Gas-Phase Water in the Surface Layer of Protoplanetary Disks

Recent observations of the ground-state transition of HDO at 464 GHz toward the protoplanetary disk of DM Tau have detected the presence of water vapor in the regions just above the outer disk midplane (Ceccarelli et al.). In the absence of nonthermal desorption processes, water should be almost entirely frozen onto the grain mantles, and HDO undetectable. In this Letter we present a chemical model that explores the possibility that the icy mantles are photodesorbed by FUV (6 eV ≤ h ν ≤ 13.6 eV) photons. We show that the average interstellar FUV field is enough to create a layer of water vapor above the disk midplane over the entire disk. Assuming a photodesorption yield of 10 -3 , the water abundance in this layer is predicted to be ~3 × 10 -7 , and the average H 2 O column density is ~1.6 × 10 15 cm -2 . The predictions are very weakly dependent on the details of the model, like the incident FUV radiation field, and on the gas density in the disk. Based on this model, we predict a gaseous HDO/H 2 O ratio in DM Tau of ~1%. In addition, we predict the ground-state transition of water at 557 GHz to be undetectable with Odin and/or with the Herschel Space Observatory Heterodyne Instrument for the Far Infrared (HIFI).