Water abundance variations around high-mass protostars: Herschel-HIFI
  observations of the DR21 region
  Water is a key molecule in the star formation process, but its spatial
distribution in star-forming regions is not well known. We study the
distribution of dust continuum and H2O and 13CO line emission in DR21, a
luminous star-forming region with a powerful outflow and a compact HII region.
Herschel-HIFI spectra near 1100 GHz show narrow 13CO 10-9 emission and H2O
1(11)-0(00) absorption from the dense core and broad emission from the outflow
in both lines. The H2O line also shows absorption by a foreground cloud known
from ground-based observations of low-J CO lines. The dust continuum emission
is extended over 36" FWHM, while the 13CO and H2O lines are confined to ~24" or
less. The foreground absorption appears to peak further North than the other
components. Radiative transfer models indicate very low abundances of ~2e-10
for H2O and ~8e-7 for 13CO in the dense core, and higher H2O abundances of
~4e-9 in the foreground cloud and ~7e-7 in the outflow. The high H2O abundance
in the warm outflow is probably due to the evaporation of water-rich icy grain
mantles, while the H2O abundance is kept down by freeze-out in the dense core
and by photodissociation in the foreground cloud.
