Instability and Angular Momentum Transfer in Thick Circumbinary Disks

Utilizing the DISCO code we solve the 2D hydrodynamic equations for an equal mass circular restricted binary with an isothermal circumbinary disk (CBD). This work explores the eccentric instability in the CBD and angular momentum transfer, and their dependence on disk thickness and viscosity. The focus is specifically on thicker CBDs, with Mach number, (M at most 14). The growth rate of the disk cavity's eccentricity and the angular momentum transport from accretion is calculated as diagnostics for binary-disk morphology evolution. Calculations of CBD torques must run long enough to allow saturation of the disk's eccentric instability. The torque on the binary rapidly flips sign once the instability saturates. Thus we connect eccentricity saturation to a reversal in migration behavior for the binary. The binary experiences inward migration while the disk is in an evolving state, and outward migration once the disk eccentricity has saturated. The saturation time was found to be non-linearly dependent on Mach number and thicker CBDs require longer evolution time for a quasi-steady state to appear. The eccentric instability is not present for M 5, and therefore migration reverses direction, leading to robustly inward migration for very thick disks h/r 0.2. The results of this work were found to be consistent with community results for Mach numbers M 10.