Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks

TL;DR

This study uses resistive MHD simulations to show how non-uniform MRI growth in protoplanetary disks can create super-Keplerian regions, influencing dust accumulation and planet formation.

Contribution

It demonstrates that inhomogeneous MRI growth can modify angular velocity profiles, leading to super-Keplerian zones that impact planetesimal formation.

Findings

Super-Keplerian regions emerge due to MRI turbulence in non-uniform magnetic fields.

The velocity profile becomes more rigid or flattened in certain regions.

Outer edges of MRI dead zones are potential sites for planetesimal accumulation.

Abstract

We have investigated evolution of magneto-rotational instability (MRI) in protoplanetary disks that have radially non-uniform magnetic field such that stable and unstable regions coexist initially, and found that a zone in which the disk gas rotates with a super-Keplerian velocity emerges as a result of the non-uniformly growing MRI turbulence. We have carried out two-dimensional resistive MHD simulations with a shearing box model. We found that if the spatially averaged magnetic Reynolds number, which is determined by widths of the stable and unstable regions in the initial conditions and values of the resistivity, is smaller than unity, the original Keplerian shear flow is transformed to the quasi-steady flow such that more flattened (rigid-rotation in extreme cases) velocity profile emerges locally and the outer part of the profile tends to be super-Keplerian. Angular momentum and mass transfer due to temporally generated MRI turbulence in the initially unstable region is responsible for the transformation. In the local super-Keplerian region, migrations due to aerodynamic gas drag and tidal interaction with disk gas are reversed. The simulation setting corresponds to the regions near the outer and inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of dead zone, as well as the inner edge, would be a favorable site to accumulate dust particles to form planetesimals and retain planetary embryos against type I migration.