Herschel-HIFI observations of H

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 (10-10−10-9) in foreground clouds and protostellar envelopes is due to rapid photodissociation and freeze-out on dust grains, respectively. The outflows show higher H2O abundances (10-7−10-6) due to grain mantle evaporation and (probably) neutral-neutral reactions