Water abundance variations around high-mass protostars: HIFI observations of the DR21 region
de Graauw, Th.; Liseau, R.; Braine, J.; Codella, C.; Dominik, C.; Hogerheijde, M.; Teyssier, D.; Bontemps, S.; Benedettini, M.; van Dishoeck, E. F.; Bachiller, R.; Larsson, B.; Cernicharo, J.; Daniel, F.; Encrenaz, P.; Johnstone, D.; Plume, R.; van der Tak, F. F. S.; Olberg, M.; Melnick, G.; Risacher, C.; Fuente, A.; Benz, A. O.; Bruderer, S.; Siebertz, O.; Wyrowski, F.; Shipman, R.; Fich, M.; Herpin, F.; McCoey, C.; Pearson, J.; Ravera, L.; Bjerkeli, P.; Yıldız, U.; Nisini, B.; Caselli, P.; Lis, D.; Tafalla, M.; Giannini, T.; Neufeld, D.; Saraceno, P.; Baudry, A.; Bergin, E.; di Giorgio, A. M.; Helmich, F.; Parise, B.; Visser, R.; Frieswijk, W.; Roelfsema, P.; van Kempen, T.; Herczeg, G.; Blake, G.; Santiago, J.; Wampfler, S.; Marseille, M. G.; Kristensen, L.; Doty, S.; Goicoechea, J.; Jørgensen, J.
Netherlands, France, Germany, United States, Switzerland, United Kingdom, Canada, Sweden, Spain, Italy, Chile, Denmark
Abstract
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.