Eccentric Binaries Accreting from Thin Disks: Orbital Evolution

TL;DR

This study systematically explores how thin circumbinary disks influence the orbital evolution of eccentric binaries, revealing that thinner disks cause rapid inspiral and higher eccentricity excitation, with implications for gravitational wave detection.

Contribution

First systematic analysis of eccentric binary evolution with thin accretion disks, highlighting differences from thick disks in eccentricity growth and orbital dynamics.

Findings

Thinner disks drive rapid binary inspiral and higher eccentricities.

Thinner disks can pump eccentricities beyond 0.6, unlike thick disks.

Disk thickness and viscosity influence eccentricity damping and excitation.

Abstract

Circumbinary disks crucially affect the orbital and electromagnetic properties of binary systems across the universe, from stars in our galactic neighborhood to supermassive black hole binaries formed as the result of tumultuous galactic mergers. Previous simulations have focused nearly exclusively on thick accretion disks, appropriate for studying stellar binaries, and have found encouraging agreement with observations thereof. We present herein the first systematic study of eccentric binary systems accreting from thin disks, focusing on binary orbital evolution. Our main results are that (1) thinner disk not only drive binaries to rapidly inspiral, but also excite binary eccentricities at much higher rates; (2) while thick disks may drive binaries to a stable fixed point of $e\approx0.425$, thinner disks pump binary eccentricities to $e\gtrsim0.6$; (3) the range of near-zero eccentricities that are damped towards zero depends on both disk thickness and viscosity, thinner disks and those with $\alpha$ viscosities driving binaries towards circularity over a much narrower range of eccentricities. These differences follow largely from the effects of pressure support on accretion streams and shocks within the inner regions of the accretion flow. Our results suggest that accreting binary black holes should have high eccentricities well into the frequency range probed by pulsar timing arrays and space-based gravitational wave interferometers, affecting the spectrum and isotropy of the gravitational wave background. Our results also suggest that circumbinary disks may play an important role in shaping the orbits of close binary stars, but much less so those of wider binaries.