Deuterated water abundance in the young hot core RCW 120 S2

TL;DR

This study measures the HDO abundance in the hot core RCW 120 S2 using submillimeter observations, revealing low deuterated water levels indicative of early protostellar heating stages.

Contribution

First detection of HDO lines in RCW 120 S2, providing key data on water deuteration in an early-stage hot core.

Findings

HDO lines detected with excitation temperature ~286 K

HDO abundance is among the lowest in hot cores

Protostellar heating in RCW 120 S2 is in early stages

Abstract

Since the emission of water molecules cannot be observed from Earth, less abundant isotopologues, such as H$_2^{18}$O and HDO, are used to trace water in star-forming regions. The main aim of this study is to determine HDO abundance in the hot core RCW 120 S2. We performed observations of the hot core in the 200-255~GHz range using the nFLASH230 receiver on the APEX telescope. Two HDO lines were detected toward RCW 120 S2. Their intensities are described by excitation temperature $286\pm2$~K and gas number density $\geq 10^9$~cm$^{-3}$. The emission originates from the hot core rather than the warm dense envelope surrounding a central young stellar object. The HDO column density ranges from $(3.9-7.9)\times10^{13}$~cm$^{-3}$ with a most probable value $5.6\times10^{13}$~cm$^{-3}$. The HDO abundance relative to hydrogen nuclei is $1.5\times10^{-9}$. This HDO abundance value is among the lowest reported for hot cores. Combined with the non-detection of the H$_2^{18}$O line, we conclude that protostellar heating in RCW 120 S2 is still in its early stages.