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

Surface Layer Accretion in Transitional and Conventional Disks: From Polycyclic Aromatic Hydrocarbons to Planets