Unequal mass binary evolution driven by high-Mach circumbinary discs

Madeline Clyburn

doi:10.1093/mnras/stag567

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Unequal mass binary evolution driven by high-Mach circumbinary discs

2026-05-01

Madeline Clyburn

Jonathan Zrake *

DOI：10.1093/mnras/stag567

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Abstract

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We present a study of the gas-driven orbital evolution of unequal mass black hole binaries with circumbinary gas discs (CBDs), varying Mach number ($\mathcal {M}$), and viscosity ($\mathcal { u }$). 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 $\mathcal {M}$ 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-$\mathcal {M}$ discs but transition to negative torques for $\mathcal {M} \gtrsim 25$. As q decreases, the transition moves to higher Mach numbers. For $q\lt 0.1$, we find no torque sign reversal below $\mathcal {M}\approx 52$, except in sufficiently low-viscosity discs. 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 discs therefore tend to outspiral, although inspiral can occur in less viscous environments. We also find that binaries with mass ratios in the range of $0.25 ext{-} 0.5$ can show preferential accretion favouring the primary when the gas viscosity is low, exemplifying an exception to the established rule of thumb that accretion favours 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 in CBDs may be less frequent, but this depends sensitively on the interplay between mass ratio, disc temperature, and viscosity.

Keywords:

accretion, accretion discs

black hole physics

hydrodynamics

black hole mergers