Surface Layer Accretion in Conventional and Transitional Disks Driven by
  Far-Ultraviolet Ionization
  Whether protoplanetary disks accrete at observationally significant rates by
the magnetorotational instability (MRI) depends on how well ionized they are.
Disk surface layers ionized by stellar X-rays are susceptible to charge
neutralization by small condensates, ranging from ~0.01-micron-sized grains to
angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in
X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI.
Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables
full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic
carbon and sulfur produces a plasma so dense that it is immune to ion
recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01--0.1
g/cm^2, behaves in the ideal magnetohydrodynamic limit and can accrete at
observationally significant rates at radii > 1--10 AU. Surface layer accretion
driven by FUV ionization can reproduce the trend of increasing accretion rate
with increasing hole size seen in transitional disks. At radii < 1--10 AU,
FUV-ionized surface layers cannot sustain the accretion rates generated at
larger distance, and unless turbulent mixing of plasma can thicken the
MRI-active layer, an additional means of transport is needed. In the case of
transitional disks, it could be provided by planets.
