JWST detections of amorphous and crystalline HDO ice toward massive protostars
Authors: Slavicinska Katerina , van Dishoeck Ewine F. , Tychoniec Łukasz , Nazari Pooneh , Rubinstein Adam E. , Gutermuth Robert , Tyagi Himanshu , Chen Yuan , Brunken Nashanty G. C. , Rocha Will R. M. , Manoj P. , Narang Mayank , Thomas Megeath S. , Yang Yao-Lun , Looney Leslie W. , Tobin John J. , Beuther Henrik , Bourke Tyler L. , Linnartz Harold , Federman Samuel , Watson Dan M. , Linz Hendrik 
Published: May 2024

Context. Tracing the origin and evolution of interstellar water is key to understanding many of the physical and chemical processes involved in star and planet formation. Deuterium fractionation offers a window into the physicochemical history of water due to its sensitivity to local conditions.

Aims. The aim of this work is to utilize the increased sensitivity and resolution of the James Webb Space Telescope (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.

Methods. 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 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. New laboratory IR spectra of ice mixtures containing HDO, H2O, CH3OH, and CO were collected to aid in the fitting and chemical interpretation of the observed spectra.

Results. HDO ice is detected above the 3σ level in both sources. It requires a minimum of two components, one amorphous and one crystalline, to obtain satisfactory fits. The H2O ice band at 3 µm similarly requires both amorphous and crystalline components. The observed peak positions of the crystalline HDO component are consistent with those of annealed laboratory ices, which could be evidence of heating and subsequent recooling of the ice envelope (i.e., thermal cycling). The CH3OH 3.53 µm band is fit best with two cold components, one consisting of pure CH3OH and the other of CH3OH in an H2O-rich mixture. 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.

Conclusions. 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 and 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.