Grid-Based Simulations of Polar Circumbinary Disks: Polar Alignment and Vortex Formation

We describe the first grid-based simulations of the polar alignment of a circumbinary disk. We simulate the evolution of an inclined disk around an eccentric binary using the grid-based code ATHENA++. The use of a grid-based numerical code allows us to explore lower disk viscosities than have been examined in previous studies. We find that the disk aligns to a polar orientation when the $\alpha$ viscosity is high, while disks with lower viscosity nodally precess with little alignment over 1000 binary orbital periods. The timescales for polar alignment and disk precession are compared as a function of disk viscosity, and are found to be in agreement with previous studies. At very low disk viscosities (e.g. $\alpha = 10^{-5}$), anticyclonic vortices are observed along the inner edge of the disk. These vortices can persist for thousands of binary orbits, creating azimuthally localized overdensities as well as multiple pairs of spiral arms. The vortex is formed at $\sim 3-4$ times the binary semi-major axis, close to the inner edge of the disk, and orbits at roughly the local Keplerian speed. The presence of a vortex in the disk may play an important role in the evolution of circumbinary systems, such as driving episodic accretion and accelerating the formation of polar circumbinary planets.