Other
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21931401b&link_type=abstract
American Astronomical Society, AAS Meeting #219, #314.01
Other
Scientific paper
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 show that the MRI becomes very inefficient at the inner disk with optimistically predicted accretion rate at least one order of magnitude too small compared with typical observed accretion rates, while angular momentum transport by magnetized wind is likely to be a favorable alternative. 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. This work is supported by NASA headquarters under NASA Earth and Space Science Fellowship awarded to XNB.
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