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

¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: vdtak@sron.nl
² Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
³ Université de Bordeaux, Bordeaux, France
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 6 July 2018
Accepted: 18 March 2019

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.