THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS OF PLANET MIGRATION IN TURBULENT STRATIFIED DISKS

We performed three-dimensional magnetohydrodynamic simulations of planet migration in stratified disks using the Godunov code PLUTO, where the disk is turbulent due to the magnetorotational instability. We study the migration for planets with different planet-star mass ratios q = M-p/M-s. In agreement with previous studies, for the low-mass planet cases (q = 5 x 10(-6) and 10(-5)), migration is dominated by random fluctuations in the torque. For a Jupiter-mass planet (q = M-p/M-s = 10(-3) for M-s = 1M(circle dot)), we find a reduction of the magnetic stress inside the orbit of the planet and around the gap region. After an initial stage where the torque on the planet is positive, it reverses and we recover migration rates similar to those found in disks where the turbulent viscosity is modeled by an a viscosity. For the intermediate-mass planets (q = 5 x 10(-5), 10(-4), and 2 x 10(-4)), we find a new and so far unexpected behavior. In some cases they experience sustained and systematic outward migration for the entire duration of the simulation. For this case, the horseshoe region is resolved and torques coming from the corotation region can remain unsaturated due to the stresses in the disk. These stresses are generated directly by the magnetic field. The magnitude of the horseshoe drag can overcome the negative Lindblad contribution when the local surface density profile is flat or increasing outward, which we see in certain locations in our simulations due to the presence of a zonal flow. The intermediate-mass planet is migrating radially outward in locations where there is a positive gradient of a pressure bump (zonal flow).