Unequal Mass Binary Evolution Driven by High Mach Circumbinary Disks

TL;DR

This study investigates how gas disks influence the orbital evolution of unequal mass black hole binaries, revealing complex torque behaviors and accretion patterns that depend on Mach number, mass ratio, and viscosity.

Contribution

The paper provides new insights into the gas-driven evolution of unequal mass black hole binaries, especially in regimes with high Mach numbers and varying viscosities, extending previous research.

Findings

Equal mass binaries receive positive torques at low Mach but switch to negative at high Mach.

Low mass ratio binaries tend to outspiral due to accretion patterns, especially in high viscosity disks.

Binaries with mass ratios 0.25-0.5 can favor primary accretion under certain conditions.

Abstract

We present a study of the gas-driven orbital evolution of unequal mass black hole binaries with circumbinary gas disks (CBDs), varying Mach number and viscosity (nu). Using two-dimensional grid-based hydrodynamics simulations spanning a thousand binary orbits at fixed separation, we explore low to moderate mass ratios (q = 0.05-1.0) and examine how variations in Mach and q affect the torques and component accretion rates exerted by the CBD and consequently the binary evolution. Equal mass binary systems receive positive torques in low-mach disks but transition to negative torques for Mach >25. As q decreases, the transition moves to higher Mach numbers. For q<0.1, we find no torque sign reversal below Mach~52, except in sufficiently low-viscosity disks. We find that the secondary black hole cannot effectively repel the CBD, it instead accretes most of the inflowing gas from the CBD; these low mass ratio binaries in high viscosity disks therefore tend to outspiral, although inspiral can occur in less viscous environments. We also find that binaries with mass ratios in the range of 0.25 - 0.5 can show preferential accretion favoring the primary when the gas viscosity is low, exemplifying an exception to the established rule of thumb that accretion favors the secondary. We discuss differences between our results and those reported in the literature on the orbital evolution and preferential accretion, and emphasize that our simulations extend into a regime that remains largely unexplored. Overall, our results suggest that intermediate mass ratio inspirals (IMRIs) in CBDs may be less frequent, but this depends sensitively on the interplay between mass ratio, disk temperature, and viscosity.