H ₂ Ortho–Para Spin Conversion on Inhomogeneous Grain Surfaces. II. Impact of the Rotational Energy Difference between Adsorbed Ortho-H ₂ and Para-H ₂ and Implication for Deuterium Fractionation Chemistry

Abstract We investigate how the H 2 ortho-to-para ratio (OPR) and deuterium fractionation in star-forming regions are affected by nuclear spin conversion (NSC) on dust grains. Particular focus is placed on the rotational energy difference between ortho-H 2 (o-H 2 ) and para-H 2 (p-H 2 ) on grain surfaces. While the ground state of o-H 2 has a higher rotational energy than that of p-H 2 by 170.5 K in the gas phase, this energy difference is expected to become smaller on solid surfaces, where interactions between the surface and adsorbed H 2 molecules affect their rotational motion. A previous study by K. Furuya et al. developed a rigorous formulation of the rate for the temporal variation of the H 2 OPR via the NSC on grains, assuming that adsorbed o-H 2 has higher rotational energy than adsorbed p-H 2 by 170.5 K, as in the gas phase. In this work, we relax the assumption and reevaluate the rate, varying the rotational energy difference between their ground states. The reevaluated rate is incorporated into a gas-ice astrochemical model to study the evolution of the H 2 OPR and the deuterium fractionation in prestellar cores and the outer, cold regions of protostellar envelopes. The inclusion of the NSC on grains reduces the timescale of the H 2 OPR evolution and thus the deuterium fractionation, at densities of ≳10 4 cm −3 and temperatures of ≲14–16 K (depending on the rotational energy difference), when the ionization rate of H 2 is 10 −17 s −1 .