Correlation between nuclear spin ratio of cyclic C3H2 and chemical evolution in TMC-1 cores

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C ₃H₂) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (JK,KC = 212-101,3123₀₃, 3₂₁-3₁₂, and 3 ₃₀-3₂₁) and four para lines (J_KaKc = 2 ₀₂-1₁₁, 2₁₁-2₀₂,2 ₂₀-2₁₁, and 3₂₂-3₁₃) of cyclic C₃H₂ in six cores in TMC-1. A statistical equilibrium analysis of the observed intensities revealed apparent correlation between the ortho-to-para ratio of cyclic 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 momenta in chemical reactions relevant to the production, destruction, and reproduction of cyclic C₃H ₂. Since cyclic C₃H₂ has been observed in a variety of sources, the ortho-to-para ratio of cyclic C₃H₂ may be a useful probe of chemical evolution of molecular clouds.