimum (1.3 hours) contrary to the minimum predicted by angular momentum loss due to gravitational radiation alone (1.1 hours). We introduce a method to infer the time-averaged mass accretion rate and derive mass-period relation for different evolutionary states of the secondary. 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.