Simulations of Eccentricity Growth in Compact Binary Accretion Disks with MHD Turbulence

TL;DR

This study uses MHD simulations to explore how accretion disks in compact binary systems develop eccentricity, revealing complex behaviors and challenges in disk spreading not captured by traditional alpha-disk models.

Contribution

It demonstrates that MRI turbulence does not prevent eccentricity growth and uncovers new phenomena like disk breaking and eccentric wave formation in MHD disks.

Findings

Eccentricity growth occurs similarly in MHD and hydrodynamical disks with net vertical field.

MHD disks exhibit episodes of radial disk breaking and standing eccentric waves.

Spreading of MRI-turbulent disks to the resonant radius is more difficult than in alpha-disk models.

Abstract

We present the results of four magnetohydrodynamic simulations and one alpha-disk simulation of accretion disks in a compact binary system, neglecting vertical stratification and assuming a locally isothermal equation of state. We demonstrate that in the presence of net vertical field, disks that extend out to the 3:1 mean motion resonance grow eccentricity in full MHD in much the same way as in hydrodynamical disks. Hence turbulence due to the magnetorotational instability (MRI) does not impede the tidally-driven growth of eccentricity in any meaningful way. However, we find two important differences with alpha-disk theory. First, in MHD, eccentricity builds up in the inner disk with a series of episodes of radial disk breaking into two misaligned eccentric disks, separated by a region of circular orbits. Standing eccentric waves are often present in the inner eccentric disk. Second, the successful spreading of an accretion disk with MRI turbulence out to the resonant radius is nontrivial, and much harder than spreading an alpha-disk. This is due to the tendency to develop over-dense rings in which tidal torques overwhelm MRI transport and truncate the disk too early. We believe that the inability to spread the disk sufficiently was the reason why our previous attempt to excite eccentricity via the 3:1 mean motion resonance with MHD failed. Exactly how MHD disks successfully spread outward in compact binary systems is an important problem that has not yet been understood.