Herschel-HIFI view of mid-IR quiet massive protostellar objects

摘要

Aims. We present Herschel /HIFI observations of 14 water lines in a small sample of Galactic massive protostellar objects: NGC 6334I(N), DR21(OH), IRAS 16272-4837, and IRAS 05358+3543. Using water as a tracer of the structure and kinematics, we individually study each of these objects with the aim to estimate the amount of water around them, but to also to shed light on the high-mass star formation process. Methods. We analyzed the gas dynamics from the line profiles using Herschel -HIFI observations acquired as part of the WISH key-project of 14 far-IR water lines (H hbox{$_2^{16}$} O, H hbox{$_2^{17}$} O, H hbox{$_2^{18}$} O) and several other species. Then through modeling the observations using the RATRAN radiative transfer code, we estimated outflow, infall, turbulent velocities, and molecular abundances and investigated the correlation with the evolutionary status of each source. Results. The four sources (and the previously studied W43-MM1) have been ordered in terms of evolution based on their spectral energy distribution from youngest to older: 1) NGC 64334I(N); 2) W43-MM1; 3) DR21(OH); 4) IRAS?16272-4837; 5) IRAS?05358+3543. The molecular line profiles exhibit a broad component coming from the shocks along the cavity walls that is associated with the protostars, and an infalling (or expanding, for IRAS?05358+3543) and passively heated envelope component, with highly supersonic turbulence that probably increases with the distance from the center. Accretion rates between 6.3 × 10~(-5) and 5.6 × 10~(-4) M _(⊙) ?yr ~(-1) are derived from the infall observed in three of our sources. The outer water abundance is estimated to be at the typical value of a few 10~(-8) , while the inner abundance varies from 1.7 × 10~(-6) to 1.4 × 10~(-4) with respect to H _(2) depending on the source. Conclusions. We confirm that regions of massive star formation are highly turbulent and that the turbulence probably increases in the envelope with the distance to the star. The inner abundances are lower than the expected, 10~(-4) , perhaps because our observed lines do not probe deep enough into the inner envelope or because photodissociation through protostellar UV photons is more efficient than expected. We show that the higher the infall or expansion velocity in the protostellar envelope, the higher the inner abundance. This may indicate that higher infall or expansion velocities generate shocks that will sputter water from the ice mantles of dust grains in the inner region. High-velocity water must be formed in the gas phase from shocked material.