Gas phase Elemental abundances in Molecular cloudS (GEMS)

Abstract

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.