ANGULAR MOMENTUM LOSS MECHANISMS IN CATACLYSMIC VARIABLES BELOW THE PERIOD GAP

Mass transfer in cataclysmic variables (CVs) is usually considered to be caused by angular momentum loss (AML) driven by magnetic braking and gravitational radiation (GR) above the period gap, and solely by GR below the period gap. The best-fit revised model of CV evolution recently presented by Knigge et al., however, indicates that the AML rate below the period gap is 2.47(± 0.22) times the GR rate, suggesting the existence of some other AML mechanisms. We consider several kinds of consequential AML mechanisms often invoked in the literature: isotropic wind from the accreting white dwarfs, outflows from the Lagrangian points, and the formation of a circumbinary disk. We find that neither isotropic wind from the white dwarf nor outflow from the L1 point can explain the extra AML rate, while outflow from the L2 point or a circumbinary disk can effectively extract the angular momentum provided that ∼(15-45)% of the transferred mass is lost from the binary. A more promising mechanism is a circumbinary disk exerting gravitational torque on the binary. In this case, the mass-loss fraction can be as low as ∼< 10–3.