Turbulence in the Outer Regions of Protoplanetary Disks. II. Strong Accretion Driven by a Vertical Magnetic Field
Abstract
We carry out a series of local, vertically stratified shearing box simulations of protoplanetary disks that include ambipolar diffusion and a net vertical magnetic field. The ambipolar diffusion profiles we employ correspond to 30 AU and 100 AU in a minimum mass solar nebula (MMSN) disk model, which consists of a far-ultraviolet-ionized surface layer and low-ionization disk interior. These simulations serve as a follow-up to Simon et al., in which we found that without a net vertical field, the turbulent stresses that result from the magnetorotational instability (MRI) are too weak to account for observed accretion rates. The simulations in this work show a very strong dependence of the accretion stresses on the strength of the background vertical field; as the field strength increases, the stress amplitude increases. For a net vertical field strength (quantified by β0, the ratio of gas to magnetic pressure at the disk mid-plane) of β0 = 104 and β0 = 105, we find accretion rates \dot{M} \sim 10^{-8}-10-7 M ⊙ yr-1. These accretion rates agree with observational constraints, suggesting a vertical magnetic field strength of ~60-200 μG and 10-30 μG at 30 AU and 100 AU, respectively, in a MMSN disk. Furthermore, the stress has a non-negligible component due to a magnetic wind. For sufficiently strong vertical field strengths, MRI turbulence is quenched, and the flow becomes largely laminar, with accretion proceeding through large-scale correlations in the radial and toroidal field components as well as through the magnetic wind. In all simulations, the presence of a low-ionization region near the disk mid-plane, which we call the ambipolar damping zone, results in reduced stresses there.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2013
- DOI:
- 10.1088/0004-637X/775/1/73
- arXiv:
- arXiv:1306.3222
- Bibcode:
- 2013ApJ...775...73S
- Keywords:
-
- accretion;
- accretion disks;
- magnetohydrodynamics: MHD;
- protoplanetary disks;
- turbulence;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- accepted to ApJ after very minor revisions