Detection of Triply Deuterated Ammonia in the Barnard 1 Cloud

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Abstract
We report the detection of the ground-state rotational transition JK = 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.