2016A&A...596A..35L

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 ≳20-25K, depending on time, and values above three but lower than the thermal limit arise at still lower temperatures.