Magnetic braking and the evolution of cataclysmic binaries

Abstract

We study the evolution and the period distribution of cataclysmic variables using a magnetic braking law proposed recently by Mestel & Spruit. The application of this law (with a suitable choice of parameters) gives a predicted period distribution that matches the observed one. The computed mass-transfer rates are in much better agreement with the observations than those resulting from the application of the prescription of Verbunt & Zwaan. We also show that the observed accumulation of AM Her systems with periods in the range 114–115 min can be explained in terms of evolution, as a result of the adiabatic expansion of the secondary when accretion resumes at the end of the period gap provided that the masses of the white dwarfs in AM Her systems are in a restricted range. We examine finally the recent claim by Lamb & Melia that when the magnetic white dwarf synchronizes with the orbit, the system detaches, and enters a prolonged gap. We show that the proposed mechanism does not work, and that synchronization has a relatively small effect on the evolution of these systems.