The physical conditions in IRDC clumps from Herschel HIFI observations of H2O

TL;DR

This study investigates the physical conditions and kinematic structures of massive star-forming cloud clumps using Herschel HIFI water line observations, revealing high densities, infall motions, and an evolutionary sequence among the clumps.

Contribution

First detailed analysis of water line profiles in IRDC clumps to determine their physical and kinematic states, establishing an evolutionary sequence based on molecular tracers.

Findings

Clumps have high gas densities (~10^7 cm^-3) and show infall signatures.

Most clumps exhibit protostellar outflows, indicating active star formation.

An evolutionary sequence is proposed from less to more evolved clumps.

Abstract

Context. The earliest phases of high-mass star formation are poorly understood. Aims. Our goal is to determine the physical conditions and kinematic structure of massive star-forming cloud clumps. Methods. We analyze H$_2$O 557 GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps . By comparison with ground-based C$^{17}$O, N$_2$H$^+$, CH$_3$OH and NH$_3$ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H$_2$O and N$_2$H$^+$. Results. The observed water lines are complex with emission and absorption components. The absorption is red shifted and consistent with a cold envelope, while the emission is interpreted as resulting from protostellar outflows. The gas density in the clumps is $\sim$ 10$^7$ cm$^{-3}$. The o-H2O outflow column density is 0.3 to 3.0 10$^{14}$ cm$^{-2}$, the o-H2O absorption column density is between 1.5 10$^{14}$ and 2.6 10$^{15}$ cm$^-2$ with cold o-H2O abundances between 1.5 10$^{-9}$ and 3.1 10$^{-8}$. Conclusions. All clumps have high gas densities ($\sim$ 10$^7$ cm$^{-3}$) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H$_2$O N$_2$H$^+$, NH$_3$ and CH$_3$OH. We find that G28-MM is the most evolved, followed by G11-MM then G28-NH3. The least evolved clump is G11-NH3 which shows no sign-posts of star-formation. G11-NH3 is a high-mass pre-stellar core.