Water abundance variations around high-mass protostars: HIFI observations of the DR21 region [Letter]
Context. Water is a key molecule in the star formation process, but its spatial distribution in star-forming regions is not well known. 
Aims. 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 H ii region. 
Methods. Herschel-HIFI spectra near 1100 GHz show narrow 13CO 10–9 emission and H2O 111–000 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. 
Results. 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 ~2×10-10 for H2O and ~8×10-7 for 13CO in the dense core, and higher H2O abundances of ~4×10-9 in the foreground cloud and ~7×10-7 in the outflow. 
Conclusions. 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.