Triply deuterated ammonia in NGC 1333

Abstract

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND3, through its JK = 1oa → 0os 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 × 10111 cm-2 for Tex = 10 K and twice as high for Tex = 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 × 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 1.0 × 10-11. To reproduce the observed NH3/NND3 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.