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

TL;DR

This study uses Herschel/HIFI observations to analyze water line profiles in massive protostars, revealing details about gas dynamics, turbulence, and chemical abundances, and how these relate to star formation stages.

Contribution

It provides the first detailed modeling of water lines in multiple massive protostars, linking gas dynamics and chemical abundances to evolutionary status.

Findings

Regions of massive star formation are highly turbulent.

Inner water abundance varies significantly among sources.

Higher infall/expansion velocities correlate with increased inner water abundance.

Abstract

We present Herschel/HIFI observations (WISH KP) of 14 water lines in a small sample of galactic massive protostellar objects: NGC6334I(N), DR21(OH), IRAS16272-4837, and IRAS05358+3543. We analyze the gas dynamics from the line profiles. Through modeling of the observations using RATRAN, we estimate outflow, infall, turbulent velocities, molecular abundances, and investigate any correlation with the evolutionary status of each source. The molecular line profiles exhibit a broad component coming from the shocks along the cavity walls associated with the protostars, and an infalling (or expansion for IRAS05358+3543) and passively heated envelope component, with highly supersonic turbulence likely increasing with the distance from the center. Accretion rates between 6.3 10^{-5} and 5.6 10^{-4} \msun 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 H2 depending on the source. We confirm that regions of massive star formation are highly turbulent and that the turbulence likely 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/expansion velocity in the protostellar envelope, the higher is the inner abundance, maybe indicating that larger infall/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.