SIMBAD references

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C₃H₂) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (J_Ka_Kc=2₁₂-1₀₁, 3₁₂-3₀₃, 3₂₁-3₁₂, and 3₃₀-3₂₁) and four para lines (J_Ka_Kc=2₀₂-1₁₁, 2₁₁-2₀₂, 2₂₀-2₁₁, and 3₂₂-3₁₃) of cyclic C₃H₂in 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 C₃H₂. Since cyclic C₃H₂has been observed in a variety of sources, the ortho-to-para ratio of cyclic C₃H₂may be a useful probe of chemical evolution of molecular clouds.