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宇宙電波観測を用いた、暗黒星雲コアにおける環状C₃H₂の オルト・パラ比と化学進化の相関に関する研究

Correlation between Nuclear Spin Ratio of cyclic-C₃H₂ and Chemical Evolution in TMC-1

森澤 勇介
Yusuke MORISAWA

野辺山45 m電波望遠鏡を用いて，暗黒星雲TMC-1の５つのコアにおける環状C₃H₂のオルト遷移４本，パラ遷移４本，計８つの純回転遷移を観測することにより，環状C₃H₂のオルト／パラ比の分布を求めた。その結果，オルト／パラ比はコアの化学進化の進化度と相関があることがわかった。化学モデル計算から，この相関は化学反応における核スピン保存則を使って説明できることを示した。

Ortho-to-para nuclear spin ratio of cyclopropenylidene (c-C₃H₂) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (J_KaKc = 2₁₂-1₀₁, 3₁₂-3₀₃, 3₂₁-3₁₂, and 3₃₀-3₂₁) and four para lines (J_KaKc = 2₀₂1₁₁, 2₁₁-2₀₂, 2₂₀-2₁₁, and 3₂₂-3₁₃) of c-C₃H₂ in six cores in TMC-1. A statistical equilibrium analysis of the observed intensities revealed clear correlation between the ortho-to-para ratio of c-C₃H₂ and chemical evolution of the molecular cloud; the ortho-to-para ratio in younger cores is less than 2, while that in older cores is close to the statistical ratio of 3. A simple chemical model calculation indicates that the correlation is ascribed to the conservation of nuclear spin angular moment in chemical reactions relevant to the production, destruction, and re-production of c-C₃H₂.

Keywords:Astronomical observation, Millimeter-wave, Chemical evolution, Interstellar molecule, Nuclear spin modification