A modest change in magnetic braking at the fully convective boundary explains the evolution of cataclysmic variables

Context. For decades, reproducing the orbital period distribution of nonmagnetic cataclysmic variables (CVs) seemed to require a drastic decrease, usually termed disruption, in the angular momentum loss through magnetic braking at the fully convective boundary, which argued for a change in the dynamo mechanism operating in fully and partially convective stars. However, recent studies showed that the prescription for magnetic braking that is traditionally used in CV evolution theory is clearly outdated because saturation, that is, a weak period dependence for rapidly rotating stars, is not included.

Aims. We test an updated version of a prescription for saturated magnetic braking that has been developed to explain the spin-down of single stars in the context of CV evolution. This prescription contains a boosting and a disruption parameter that represent the change in the strength of magnetic braking at the fully convective boundary.

Methods. We performed dedicated MESA simulations for CVs with the revised prescription for saturated magnetic braking.

Results. As in previous studies, we found that magnetic braking needs to be stronger in close binaries than in single stars, and that, in contrast to what is observed in single stars, magnetic braking needs to be reduced at the fully convective boundary. However, in contrast to previous studies of CV evolution, only a moderate disruption by a factor of 2–3 is sufficient to explain key features of the CV orbital period distribution and the measured mass-radius relation for CV donors.

Conclusions. The relatively small decrease in the efficiency of magnetic braking at the fully convective boundary might have implications for our understanding of dynamo models for fully and partially convective stars.