Circumbinary Accretion from Finite and Infinite Disks

TL;DR

This study uses 2D hydrodynamics simulations to analyze how circumbinary disks transfer mass and angular momentum to binary systems, revealing that similar-mass binaries tend to expand over time, impacting black hole and star evolution.

Contribution

It provides detailed insights into accretion dynamics from both infinite and finite disks, highlighting the positive angular momentum transfer and its implications for binary evolution.

Findings

Accretion variability frequency depends on mass ratio.

Net angular momentum transfer always positive, leading to binary expansion.

Finite disk accretion phases include transient and viscous pseudo-stationary stages.

Abstract

We carry out 2D viscous hydrodynamics simulations of circumbinary disk (CBD) accretion using {\footnotesize AREPO}. We resolve the accretion flow from a large-scale CBD down to the streamers and disks around individual binary components. Extending our recent studies \citep{mun19}, we consider circular binaries with various mass ratios ($0.1\leq q_{\rm{b}}\leq1$) and study accretion from ``infinite'', steady-supply disks and from finite-sized, viscously spreading tori. For ``infinite'' disks, a global steady state can be reached, and the accretion variability has a dominant frequency ${\sim}0.2\Omega_{\rm{b}}$ for $q_{\rm{b}}>0.5$ and $\Omega_{\rm{b}}$ for $q_{\rm{b}}<0.5$, ($\Omega_{\rm{b}}$ is the binary angular frequency). We find that the accretion ``eigenvalue'' $l_0$ -- the net angular momentum transfer from the disk to the binary per unit accreted mass -- is always positive and falls in the range ($0.65$-$0.85)a_{\rm b}^2\Omega_{\rm{b}}$ (with $a_{\rm{b}}$ the binary separation), depending weakly on the mass ratio and viscosity. This leads to binary expansion when $q_{\rm{b}}\gtrsim0.3$. Accretion from a finite torus can be separated into two phases: an initial transient phase, corresponding to the filling of the binary cavity, followed by a viscous pseudo-stationary phase, during which the torus viscously spreads and accretes onto the binary. In the viscous phase, the net torque on the binary per unit accreted mass is close to $l_0$, the value derived for ``infinite'' disks. We conclude that similar-mass binaries accreting from CBDs gain angular momentum and expand over long time scales. This result significantly impacts the coalescence of supermassive binary black holes and newly formed binary stars. We offer a word of caution against conclusions drawn from simulations of transient accretion onto empty circumbinary cavities.