Collisional excitation of doubly and triply deuterated ammonia ND2H and ND3 by H2

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 ofmultiply deuterated ammonia, namely the ND2H and ND3 isotopologues. These calculations are based on the most accurate NH3-H2 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 NH2D, ND2H and ND3 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 NH2D and NHD2 are found to be consistent with the statistical ratios.