Layered Accretion in T Tauri Disks

We put forward a model for accretion disks around T Tauri stars. The model assumes that angular momentum transport is driven by magnetic fields and can occur only in those parts of the disk that are sufficiently ionized that the gas can couple to the magnetic field. These regions lie at R ≲ 0.1 AU, where collisional ionization is effective, and at R ≲ 0.1 AU in a layer of thickness ≍ 100 g cm-2 at the surface of the disk where cosmic-ray ionization is effective. The model predicts that the stellar accretion rate is about 10-8 Msun yr-1, independent of the rate of infall onto the disk. Matter that is not accreted onto the star accumulates in the inner few AU of the disk at a rate of about 10-3 Msun in 104 yr. Given this buildup it is unlikely that accretion is steady. The effective temperature profile is Te ∼ r-1/2 outside of 0.1 AU, which differs from the canonical Te ∼ r-3/4. We calculate the expected spectral energy distribution for the disk and show that this temperature profile produces an infrared excess. Finally, we discuss some of the leading uncertainties in the theory.