How two cohabiting magnetic dynamos explain the secular evolution of cataclysmic variables

Abstract

We show how a combination of two α-Ω dynamos, one in the convective envelope and the other at the boundary with a slowly rotating radiative core, can nicely explain the secular evolution of cataclysmic variables. The boundary-layer dynamo is dominant when the convective core is small. There it drives magnetic braking which leads to mass-transfer rates of the order of 10-9M⊙yr-1. When the star becomes fully convective, this dynamo switches off. This allows the star to contract inside its Roche lobe creating a period gap at about 3 h. The envelope dynamo is dominant at larger periods, while its effects are comparable with gravitational radiation below 3 h. It does not, therefore, have much effect on the edges of the period gap. The boundary-layer dynamo must maintain a universal rate of angular momentum loss for all cataclysmic variables if it is to produce the well-defined, observed period gap. The envelope dynamo, on the other hand, may fluctuate considerably and hence give rise to the wide range of mass-transfer rates implied for systems away from the period gap. © 1997 RAS.