Detection of triply deuterated ammonia : the first triply deuterated molecule in the interstellar medium

For the first time, a molecule in which hydrogen is substituted by deuterium 3 times, has been discovered in the interstellar medium, by an international group including astronomers from Paris Observatory. The molecule is ammonia NH3, and its deuterated version ND3. Given that the relative abundance of deuterium relative to hydrogen is expected from the Big-Bang nucleosynthesis to be very low (D/H 2 10-5), the existence of such a molecule is a surprise. Already deuterated molecules, with one or two hydrogens substituted with deuterium, have been observed in the interstellar medium with a much enhanced abundance, due to an enrichment process called fractionation. But a triply deuterated molecule requires an extreme enrichment. ND3 has been detected through its first rotational transition at a frequency near 310 GHz, in two interstellar sources, the Barnard 1 cloud and the NGC 1333 star forming cloud. Towards both sources the deuterium fractionation is very high, and the abundance ratio [NH3]/[ND3] is approximatively 1000. Preliminary chemical models indicate that the deuterium fractionation of ammonia is achieved mainly in the gas phase through ion-molecule reactions. A better agreement is obtained when dissociative recombinations of partially deuterated ions result in a somewhat higher probability for the ejection of hydrogen atoms rather than deuterium.

Why deuterated molecules

Deuterium is a relative rare isotope of hydrogen, with an abundance ratio fairly well determined in the interstellar medium of D/H = 1.5 10-5. A rich and active chemistry is present in cold dense molecular clouds, where the temperature is only 10 K and the density 10 000 molecules per cubic centimeter. The main molecule is molecular hydrogen, but many other molecules have been detected (cf a list of interstellar molecules). These molecules are important tracers of the physical conditions and evolution of the dense interstellar medium. Indeed, molecules are the best probes of the very cold phases of molecular clouds prior to star formation and in the very early evolution of newly formed stars. Despite the low deuterium abundance, most deuterated counterparts of hydrogenated interstellar molecules are now observed, such as DCN for HCN. The abundance ratio of the deuterated to hydrogenated molecule (for example [DCN]/[HCN]) is always much larger than for the atoms, with typical values of several percents. This very large fractionation is understood as the result of ion molecule reactions which favour the substitution of an hydrogen atom by a deuterium at very low temperature. For example, at temperature below 50 K, the reaction

H3+ + HD H2D+ + H2 Researchers from the Paris Observatory participate actively in the field of astrochemistry, and are particularly interested in developing chemical models of interstellar clouds (E. Roueff and co-workers - LUTH) and performing observations (M. Gerin et al. - LERMA). Ammonia is among the first molecules detected in the interstellar medium. It is now established that ammonia is present in dense cores, either before star formation or just after the formation of new stars. Surveys of deuterated ammonia NH2D have been performed recently, establishing that the fractionation is quite high in a large variety of sources (Tiné et al. 2000, Saito et al. 2000, Shah and Wootten 2001). Doubly deuterated ammonia has been subsequently detected in two cold dense cores by the same team (Roueff et al., 2000) and other French researchers (Loinard et al. 2001). The detection of triply deuterated ammonia was the next challenge. N1333_nd3.jpg B1_nd3.jpg Figure 1. Spectra of ND3, the triply deuterated ammonia, towards the NGC 1333 molecular cloud (left), and towards Barnard 1 (right). Click on the figures to enlarge.