The distribution of water in the high-mass star-forming region NGC 6334I

TL;DR

This study uses Herschel/HIFI observations of water isotopologues in NGC 6334 I to analyze physical conditions, water distribution, and chemical processes in different gas components of this high-mass star-forming region.

Contribution

It provides detailed insights into water abundance, excitation, and isotopic ratios across different gas components in NGC 6334 I, revealing the effects of freeze-out and temperature on water chemistry.

Findings

Water abundance varies from 4×10^-5 to 10^-8 across components.

Hot core temperature is approximately 200 K.

Ortho/para water ratio is about 1.6 in cold gas and 2.5 in outflow.

Abstract

We present observations of twelve rotational transitions of H2O-16, H2O-18, and H2O-17 toward the massive star-forming region NGC 6334 I, carried out with Herschel/HIFI as part of the guaranteed time key program Chemical HErschel Surveys of Star forming regions (CHESS). We analyze these observations to obtain insights into physical processes in this region. We identify three main gas components (hot core, cold foreground, and outflow) in NGC 6334 I and derive the physical conditions in these components. The hot core, identified by the emission in highly excited lines, shows a high excitation temperature of 200 K, whereas water in the foreground component is predominantly in the ortho- and para- ground states. The abundance of water varies between 4 10^-5 (outflow) and 10^-8 (cold foreground gas). This variation is most likely due to the freeze-out of water molecules onto dust grains. The H2O-18/H2O-17 abundance ratio is 3.2, which is consistent with the O-18/O-17 ratio determined from CO isotopologues. The ortho/para ratio in water appears to be relatively low 1.6(1) in the cold, quiescent gas, but close to the equilibrium value of three in the warmer outflow material (2.5(0.8)).