THE ROLE OF TINY GRAINS ON THE ACCRETION PROCESS IN PROTOPLANETARY DISKS

Tiny grains such as polycyclic aromatic hydrocarbons (PAHs) have been thought to dramatically reduce the coupling between the gas and magnetic fields in weakly ionized gas such as in protoplanetary disks (PPDs) because they provide a 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 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) over bar /n(e) > 1,where n(e) and (n) over 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 by electrons when (n) over bar /n(e) exceeds about 10(3), and Hall and ambipolar diffusion (AD) coefficients are reduced by a factor of ((n) over bar /n(e))(2) in the AD-dominated regime relative to that in the Ohmic regime. Applying the methodology of Bai, we find that in PPDs, when PAHs are sufficiently abundant (greater than or similar to 10(-9) per H-2 molecule), 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) over 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) over dot. with solar-abundance 0.1 mu m grains. At r > r(trans), we find that, remarkably, the predicted M. exceeds the grain-free case due to a net reduction of AD by charged tiny grains and reaches a few times 10(-8) M-circle dot yr(-1). This is sufficient to account for the observed (M) over dot in transitional disks. Larger grains (greater than or similar to 0.1 mu m) are too massive to reach such high abundance as tiny grains and to facilitate the accretion process.