# Hydrodynamic Torques in Circumbinary Accretion Disks

## TL;DR

This study uses 3D hydrodynamic simulations to analyze how circumbinary disks influence the orbital evolution of equal-mass binary systems, finding that disks tend to cause the binaries to grow in semi-major axis.

## Contribution

First detailed 3D hydrodynamic simulations of circumbinary disks showing they cause binary orbital expansion regardless of disk alignment.

## Key findings

- Total torque on the binary is positive, leading to orbital expansion.
- Binary growth occurs in both aligned and misaligned disk configurations.
- Disk realignment timescale matches the viscous timescale of the disk.

## Abstract

Gaseous disks have been proposed as a mechanism for facilitating mergers of binary black holes. We explore circumbinary disk systems to determine the evolution of the central binary. To do so, we perform 3D, hydrodynamic, locally isothermal simulations of circumbinary disks on a Cartesian grid. We focus on binaries of equal mass ratios on fixed circular orbits. To investigate the orbital evolution of the binary, we examine the various torques exerted on the system. For the case where the disk plane and binary orbital plane are aligned, we find that the total torque is positive so that the semi-major axis of the binary increases. For the misaligned case, we run simulations with the binary orbital plane and disk midplane misaligned by 45\degree and find the same results - the binary grows. The timescale for the circumbinary disk to realign to the plane of the binary is consistent with the global viscous timescale of the disk.