Orbital Evolution of Equal-mass Eccentric Binaries due to a Gas Disk: Eccentric Inspirals and Circular Outspirals

TL;DR

This study models how circumbinary gas disks influence the orbital evolution of equal-mass eccentric binaries, revealing attractors at specific eccentricities and complex oscillatory behaviors affecting binary separation and shape.

Contribution

It provides a detailed analysis of binary orbital evolution due to gas disk interactions, highlighting the transition between symmetric and asymmetric disk states and their impact on eccentricity and semimajor axis.

Findings

Binaries with initial eccentricity less than 0.1 tend to circularize and expand.

Binaries with initial eccentricity greater than 0.1 evolve toward and oscillate around e≈0.4.

Disk state transitions cause oscillations in orbital parameters and semimajor axis drift.

Abstract

We solve the equations of two-dimensional hydrodynamics describing a circumbinary disk accreting onto an eccentric, equal-mass binary. We compute the time rate of change of the binary semimajor axis $a$ and eccentricity $e$ over a continuous range of eccentricities spanning $e=0$ to $e=0.9$. We find that binaries with initial eccentricities $e_0 \lesssim 0.1$ tend to $e=0$, where the binary semimajor axis expands. All others are attracted to $e \approx 0.4$, where the binary semimajor axis decays. The $e \approx 0.4$ attractor is caused by a rapid change in the disk response from a nearly origin-symmetric state to a precessing asymmetric state. The state change causes the time rates of change $\dot{a}$ and $\dot{e}$ to steeply change sign at the same critical eccentricity resulting in an attracting solution where $\dot{a} = \dot{e} = 0$. This does not, however, result in a stalled, eccentric binary. The finite-transition time between disk states causes the binary eccentricity to evolve beyond the attracting eccentricity in both directions resulting in oscillating orbital parameters and a drift of the semimajor axis. For the chosen disk parameters, binaries with $e_0 \gtrsim 0.1$ evolve toward and then oscillate around $e \approx 0.4$ where they shrink in semimajor axis. Because unequal mass binaries grow toward equal mass through preferential accretion, our results are applicable to a wide range of initial binary mass ratios. Hence, these findings merit further investigations of this disk transition; understanding its dependence on disk parameters is vital for determining the fate of binaries undergoing orbital evolution with a circumbinary disk.