First detection of doubly deuterated hydrogen sulfide

This work was carried out using the Caltech Submillimeter Observatory and presents the observational study of HDS and D2S toward a sample of Class 0 sources and dense cores. We report the first detection of doubly deuterated hydrogen sulfide (D2S) in two dense cores and analyze the chemistry of these molecules, aiming to help understand the deuteration processes in the interstellar medium. The observed values of the D2S/HDS ratio and upper limits require an atomic D/H ratio in the accreting gas of 0.1-1. The study presented in this Letter supports the hypothesis that formaldehyde, methanol, and hydrogen sulfide are formed on the grain surfaces, during the cold prestellar core phase, where the CO-depleted gas has large atomic D/H ratios. The high values for the D/H ratios are consistent with the predictions of a recent gas-phase chemical model that includes H-3(+) and its deuterated isotopomers, H2D+, D2H+, and D-3(+) (Roberts, Herbst, Millar).