The ortho-to-para ratio of interstellar NH₂: quasi-classical trajectory calculations and new simulations

¹ Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904, USA
e-mail: romane.legal@virginia.edu
² Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA
³ Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, Ministry of Education, College of Chemistry and Materials Science, Northwest University, 710127 Xi’an, PR China

Received: 13 June 2016
Accepted: 6 September 2016

Abstract

Based on recent Herschel results, the ortho-to-para ratio (OPR) of NH₂ has been measured towards the following high-mass star-forming regions: W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. The OPR at thermal equilibrium ranges from the statistical limit of three at high temperatures to infinity as the temperature tends toward zero, unlike the case of H₂. Depending on the position observed along the lines-of-sight, the OPR was found to lie either slightly below the high temperature limit of three (in the range 2.2–2.9) or above this limit (~3.5, ≳ 4.2, and ≳5.0). In low temperature interstellar gas, where the H₂ is para-enriched, our nearly pure gas-phase astrochemical models with nuclear-spin chemistry can account for anomalously low observed NH₂-OPR values. We have tentatively explained OPR values larger than three by assuming that spin thermalization of NH₂ can proceed at least partially by H-atom exchange collisions with atomic hydrogen, thus increasing the OPR with decreasing temperature. In this paper, we present quasi-classical trajectory calculations of the H-exchange reaction NH₂ + H, which show the reaction to proceed without a barrier, confirming that the H-exchange will be efficient in the temperature range of interest. With the inclusion of this process, our models suggest both that OPR values below three arise in regions with temperatures ≳20–25 K, depending on time, and values above three but lower than the thermal limit arise at still lower temperatures.