Triply deuterated ammonia in NGC 1333

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND₃, through its J_K=1₀^a ->0₀^s transition near 310 GHz. Emission is found in the NGC 1333 region, both towards IRAS 4A and a position to the South-East where DCO⁺ peaks. In both cases, the hyperfine ratio indicates that the emission is optically thin. Column densities of ND₃ are 3-6x 10¹¹ cm^-2 for T_ex,= 10 K and twice as high for T_ex,= 5 K. Using a Monte Carlo radiative transfer code and a model of the structure of the IRAS source with temperature and density gradients, the estimated ND₃ abundance is 3.2x 10^-12 if ND₃/H₂ is constant throughout the envelope. In the more likely case that ND₃/H₂D⁺ is constant, ND₃/H₂ peaks in the cold outer parts of the source at a value of 1.0x 10^-11. To reproduce the observed NH₃/ND₃ abundance ratio of ~ 1000, grain surface chemistry requires an atomic D/H ratio of ~0.15 in the gas phase, >10 times higher than in recent chemical models. More likely, the deuteration of NH₃ occurs by ion-molecule reactions in the gas phase, in which case the data indicate that deuteron transfer reactions are much faster than proton transfers.