Deuterated Water Abundance in the Young Hot Core RCW 120 S2

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 \({\approx}290\) 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 \((3.9{-}7.9)\times 10^{13}\) cm\({}^{-3}\) with the best-fit model value of \(5.6\times 10^{13}\) cm\({}^{-3}\). The HDO abundance relative to hydrogen is \(1.7\times 10^{-9}\). 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.