Triply deuterated ammonia in NGC 1333

Abstract

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND_3, through its J_K = 1^a_0 → 0^s_0 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 ND_3 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 ND_3 abundance is 3.2 x 10^(-12) if ND_3/H_2 is constant throughout the envelope. In the more likely case that ND_3/H_2D^+ is constant, ND_3/H_2 peaks in the cold outer parts of the source at a value of 1.0 x 10^(-11). To reproduce the observed NH_3/ND_3 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 NH_3 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.

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