Collisional excitation of doubly and triply deuterated ammonia ND_2H 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-local thermodynamic equilibrium 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_2H 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 = 50 K. For the various isotopologues/symmetries, we provide rate coefficients for the energy levels below ∼100 cm^(−1). Subsequently, these new rate coefficients are used in astrophysical models aimed at reproducing the NH_2D, ND_2H and ND_3 observations previously reported towards the pre-stellar 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_2D and NHD_2 are found to be consistent with the statistical ratios.

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