Origin of the low-eccentricity precession-to-locking transition

Determine the physical origin of the transition from freely precessing to apsidally locked circumbinary disks at low binary eccentricity, particularly for equal-mass binaries lacking an octupole potential and subject to weakened eccentric Lindblad resonances.

Background

The simulations show that the circumbinary disk can be freely precessing at low binary eccentricity but locked to the binary’s apsidal line at intermediate eccentricity. This behavior was previously identified for equal-mass binaries, for which standard secular test-particle theory predicts no apsidal alignment because the octupole component of the binary potential vanishes.

The paper notes that eccentric Lindblad resonances, which could otherwise pump disk eccentricity, become less effective relative to viscous damping at higher eccentricity. The competing explanations do not establish why the transition from low-eccentricity precession to intermediate-eccentricity locking occurs, leaving its physical mechanism unresolved.