Understanding the ${\mathrm{C}}_3{\mathrm{H}}_2$ cyclic-to-linear ratio in L1544

Abstract

Aims. We aim to understand the high cyclic-to-linear ${\mathrm{C}}_3{\mathrm{H}}_2$ ratio ($32\pm 4$) observed toward L1544 by Spezzano et al. (2016). Methods. We combine a gas-grain chemical model with a physical model for L1544 to simulate the column densities of cyclic and linear ${\mathrm{C}}_3{\mathrm{H}}_2$ observed toward L1544. The most important reactions for the formation and destruction of both forms of ${\mathrm{C}}_3{\mathrm{H}}_2$ are identified, and their relative rate coefficients are varied to find the best match to the observations. Results. We find that the ratio of the rate coefficients of ${\mathrm{C}}_3{\mathrm{H}}_3^{+}+{\mathrm{e}}^{-}⟶{\mathrm{C}}_3{\mathrm{H}}_2+\mathrm{H}$ for cyclic and linear ${\mathrm{C}}_3{\mathrm{H}}_2$ must be $\sim 20$ in order to reproduce the observations, depending on the branching ratios assumed for the ${\mathrm{C}}_3{\mathrm{H}}_3^{+}+{\mathrm{e}}^{-}⟶{\mathrm{C}}_3\mathrm{H}+{\mathrm{H}}_2$ reaction. In current astrochemical networks it is assumed that cyclic and linear ${\mathrm{C}}_3{\mathrm{H}}_2$ are formed in a 1:1 ratio in the aforementioned reactions. Laboratory studies and/or theoretical calculations are needed to confirm the results of our chemical modeling based on observational constraints.

5 pages, 2 figures; accepted as an A&A Letter