2023A&A...679A.120R

Context. H₂S is predicted to form by hydrogenation of atomic sulphur on grains and is thought to be the main sulphur reservoir in interstellar ice, being therefore a key molecule to understanding sulphur chemistry in the star formation process and to solving the missing sulphur problem in molecular clouds and star-forming regions. The study of the H₂S deuterium fraction can be used to constrain its molecule formation pathways.
Aims. The aim of this work is to investigate for the first time the deuteration of H₂S in a large sample of starless cores.
Methods. We used observations of the GEMS IRAM 30 m Large Program and complementary IRAM 30 m telescope observations. We considered a sample of 19 starless cores located in the Taurus, Perseus, and Orion molecular clouds, detecting HDS in ten of these starless cores, and D₂S in five. The single and double H₂S deuterium fractions were analysed with regard to their relation with the cloud physical parameters, comparisons with values obtained for other interstellar sources, and comparisons with deuterium fractions in early-stage star-forming sources of abundant molecules: c-C₃H₂, H₂CS, H₂O, H₂CO, and CH₃OH.
Results. We obtain a range of X(HDS)/X(H₂S) ∼ 0.025-0.2 in the starless cores with HDS detections. The five starless cores with D₂S detections show values of X(D₂S)/X(HDS) ∼ 0.05-0.3. H₂S single deuteration shows an inverse relation with the cloud kinetic temperature, but no trend is found with molecular hydrogen density or visual extinction. H₂S deuteration values in starless cores are similar to those observed in Class 0, although this may be a consequence of an observational bias due to the limited spatial resolution. Comparison with c-C₃H₂, H₂CS, H₂O, H₂CO, and CH₃OH in other interstellar sources reveals a general trend of decreasing deuteration with increasing temperature, with lower values for Class I and massive star-forming sources. In starless cores and Class 0 objects, H₂CS and H₂CO present higher deuteration fractions than c-C₃H₂, H₂S, H₂O, and CH₃OH. H₂O shows single and double deuteration values one order of magnitude lower than those of H₂S and CH₃OH.
Conclusions. Differences between c-C₃H₂, H₂CS and H₂CO deuterium fractions and those of H₂S, H₂O, and CH₃OH are related to deuteration processes produced in gas or solid phases, respectively. We interpret the differences between H₂S and CH₃OH deuterations and that of H₂O as a consequence of differences in the formation routes in the solid phase, which can particularly be explained in terms of the different occurrence of the D-H and H-D substitution reactions in the ice, together with the chemical desorption processes. Further interferometric observations and laboratory experiments are needed to understand the deuteration processes.