Deuterium fractionation in dense ammonia cores

We report observations of several deuterated species in the dark clouds L13 4N 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 e mission has been confirmed towards L134N. The deuterium fractionation is ve ry different in these two clouds, with abundance ratio [NH2D]/[NH3]similar to 0.1 and similar to 0.02, [N2D+]/[N2H+]similar to 0.35 and similar to 0.0 8 and [DCO+]/[HCO+]similar to 0.18 and similar to 0.02, towards L134N and T MC1-N, respectively. For both clouds, but most clearly for L134N, the regio n of high deuterium fractionation is more localized and much more compact t han the high column density region traced by (CO)-O-18 and (CO)-O-17 emissi on. We compare the observed deuterium fractionation with steady-state chemi cal models and we find that a better match is obtained when C and O are hea vily depleted from the gas phase.