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Published online by Cambridge University Press: 25 November 2011
This paper reviews the first results of observations of H2O line emission withHerschel-HIFI towards high-mass star-forming regions, obtained within the WISH guaranteedtime program. The data reveal three kinds of gas-phase H2O: “cloud water” incold 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 isdue to rapid photodissociation and freeze-out on dust grains, respectively. The outflowsshow higher H2O abundances (10-7−10-6) due to grainmantle evaporation and (probably) neutral-neutral reactions.