TRACKING WATER, O2 AND ICE IN MOLECULAR CLOUDS: PDRS MODELS WITH PHOTODESORPTION AND GRAIN CHEMISTRY

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 O-2, peak in abundance at A(V) similar to few into the cloud. Most of the gas-phase column, and most of the emission. from these species arises in die peak. Closer to the surface, H2O and O-2 are photodissociated. while deeper into the cloud, they freeze out onto grain surfaces. The result is H2O and O-2 column densities that are nearly constant for a wide range of gas densities, n, and for FUV fields G(0) less than or similar to 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 O-2