Modeling the ortho-to-para abundance ratio of cyclic C3H2 in cold dense   cores
  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 the 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 Hee Park, Valentine Wakelam and Eric Herbst