Triply deuterated ammonia in NGC 1333

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND3, through its J(K) = 1(0)(a) --> 0(0)(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 3-6 x 10(11) 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 3.2 x 10(-12) if ND3/H-2 is constant throughout the envelope. In the more likely case that ND3/H2D+ is constant, ND3/H-2 peaks in the cold outer parts of the source at a value of 1.0 x 10(-11). To reproduce the observed NH3/ND3 abundance ratio of similar to1000, grain surface chemistry requires an atomic D/H ratio of approximate to0.15 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.