Deuterated water abundance in the young hot core RCW 120 S2

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.