摘要：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.
