2005A&A...438..585R

We use spectra and maps of NH₂D, ND₂H, and ND₃, obtained with the CSO, IRAM 30 m and Arecibo telescopes, to study deuteration processes in dense cores. The data include the first detection of the hyperfine structure in ND₂H. The emission of NH₂D and ND₃ does not seem to peak at the positions of the embedded protostars, but instead at offset positions, where outflow interactions may occur. A constant ammonia fractionation ratio in star-forming regions is generally assumed to be consistent with an origin on dust grains. However, in the pre-stellar cores studied here, the fractionation varies significantly when going from NH₃ to ND₃. We present a steady state model of the gas-phase chemistry for these sources, which includes passive depletion onto dust grains and multiply saturated deuterated species up to five deuterium atoms (e.g. CD₅⁺). The observed column density ratios of all four ammonia isotopologues are reproduced within a factor of 3 for a gas temperature of 10K. We also predict that deuterium fractionation remains significant at temperatures up to about 20K. ND and NHD, which have rotational transitions in the submillimeter domain are predicted to be abundant.