Magnetorotational Instability and the Formation of Meteorites, Planetesimals, and Planets
Magnetorotational instability (MRI) structures planet-forming disks at both large and small scales. I will discuss five aspects of this interaction. First, MRI can be maintained at realistically low values of the magnetic Prandtl number (the ratio of viscosity to resistivity). Direct numerical simulation demonstrates that the key issue for self-sustaining MRI is not the value of the magnetic Prandtl number, as has been claimed by previous workers, but rather that the magnetic Reynolds number remain above a critical value, one that is easily reached in astrophysical disks. Second, magnetized turbulence driven by MRI forms current sheets. When Ohmic heating in these structures starts ionizing alkali metals, a short circuit instability sets in that can raise temperatures to rock-melting values, perhaps offering an explanation for the formation of high-temperature minerals, including meteoritic chondrules, far out in protoplanetary disks. Third, MRI causes density perturbations that can stir up planetesimal orbits. However, relative velocities appear not to reach levels able to grind down planetesimals as has been suggested. Fourth, Rossby wave instabilities form vortices at the edge of dead zones in which ionization at the disk midplane drops too low for MRI to occur. Our global models include self-consistent MRI, and confirm earlier work using imposed viscosity variations that showed these vortices can grow large enough to drive planet formation in their centers. Finally, relaxing the assumption of radial isothermality allows planet migration to occur both inwards and outwards. Calculations including migration and turbulent perturbations show that planetoids tend to be forced towards equilibrium orbits where they collide to form massive planet cores, potentially large enough to start accreting gas well within disk lifetimes.