Deuterated Molecules in Star-Forming Regions

Imaging of the spatial distribution of deuterated molecules in several nearby low-mass star forming regions, as traced by the DCO+ (3-2) emission, suggests that the deuterated material is often displaced with respect to the highest column density regions, as traced by submillimeter dust continuum emission and sometimes appears to be associated with the high-velocity CO (2-1) emission, tracing outflow activity. High deuteration levels in isolated prestellar cores can be explained by gas-phase or grain-surface chemistry in dense, cold regions, where abundant gas-phase species are frozen onto dust grains - a process that is now relatively well understood. We argue that an alternative mechanism may operate in dense gas in the vicinity of embedded young stellar objects, where slow C-type shocks associated with molecular outflows may produce conditions favorable for deuterium fractionation either by increasing the gas volume density in the post-shock gas and thus, shortening the depletion and gas-phase reaction time-scales, or by evaporating grain mantle ices.