Monday, June 3, 2013

1305.7232 (Xue-Ning Bai)

Wind-driven Accretion in Protoplanetary Disks --- II: Radial Dependence and Global Picture    [PDF]

Xue-Ning Bai
Non-ideal magnetohydrodynamical effects play a crucial role in determining the mechanism and efficiency of angular momentum transport as well as the level of turbulence in protoplanetary disks (PPDs), which are key to understanding PPD evolution and planet formation. It was shown in our previous work that at 1 AU, the magnetorotational instability (MRI) is completely suppressed when both Ohmic resistivity and ambipolar diffusion (AD) are taken into account, resulting in a laminar flow with accretion driven by magnetocentrifugal wind. In this work, we study the radial dependence of the laminar wind solution using local shearing-box simulations. Scaling relation on the angular momentum transport for the laminar wind is obtained, and we find that the wind-driven accretion rate can be approximated as M_dot~0.91x10^(-8)R_AU^(1.21)(B_z/10mG)^(0.93)M_Sun/yr, where B_z is the strength of the large-scale vertical magnetic field threading the disk. The result is independent of disk surface density. Four criteria are outlined for the existence of the laminar wind solution: 1). Ohmic resistivity dominated midplane region; 2). AD dominated disk upper layer; 3). Presence of (not too weak) net vertical magnetic flux. 4). Sufficiently well ionized gas beyond disk surface. All these criteria are likely to be met in the inner region of the disk from ~0.3 AU to about 5-10 AU for typical PPD accretion rates. Beyond this radius, angular momentum transport is likely to proceed due to a combination of the MRI and disk wind, and eventually dominated by the MRI (in the presence of strong AD) in the outer disk. Our simulation results provide key ingredients for a new paradigm on the accretion processes in PPDs.
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