星間ダスト表面におけるH2オルソ・パラ転換効率の評価 －吸着水素分子の回転エネルギー差が化学進化時間に影響－

2026-04-17 分子科学研究所

星間空間（分子雲）のダスト表面で起こる水素分子（H₂）のオルソ・パラ核スピン転換（背景画像は©NASA）

＜関連情報＞

• https://www.ims.ac.jp/news/2026/04/0417.html
• https://iopscience.iop.org/article/10.3847/1538-4357/ae43e3

不均一な結晶粒表面におけるH₂オルト-パラスピン変換。II. 吸着オルト-H₂とパラ-H₂の回転エネルギー差の影響と重水素分画化学への示唆 H2 Ortho–Para Spin Conversion on Inhomogeneous Grain Surfaces. II. Impact of the Rotational Energy Difference between Adsorbed Ortho-H2 and Para-H2 and Implication for Deuterium Fractionation Chemistry

Kenji Furuya, Toshiki Sugimoto, Kazunari Iwasaki, Masashi Tsuge, and Naoki Watanabe
The Astrophysical Journal  Published: 2026 April 6
DOI:10.3847/1538-4357/ae43e3

Abstract

We investigate how the H₂ 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-H₂ (o-H₂) and para-H₂ (p-H₂) on grain surfaces. While the ground state of o-H₂ has a higher rotational energy than that of p-H₂ 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 H₂ 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 H₂ OPR via the NSC on grains, assuming that adsorbed o-H₂ has higher rotational energy than adsorbed p-H₂ 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 H₂ 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 H₂ OPR evolution and thus the deuterium fractionation, at densities of ≳10⁴ cm⁻³ and temperatures of ≲14–16 K (depending on the rotational energy difference), when the ionization rate of H₂ is 10⁻¹⁷ s⁻¹.