Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-10-21
Astrophys.J.635:L85-L88,2005
Astronomy and Astrophysics
Astrophysics
13 pages, 4 figures. Astrophysical Journal Letters, in press
Scientific paper
10.1086/498942
Recent observations of the ground state transition of HDO at 464 GHz towards 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 2005). In the absence of non-thermal 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 photo-desorbed by FUV (6eV < h nu < 13.6eV) 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 photo-desorption yield of 10^{-3}, the water abundance in this layer is predicted to be ~ 3 x 10^{-7} and the average H2O column density is ~ 1.6x 10^{15} cm^{-2}. The predictions are very weakly dependent on the details of the model, like the incident FUV radiation field, and the gas density in the disk. Based on this model, we predict a gaseous HDO/H2O 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 HSO-HIFI.
Ceccarelli Cecilia
Dominik Carsten
Hollenbach David
Kaufman Maike
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