| 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. |
