Paper: astro-ph/0510635
Date: Fri, 21 Oct 2005 05:25:23 GMT (31kb)

Title: Gas phase water in the surface layer of protoplanetary disks

Authors: C. Dominik, C. Ceccarelli, D. Hollenbach, M.Kaufman

Comments: 13 pages, 4 figures. Astrophysical Journal Letters, in press
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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.

\\ ( http://arXiv.org/abs/astro-ph/0510635 , 31kb)