2003ApJ...593L..97V

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⁺₃and its deuterated isotopomers, H₂D⁺, D₂H⁺, and D⁺₃(Roberts, Herbst, & Millar).