Multi-line Herschel/HIFI observations of water reveal infall motions and chemical segregation around high-mass protostars

Abstract

Context. 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. Aims. 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. Methods. We used HIFI maps of the 987 GHz H2O 202–111 emission to measure the sizes and shapes of 19 high-mass protostellar envelopes. To identify infall, we used HIFI spectra of the optically thin C18O 9–8 and H218O 111–000 lines. The high-J C18O line traces the warm central material and redshifted H218O 111–000 absorption indicates material falling onto the warm core. We probe small-scale chemical differentiation by comparing H2O 752 and 987 GHz spectra with those of H218O. Results. Our measured radii of the central part of the H2O 202–111 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 H218O 111–000 line relative to C18O 9–8. The inferred infall velocities are 0.6–3.2 km s−1, and estimated mass inflow rates range from 7 × 10−5 to 2 × 10−2 M⊙ yr−1. 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 H218O lines in emission. The H218O 111–000 line profiles are remarkably similar to the differences between the H2O 202–111 and 211–202 profiles, suggesting that the H218O line and the H2O 202–111 absorption originate just inside the radius where water evaporates from grains, typically 1000–5000 au from the center. In some sources, the H218O line is detectable in the outflow, where no C18O emission is seen. Conclusions. Together, the H218O absorption and C18O 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 100 K 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.