Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI
de Graauw, Th.; Liseau, R.; Braine, J.; Codella, C.; Dominik, C.; Bontemps, S.; Benedettini, M.; van Dishoeck, E. F.; Bachiller, R.; Larsson, B.; Blake, G. A.; Caux, E.; Cernicharo, J.; Daniel, F.; Encrenaz, P.; Goicoechea, J. R.; Johnstone, D.; Pearson, J. C.; Plume, R.; Stutzki, J.; van der Tak, F. F. S.; Olberg, M.; Melnick, G.; Risacher, C.; Fuente, A.; Benz, A. O.; Bruderer, S.; Wyrowski, F.; Shipman, R.; Dieleman, P.; Fich, M.; Herpin, F.; Jellema, W.; Kester, D.; Laauwen, W.; Luinge, W.; McCoey, C.; Bjerkeli, P.; Nisini, B.; Caselli, P.; Lis, D.; Kristensen, L. E.; Hogerheijde, M. R.; Tafalla, M.; Giannini, T.; Neufeld, D.; Saraceno, P.; Herczeg, G. J.; Wampfler, S. F.; Doty, S. D.; Megej, A.; Baudry, A.; Bergin, E.; di Giorgio, A. M.; Helmich, F.; Jacq, T.; Jørgensen, J. K.; Parise, B.; Santiago-García, J.; van Kempen, T. A.; Visser, R.; Yıldız, U. A.; Frieswijk, W.; Roelfsema, P.; Gaier, T.; Chavarría, L.; Marseille, M. G.; Pietropaoli, B.; Jackson, B.; Javadi, H.; Siegel, P.
Netherlands, France, Germany, Spain, Italy, Switzerland, United States, Sweden, United Kingdom, Canada, Denmark
Abstract
Aims: We derive the dense core structure and the water abundance in four massive star-forming regions in the hope of understanding the earliest stages of massive star formation.
Methods: We present Herschel/HIFI observations of the para-H2O 111-000 and 202-111 and the para-H_218O 111-000 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modeled with Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), and the water abundance and the turbulent velocity width as free parameters.
Results: While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5×10-10 to 4×10-8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel/HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight.
Conclusions: The low H2O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.