Deuterated Species in Star-Forming Regions

Abstract

A number of multiply deuterated molecular species have been recently detected in the ISM [1]–[6] with abundances far exceeding earlier theoretical expectations. Detailed formation mechanisms for these species have not yet been satisfactorily established, but recent computations of Roberts et al. [7] show that the inclusion of HD+2 and D+3 in the gas-phase chemical network significantly enhances the fractionation of ionic and neutral species in regions of high density and heavy depletion. These models also predict the very high atomic D/H ratios necessary for grain-surface chemistry to reproduce the high formaldehyde and methanol fractionation seen in star forming regions. We argue that slow C-type shocks associated with molecular outflows may produce conditions favorable for deuterium fractionation reactions in star forming regions by (i) increasing the gas volume density and thus shortening the depletion and gas-phase reaction time-scales or (ii) evap orating grain mantle ices. In the case of triply deuterated ammonia, an extensive observational search carried out at the CSO has resulted in five detections, including only two strong detections (Barnard 1 [3] and L1689N [8]). As discussed below, in both these regions there are indications of shock interactions between molecular outflows and the ambient gas.