JWST detections of amorphous and crystalline HDO ice toward massive protostars

This work aims to utilize the increased sensitivity and resolution of the JWST to quantify the HDO/H₂O ratio in ices toward young stellar objects (YSOs) and to determine if the HDO/H₂O ratios measured in the gas phase toward massive YSOs (MYSOs) are representative of the ratios in their ice envelopes. Two protostars observed in the Investigating Protostellar Accretion (IPA) program using JWST NIRSpec were analyzed: HOPS 370, an intermediate-mass YSO (IMYSO), and IRAS 20126+4104, a MYSO. The HDO ice toward these sources was detected above the 3σ level and quantified via its 4.1 μm band. The contributions from the CH₃OH combination modes to the observed optical depth in this spectral region were constrained via the CH₃OH 3.53 μm band to ensure that the integrated optical depth of the HDO feature was not overestimated. H₂O ice was quantified via its 3 μm band. From these fits, ice HDO/H₂O abundance ratios of 4.6±1.8×10⁻³ and 2.6±1.2×10⁻³ are obtained for HOPS 370 and IRAS 20126+4104, respectively. The simultaneous detections of both crystalline HDO and crystalline H₂O corroborate the assignment of the observed feature at 4.1 μm to HDO ice. The ice HDO/H₂O ratios are similar to the highest reported gas HDO/H₂O ratios measured toward MYSOs as well as the hot inner regions of isolated low-mass protostars, suggesting that at least some of the gas HDO/H₂O ratios measured toward massive hot cores are representative of the HDO/H₂O ratios in ices. The need for an H₂O-rich CH₃OH component in the CH₃OH ice analysis supports recent experimental and observational results that indicate that some CH₃OH ice may form prior to the CO freeze-out stage in H₂O-rich ice layers.