  Transitional T Tauri disks have optically thin holes with radii > 10 AU, yet
accrete up to the median T Tauri rate. Multiple planets inside the hole can
torque the gas to high radial speeds over large distances, reducing the local
surface density while maintaining accretion. Thus multi-planet systems,
together with reductions in disk opacity due to grain growth, can explain how
holes can be simultaneously transparent and accreting. There remains the
problem of how outer disk gas diffuses into the hole. Here it has been proposed
that the magnetorotational instability (MRI) erodes disk surface layers ionized
by stellar X-rays. In contrast to previous work, we find that the extent to
which surface layers are MRI-active is limited not by ohmic dissipation but by
ambipolar diffusion, the latter measured by Am: the number of times a neutral
hydrogen molecule collides with ions in a dynamical time. Simulations by Hawley
& Stone showed that Am ~ 100 is necessary for ions to drive MRI turbulence in
neutral gas. We calculate that in X-ray-irradiated surface layers, Am typically
varies from ~0.001 to 1, depending on the abundance of charge-adsorbing
polycyclic aromatic hydrocarbons, whose properties we infer from Spitzer
observations. We conclude that ionization of H_2 by X-rays and cosmic rays can
sustain, at most, only weak MRI turbulence in surface layers 1 to 10 g/cm^2
thick, and that accretion rates in such layers are too small compared to
observed accretion rates for the majority of disks.
