MODIFICATION OF ANGULAR VELOCITY BY INHOMOGENEOUS MAGNETOROTATIONAL INSTABILITY GROWTH IN PROTOPLANETARY DISKS

We have investigated evolution of magnetorotational instability (MRI) in protoplanetary disks that have radially nonuniform 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 nonuniformly growing MRI turbulence. We have carried out two-dimensional resistive magnetohydrodynamic 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 favorable sites to accumulate dust particles to form planetesimals and retain planetary embryos against Type I migration.