Herschel-HIFI observations of H2O in high-mass star-forming regions: first results

This paper reviews the first results of observations of H2O line emission with Herschel-HIFI towards high-mass star-forming regions, obtained within the WISH guaranteed time program. The data reveal three kinds of gas-phase H2O: cloud water' in cold tenuous foreground clouds, envelope water’ in dense protostellar envelopes, and `outflow water’ in protostellar outflows. The low H2O abundance (1e-10 – 1e-9) in foreground clouds and protostellar envelopes is due to rapid photodissociation and freeze-out on dust grains, respectively. The outflows show higher H2O abundances (1e-7 – 1e-6) due to grain mantle evaporation and (probably) neutral-neutral reactions.