Planetary nurseries: vortices formed at smooth viscosity transition

TL;DR

This study uses advanced hydrodynamic simulations to demonstrate that smooth viscosity transitions at the outer edge of protoplanetary discs can still excite Rossby wave instabilities, leading to vortex formation that fosters planetesimal growth.

Contribution

It shows that even smooth viscosity transitions can generate vortices, expanding the understanding of planetary nursery formation mechanisms in protoplanetary discs.

Findings

Large-scale vortices can form at smooth viscosity transitions.

Vortices can trap significant dust mass, exceeding hundreds of Earth masses.

Both single and multiple vortices can develop depending on dust coupling.

Abstract

Excitation of Rossby wave instability and development of a large-scale vortex at the outer dead zone edge of protoplanetary discs is one of the leading theories that explains horseshoe-like brightness distribution in transition discs. Formation of such vortices requires a relatively sharp viscosity transition. Detailed modelling, however, indicates that viscosity transitions at the outer edge of the dead zone is relatively smooth. In this study, we present 2D global, non-isothermal, gas-dust coupled hydrodynamic simulations to investigate the possibility of vortex excitation at smooth viscosity transitions. Our models are based on a recently postulated scenario, wherein the recombination of charged particles on the surface of dust grains results in reduced ionisation fraction and in turn the turbulence due to magnetorotational instability. Thus, the alpha-parameter for the disc viscosity depends on the local dust-to-gas mass ratio. We found that the smooth viscosity transitions at the outer edge of the dead zone can become Rossby unstable and form vortices. A single large-scale vortex develops if the dust content of the disc is well coupled to the gas, however, multiple small-scale vortices ensue for the case of less coupled dust. As both type of vortices are trapped at the dead zone outer edge, they provide sufficient time for dust growth. The solid content collected by the vortices can exceed several hundred Earth masses, while the dust-to-gas density ratio within often exceeds unity. Thus, such vortices function as planetary nurseries within the disc, providing ideal sites for formation of planetesimals and eventually planetary systems.