  This work was carried out with using the Caltech Submillimeter Observatory
and presents the observational study of HDS and D2S towards 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
pre-stellar 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 H3+ and its deuterated
isotopomers, H2D+, D2H+ and D3+ (Roberts et al. 2003).
