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Description
The physical conditions during high-mass star formation are poorly understood. Outflow and infall motions have been detected around massive protostellar objects, but their dependence on mass, luminosity, and age is unclear. In addition, physical conditions and molecular abundances are often estimated using simple assumptions such as spherical shape and chemical homogeneity, which may limit the accuracy of the results. We aim to characterize the dust and gas distribution and kinematics of the envelopes of high-mass protostars. In particular, we search for infall motions, abundance variations, and deviations from spherical symmetry, using Herschel data from the WISH program. We used HIFI maps of the 987GHz H_2_O 2_02_-1_11_ emission to measure the sizes and shapes of 19 high-mass protostellar envelopes. To identify infall, we used HIFI spectra of the optically thin C^18^O 9-8 and H_2_^18^O 1_11_-0_00_ lines. The high-J C^18^O line traces the warm central material and redshifted H_2_^18^O 1_11_-0_00_ absorption indicates material falling onto the warm core. We probe small-scale chemical differentiation by comparing H_2_O 752 and 987GHz spectra with those of H_2_^18^O. Our measured radii of the central part of the H_2_O 2_02_-1_11_ emission are 30-40% larger than the predictions from spherical envelope models, and axis ratios are <2, which we consider good agreement. For 11 of the 19 sources, we find a significant redshift of the H_2_^18^O 1_11_-0_00_ line relative to C^18^O 9-8. The inferred infall velocities are 0.6-3.2km/s, and estimated mass inflow rates range from 7x10^-5^ to 2x10^-2^M_{sun}_/yr. The highest mass inflow rates seem to occur toward the sources with the highest masses, and possibly the youngest ages. The other sources show either expanding motions or H_2_^18^O lines in emission. The H_2_^18^O 1_11_-0_00_ line profiles are remarkably similar to the differences between the H_2_O 2_02_-1_11_ and 2_11_-2_02_ profiles, suggesting that the H_2_^18^O line and the H_2_O 2_02_-1_11_ absorption originate just inside the radius where water evaporates from grains, typically 1000-5000au from the center. In some sources, the H_2_^18^O line is detectable in the outflow, where no C^18^O emission is seen. Together, the H_2_^18^O absorption and C^18^O emission profiles show that the water abundance around high-mass protostars has at least three levels: low in the cool outer envelope, high within the 100K radius, and very high in the outflowing gas. Thus, despite the small regions, the combination of lines presented in this work reveals systematic inflows and chemical information about the outflows.
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