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The Role of Tiny Grains on the Accretion Process in Protoplanetary Disks
Tiny grains such as PAHs have been thought to dramatically reduce the
coupling between gas and magnetic fields in weakly ionized gas such as in
protoplanetary disks (PPDs) because they provide tremendous surface area to
recombine free electrons. The presence of tiny grains in PPDs thus raises the
question of whether the magnetorotational instability (MRI) is able to drive
rapid accretion to be consistent with observations. Charged tiny grains have
similar conduction properties as ions, whose presence leads to qualitatively
new behaviors in the conductivity tensor, characterized by n_bar/n_e>1, where
n_e and n_bar denote the number densities of free electrons and all other
charged species respectively. In particular, Ohmic conductivity becomes
dominated by charged grains rather than electrons when n_bar/n_e exceeds about
10^3, and Hall and ambipolar diffusion (AD) coefficients are reduced by a
factor of (n_bar/n_e)^2 in the AD dominated regime relative to that in the
Ohmic regime. Applying the methodology of Bai (2011), we find that in PPDs,
when PAHs are sufficiently abundant (>1e-9 per H_2), there exists a transition
radius r_trans of about 10-20 AU, beyond which the MRI active layer extends to
the disk midplane. At r<r_trans, the optimistically predicted MRI-driven
accretion rate M_dot is one to two orders of magnitude smaller than that in the
grain-free case, which is too small compared with the observed rates, but is in
general no smaller than the predicted M_dot with solar-abundance 0.1 micron
grains. At r>r_trans, we find that remarkably, the predicted M_dot exceeds the
grain-free case due to a net reduction of AD by charged tiny grains, and
reaches a few times 1e-8M_Sun/yr. This is sufficient to account for the
observed M_dot in transitional disks. Larger grains (>0.1 micron) are too
massive to reach such high abundance as tiny grains and to facilitate the
accretion process.Comment: 10 pages, 6 figures, accepted for publication in Ap
Wind-driven Accretion in Protoplanetary Disks. I: Suppression of the Magnetorotational Instability and Launching of the Magnetocentrifugal Wind
We perform local, vertically stratified shearing-box MHD simulations of
protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account
the effects of both Ohmic resistivity and ambipolar diffusion (AD). The
magnetic diffusion coefficients are evaluated self-consistently from a look-up
table based on equilibrium chemistry. We first show that the inclusion of AD
dramatically changes the conventional picture of layered accretion. Without net
vertical magnetic field, the system evolves into a toroidal field dominated
configuration with extremely weak turbulence in the far-UV ionization layer
that is far too inefficient to drive rapid accretion. In the presence of a weak
net vertical field (plasma beta~10^5 at midplane), we find that the MRI is
completely suppressed, resulting in a fully laminar flow throughout the
vertical extent of the disk. A strong magnetocentrifugal wind is launched that
efficiently carries away disk angular momentum and easily accounts for the
observed accretion rate in PPDs. Moreover, under a physical disk wind geometry,
all the accretion flow proceeds through a strong current layer with thickness
of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4
times the sound speed. Both Ohmic resistivity and AD are essential for the
suppression of the MRI and wind launching. The efficiency of wind transport
increases with increasing net vertical magnetic flux and the penetration depth
of the FUV ionization. Our laminar wind solution has important implications on
planet formation and global evolution of PPDs.Comment: 23 pages, 13 figures, accepted to Ap
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