2006A&A...449..631P

We report a detailed attempt to model the ortho-to-para abundance ratio of c-C₃H₂ so as to reproduce observed values in the cores of the well-known source TMC-1. According to observations, the ortho-to-para ratios vary, within large uncertainties, from a low of near unity to a high of approximately three depending on the core. We used the osu.2003 network of gas-phase chemical reactions augmented by reactions that specifically consider the formation, depletion, and interconversion of the ortho and para forms of c-C₃H₂ and its precursor ion c-C₃H₃⁺. We investigated the sensitivity of the calculated ortho-to-para ratio for c-C₃H₂ to a large number of factors. For the less evolved cores C, CP, and D, we had no difficulty reproducing the observed ortho-to-para ratios of 1-2. In order to reproduce observed ortho-to-para ratios of near three, observed for the evolved cores A and B, it was necessary to include rapid ion-catalyzed interconversion processes.