Collisional excitation of doubly and triply deuterated ammonia ND${}_2$H and ND${}_3$ by H${}_2$

Abstract

The availability of collisional rate coefficients is a prerequisite for an accurate interpretation of astrophysical observations, since the observed media often harbour densities where molecules are populated under non--LTE conditions. In the current study, we present calculations of rate coefficients suitable to describe the various spin isomers of multiply deuterated ammonia, namely the ND${}_2$H and ND${}_3$ isotopologues. These calculations are based on the most accurate NH${}_3$--H${}_2$ potential energy surface available, which has been modified to describe the geometrical changes induced by the nuclear substitutions. The dynamical calculations are performed within the close--coupling formalism and are carried out in order to provide rate coefficients up to a temperature of $T$ = 50K. For the various isotopologues/symmetries, we provide rate coefficients for the energy levels below $\sim$ 100 cm${}^{-1}$. Subsequently, these new rate coefficients are used in astrophysical models aimed at reproducing the NH${}_2$D, ND${}_2$H and ND${}_3$ observations previously reported towards the prestellar cores B1b and 16293E. We thus update the estimates of the corresponding column densities and find a reasonable agreement with the previous models. In particular, the ortho--to--para ratios of NH${}_2$D and NHD${}_2$ are found to be consistent with the statistical ratios.

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Cited by in corpus (7)

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• Collisional excitation of NH3 by atomic and molecular hydrogen
• Multipole-moment effects in ion-molecule reactions at low temperatures: part I -- Ion-dipole enhancement of the rate coefficients of the He${}^{+}$ + NH${}_3$ and He${}^{+}$ + ND${}_3$ reactions at collision energies near $0$ K
• Kinematics of a young low-mass star forming core: Understanding the evolutionary state of the First Core Candidate L1451-mm
• Star Formation and Feedback: A Molecular Outflow-Prestellar Core Interaction in L1689N
• Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NH${}_2$D chemistry in high-mass precluster and protocluster clumps
• Cl+ and HCl+ in Reaction with H2 and Isotopologues: A Glance into H Abstraction and Indirect Exchange at Astrophysical Conditions