Shielding by water and OH in FUV and X-ray irradiated protoplanetary disks

We present an integrated thermal-chemical model for the atmosphere of the inner region of a protoplanetary disk that includes irradiation by both far-ultraviolet (FUV) and X-ray radiation. We focus on how the photodissociation of H₂O and OH affects the abundances of these and related species and how it contributes to the heating of the atmosphere. The dust in the atmosphere plays several important roles, primarily as the site of H₂ formation and by absorbing the FUV. Large amounts of water can be synthesized within the inner 4 AU of a disk around a typical classical T Tauri star. OH is found primarily at the top of a warm region where the gas temperature is T _g ≈ 650-1000 K and H₂O is found below it, where the temperature is lower, T _g ≈ 250-650 K. The amounts of H₂O and OH and the temperatures of the regions in which they formed are in agreement with recent Spitzer measurements and support the notion of the in situ production of water in the inner regions of protoplanetary disks. We find that the synthesized water is effective in shielding the disk mid-plane from stellar FUV radiation.