| Description |
We try to resolve the chemical evolution of oxygen hydrides in radiatively heated dense clouds. This involves in particular H2O+ and chemically related species, which are formed by gas-phase reactions initiated by cosmic ray ionization in diffuse clouds, but may be predominantly produced by the evaporation of icy grain mantles and the subsequent ionization of water by UV radiation in heated dense clouds. We will investigate the full chain of species OH+, H2O+, H3O+, OH, and H2O, connected by gas phase reactions, to quantify the contribution of ice evaporation in dense clouds. We propose to observe these species in the dense layers known to exist in two massive star-forming regions, DR21(C), where indications of H2O+ from hot gas were already found and where the hot layer is affected both by UV radiation and a strong shock from a bipolar outflow, and Mon R2, a PDR with similar parameters but no indications of shock processing of the hot layer. Differences between the results from the two sources will provide an estimate for the impact of shocks on the H2O+ production. By comparing the observed abundances of OH+, H2O+, H3O+, OH, and H2O with steady-state PDR models, we will be able to quantify the amount of water that is fed into the gas phase by the evaporation or photodesorption of ice mantles from dust grains. |
