Counter-Rotating Accretion Discs

TL;DR

This paper investigates the dynamics of counter-rotating accretion discs through high-resolution simulations, revealing significantly increased accretion rates and complex behaviors like shear layer formation and quasi-periodic gap opening.

Contribution

It presents detailed hydrodynamic simulations of counter-rotating discs, highlighting the effects of vertical and radial separation on accretion processes and flow structures.

Findings

Accretion rates are 100 to 10,000 times higher than in single-direction discs.

Shear layers form between vertically separated components, with free-fall towards the center.

Radially separated components exhibit quasi-periodic gap opening and closing.

Abstract

Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic $\alpha-$viscosity including all terms in the viscous stress tensor. For the vertically separated components a shear layer forms between them. The middle of this layer free-falls to the disk center. The accretion rates are increased by factors $\sim 10^2-10^4$ over that of a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc co-rotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are $\gtrsim 25$ times larger than those for a disc rotating in one direction with the same viscosity.