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.
