On the Role of Gas Cooling in the Dynamics of Circumbinary Disks

TL;DR

This study investigates how different gas cooling times influence the structure, accretion variability, and angular momentum transfer in circumbinary disks around equal-mass binaries using hydrodynamic simulations.

Contribution

It introduces thermal relaxation with variable cooling times into CBD simulations, revealing effects on disk morphology, accretion variability, and binary orbital evolution.

Findings

Longer cooling times suppress accretion variability.

Disk morphology becomes more symmetric with increased cooling time.

Binary orbit tends to expand under most cooling conditions.

Abstract

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equation of state. We carry out two-dimensional viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation towards equilibrium temperature (the constant-$\beta$ cooling ansatz, where $\beta$ is the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based code Athena++ on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysterisis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity ($H/r\sim 0.1$ and $\alpha\sim 0.1$), the binary orbit expands while undergoing accretion for most $\beta$ values between 0 and 4.0 except over a narrow range of intermediate $\beta$ values. The validity of using polar grid excising the central domain is also discussed.