Detection of Triply Deuterated Ammonia in the Barnard 1 Cloud

We report the detection of the ground-state rotational transition J_K=1₀-->0₀ (0a-->0s) of triply deuterated ammonia at 309.91 GHz in the Barnard 1 cloud, obtained with the Caltech Submillimeter Observatory. The observed, integrated, line intensity of 0.307+/-0.019 K km s^-1 implies an ND₃ column density of (2+/-0.9)×10¹² cm^-2, for excitation temperatures in the range 5-10 K. Using previously published H₂ and NH₃ column density estimates in this source, we derive an ND₃ fractional abundance with respect to H₂ of (1.5+/-1)×10^-11 and an ND₃-to-NH₃ abundance ratio of ~8×10^-4. The observed abundance ratios can be explained in the framework of gas-phase chemical models, in which the dissociative recombination of partially deuterated ions results in a somewhat higher probability for the ejection of hydrogen atoms than deuterium.