The Abundance, Ortho/Para Ratio, and Deuteration of Water in the High-Mass Star Forming Region NGC 6334 I

TL;DR

This study uses Herschel/HIFI data to analyze water isotopologues in NGC 6334 I, revealing complex line profiles, water abundance variations, and isotopic ratios that inform star formation chemistry.

Contribution

First detailed analysis of water isotopologues and ratios in NGC 6334 I, highlighting abundance variations and ortho/para ratios in different components.

Findings

Ortho/para ratio of 3 +/- 0.5 in foreground clouds and outflow.

Water abundance varies from ~10^{-8} to ~10^{-6} across regions.

HDO/H2O ratio around 2 x 10^{-4} in NGC 6334 I.

Abstract

We present Herschel/HIFI observations of 30 transitions of water isotopologues toward the high-mass star forming region NGC 6334 I. The line profiles of H_2^{16}O, H_2^{17}O, H_2^{18}O, and HDO show a complex pattern of emission and absorption components associated with the embedded hot cores, a lower-density envelope, two outflow components, and several foreground clouds, some associated with the NGC 6334 complex, others seen in projection against the strong continuum background of the source. Our analysis reveals an H2O ortho/para ratio of 3 +/- 0.5 in the foreground clouds, as well as the outflow. The water abundance varies from ~10^{-8} in the foreground clouds and the outer envelope to ~10^{-6} in the hot core. The hot core abundance is two orders of magnitude below the chemical model predictions for dense, warm gas, but within the range of values found in other Herschel/HIFI studies of hot cores and hot corinos. This may be related to the relatively low gas and dust temperature (~100 K), or time dependent effects, resulting in a significant fraction of water molecules still locked up in dust grain mantles. The HDO/H_2O ratio in NGC 6334 I, ~2 10^{-4}, is also relatively low, but within the range found in other high-mass star forming regions.