Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Abstract

Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH 2 D, NHD 2 , and ND 3 in a quiescent, dense cloud. Methods. Spectral lines of NH 3 , NH 2 D, NHD 2 , ND 3 , and N 2 D + were observed towards a dense, starless core in Ophiuchus with the APEX, GBT, and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia, and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH 2 D/NH 3 ∼ 0.4, NHD 2 /NH 2 D ∼ 0.2, and ND 3 /NHD 2 ∼ 0.06 are found in the core. The observed ortho/para ratios of NH 2 D and NHD 2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but predicts uniformly too low ortho/para-NH 2 D, and too large ortho/para-NHD 2 ratios. The longevity of N 2 H + and NH 3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N 2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.

Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

“…Observationally, both ratios are close to 3, within their uncertainties (Daniel et al 2016;Harju et al 2017). If hydrogenation and deuteration were determined by statistics only -as is presumably the case in ices -both these ratios would be exactly equal to 3.…”

“…These values are consistent, within observational uncertainties, with the ratios measured in H-MM1 and listed in Table 6. The chemical model of Harju et al (2017) predicts a similar ortho:para ratio for NH 2 D, but a significantly larger one (in the range 3−5) for NHD 2 . On the other hand, the observed ratios are also consistent with the statistical values of 3 and 2, respectively.…”

“…However, as freeze-out occurs least rapidly at the densities (lower than 10 5 cm −3 ) prevailing in the outer envelope, Figure 5. Column densities of ammonia and its deuterated forms, as predicted from our L-P collapse model (full lines) and from observations (hashed regions) of the I16293E, Barnard-B1, and H-MM1 pre-stellar cores (Daniel et al 2016;Harju et al 2017) (see Table 6). The initial parameters are those of Table 1.…”

“…Column densities in the core and in the envelope are evaluated as described in Section 2. The total column densities, N tot = N core + N env , are also plotted, as are the column densities deduced by Harju et al (2017) from their observations of the starless core Ophiuchus/H-MM1 by assuming a single excitation temperature and statistical ortho:para ratios (see Table 6). …”

“…Column densities of specific isomers of ammonia and its deuterated forms have been measured towards starless and pre-stellar cores (Daniel et al 2016;Harju et al 2017). Fig.…”