2000A&A...356.1039T

We report observations of several deuterated species in the dark clouds L134N and TMC1, and in particular of NH₂D (deuterated ammonia). NH₂D has been detected for the first time towards the dense core TMC1-N and very strong emission has been confirmed towards L134N. The deuterium fractionation is very different in these two clouds, with abundance ratio [NH₂D]/[NH₃]∼0.1 and ∼0.02, [N₂D⁺]/[N₂H⁺]∼0.35 and ∼0.08 and [DCO⁺]/[HCO⁺]∼0.18 and ∼0.02, towards L134N and TMC1-N, respectively. For both clouds, but most clearly for L134N, the region of high deuterium fractionation is more localized and much more compact than the high column density region traced by C¹⁸O and C¹⁷O emission. We compare the observed deuterium fractionation with steady-state chemical models and we find that a better match is obtained when C and O are heavily depleted from the gas phase.