Deuterium fractionation in dense ammonia cores

Abstract

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 CO and CO 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.