Institute for Advanced Study Informal Astrophysics Seminar

ABSTRACT: Magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI) has long been considered as the most promising mechanism for transporting angular momentum in accretion disks. In protoplanetary disks (PPDs), however, the gas dynamics is strongly affected by non-ideal MHD effects such as Ohmic resistivity, Hall effect and ambipolar diffusion (AD) due to its weak ionization level. Most MRI calculations for PPDs done so far consider only the Ohmic resistivity, while Hall and AD effects dominate the surface and outer regions of PPDs but remain poorly explored. We perform 3D unstratified shearing-box MRI simulations with AD using a variety of magnetic field geometries and AD coefficients. We find that angular momentum transport becomes inefficient when the neutral-ion collision frequency falls below the orbital frequency. Moreover, sustained MRI turbulence requires weak magnetic field in the AD dominated regime. We present a general framework that incorporate these constraints together to predict the MRI-driven accretion rate and the corresponding magnetic field strength in PPDs. Our results indicate that MRI alone has difficulty in accounting for the observed accretion rate in a large fraction of PPDs, while angular momentum transport by magnetized wind may be a viable solution. On the other hand, for transitional disks, characterized by inner gaps or holes representing a later stage of PPD evolution, we find that MRI is able to drive sufficiently rapid accretion consistent with observations, and the presence of tiny grains even promotes accretion.