Letter to the Editor

Triply deuterated ammonia in NGC 1333

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany
³ California Institute of Technology, Downs Laboratory of Physics 320-47, Pasadena, CA 91125, USA
⁴ Lab. de Radioastronomie Millimétrique, Dépt. de Physique de l'E.N.S., 24 rue Lhomond, 75231 Paris, France
⁵ DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France

Corresponding author: F. F. S. van der Tak, vdtak@mpifr-bonn.mpg.de

Received: 1 April 2002
Accepted: 24 April 2002

Abstract

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND₃, through its 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  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 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 . 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.