Angular Momentum Loss Mechanisms in Cataclysmic Variables below the Period Gap

TL;DR

This paper investigates additional angular momentum loss mechanisms in cataclysmic variables below the period gap, finding circumbinary disks and outflows from the L2 point as effective contributors to AML beyond gravitational radiation.

Contribution

It evaluates various AML mechanisms and identifies circumbinary disks and L2 outflows as plausible sources explaining the enhanced AML rate below the period gap.

Findings

Circumbinary disks can effectively extract angular momentum with minimal mass loss.

Outflows from the L2 point can contribute to AML if a significant fraction of mass is lost.

Isotropic wind and L1 outflows are insufficient to explain the observed AML rate.

Abstract

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 by \citet{kbp11}, however, indicates that AML rate below the period gap is $2.47(\pm 0.22)$ 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 Langrangian points, and the formation of a circumbinary disk. We found that neither isotropic wind from the white dwarf nor outflow from the $L_1$ point can explain the extra AML rate, while ouflow from the $L_2$ point or a circumbinary disk can effectively extract the angular momentum provided that $\sim (15-45)%$ 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 $\lesssim 10^{-3}$.