Modeling the ortho-to-para abundance ratio of cyclic C3H2 in cold dense cores

Aims. 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. Methods. 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-C3H+3 . We investigated the sensitivity of the calculated ortho-to-para ratio for c-C3H2 to a large number of factors. Results. 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.