Long-Lived Eccentric Modes in Circumbinary Disks

Diego J. Muñoz, Yoram Lithwick

Published 2020-08-18Version 1

Hydrodynamical simulations show that circumbinary disks become eccentric, even when the binary is circular. Here we demonstrate that, in steady state, the disk's eccentricity behaves as a long-lived free mode of the disk. Consequently, both the disk's precession rate and eccentricity profile may be calculated via the simple linear theory for perturbed pressure-supported disks. By formulating and solving the linear theory we find that (i) surprisingly the precession rate is roughly determined by the binary's quadrupole, even when the quadrupole is very weak relative to pressure; (ii) the eccentricity profile is largest near the inner edge of the disk, and falls exponentially outwards; and (iii) the results from linear theory indeed agree with what is found in simulations. Understanding the development of eccentric modes in circumbinary disks is a crucial first step for understanding the long term (secular) exchange of eccentricity, angular momentum and mass between the binary and the gas. Potential applications include the search for a characteristic kinematic signature in disks around candidate binaries and precession-induced modulation of accretion over long timescales.