Orbital Evolution of Binaries in Circumbinary Disks

TL;DR

This study explores the orbital evolution of binary systems within circumbinary disks through extensive hydrodynamic simulations, revealing how eccentricity and mass ratio influence binary coalescence or expansion.

Contribution

It provides the largest parameter space exploration of binary orbital evolution in circumbinary disks and derives new orbital evolution prescriptions from simulations.

Findings

Eccentricities tend toward a steady state around 0.5 for large mass ratios.

Binaries with mass ratio q_b > 0.3 tend to coalesce, while lower ratios expand.

Steady state eccentricity can be reached within a few million years.

Abstract

We present the to-date largest parameter space exploration of binaries in circumbinary disks (CBDs), deriving orbital evolution prescriptions for eccentric, unequal mass binaries from our suite of hydrodynamic simulations. In all cases, binary eccentricities evolve towards steady state values that increase with mass ratio, and saturate at an equilibrium eccentricity $e_{\rm b, eq} \sim 0.5$ in the large mass ratio regime, in line with resonant theory. For binaries accreting at their combined Eddington limit, a steady state eccentricity can be achieved within a few Megayears. Once at their steady state eccentricities, binaries with $q_{\rm b} \gtrsim 0.3$ evolve towards coalescence, while lower mass ratio systems expand due to CBD torques. We discuss implications for population studies of massive black hole binaries, protostars in binary systems, and post-common envelope binaries observed by ground-based gravitational wave detectors.