Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI

Abstract

We derive the dense core structure and the water abundance in four massive star-forming regions which may help understand the earliest stages of massive star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00 and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modelled using Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with the water abundance and the turbulent velocity width as free parameters. 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 5E-10 to 4E-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. 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.

arXiv comment8 pages, 5 figures, accepted for publication the 15/07/2010 by Astronomy&Astrophysics as a letter in the Herschel-HIFI special issue

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• A theoretical study of the dissociative recombination of SH${}^{+}$ with electrons through the ${}^2\Pi$ states of SH
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• The physical conditions in IRDC clumps from Herschel HIFI observations of H2O
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• Observations of water with Herschel/HIFI toward the high-mass protostar AFGL 2591
• Structure and kinematics of the clouds surrounding the Galactic mini-starburst W43 MM1
• Herschel-HIFI observations of H2O in high-mass star-forming regions: first results