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