Correlation between Nuclear Spin Ratio of Cyclic C3H2and Chemical Evolution in TMC‐1 Cores

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C 3 H 2 ) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines ( J = 2 12 -1 01 , 3 12 -3 03 , 3 21 -3 12 , and 3 30 -3 21 ) and four para lines ( J = 2 02 -1 11 , 2 11 -2 02 , 2 20 -2 11 , and 3 22 -3 13 ) of cyclic C 3 H 2 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 C 3 H 2 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 C 3 H 2 . Since cyclic C 3 H 2 has been observed in a variety of sources, the ortho-to-para ratio of cyclic C 3 H 2 may be a useful probe of chemical evolution of molecular clouds.