Volume 596, December 2016
|Number of page(s)||12|
|Section||Interstellar and circumstellar matter|
|Published online||28 November 2016|
The ortho-to-para ratio of interstellar NH2: quasi-classical trajectory calculations and new simulations
1 Department of Chemistry, University of
Road, Charlottesville, VA
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
Accepted: 6 September 2016
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.
Key words: astrochemistry / ISM: molecules / submillimeter: ISM / molecular processes / line: formation
© ESO, 2016
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