Tracking Water, O₂ and Ice in Molecular Clouds: PDRs Models with Photodesorption and Grain Chemistry

Abstract

We have expanded our model of photodissociation regions (PDRs) to include the freezing of O- and C-bearing species on dust grains, simple grain surface chemistry, and desorption processes, including photodesorption, that may be important in the surface layers of diffuse, translucent, and dense molecular clouds. The main result of including these processes is that a number of important gas-phase species, including H₂O and O₂, peak in abundance at A_V ~ few into the cloud. Most of the gas-phase column, and most of the emission, from these species arises in the peak. Closer to the surface, H₂O and O₂ are photodissociated, while deeper into the cloud, they freeze out onto grain surfaces. The result is H₂O and O₂ column densities that are nearly constant for a wide range of gas densities, n, and for FUV fields G₀ $\lesssim$ 500. The roughly constant column densities of these species provides an explanation for the low line-of-sight average abundances of H₂O observed toward GMCs. The model results also suggest that regions of high FUV field are the best places to search for O₂.