Stellar models of evolved secondaries in CVs

In this paper we study the impact of chemically evolved secondaries on CV evolution. We find that when evolved secondaries are included a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. We argue that in order to explain this spread a considerable fraction of all CVs should have evolved stars as the secondaries. The evolved stars become fully convective at lower orbital periods. Therefore, even if there was an abrupt decrease in magnetic braking for fully convective stars (contrary to open cluster data) it would not be expected to produce a sharp break in the period distribution for CVs. We also explore recent proposed revisions to the angular momentum loss rate for single stars, and find that only modest increases over the saturated prescription are consistent with the overall observed spindown pattern. We compare predictions of our models with diagnostics of the mass accretion rate in WDs and find results intermediate between the saturated and the older braking prescription. Taken together these suggest that the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period, but is still significantly lower than in the traditional model. Alternative explanations for the CV period gap are discussed.Comment: 24 pages, 9 figures. Submitted to Ap

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 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.Comment: 30 pages, 6 figures. Submitted Ap