2017MNRAS.470.4075L

We report the detection of linear and cyclic isomers of C₃H and C₃H₂ towards various starless cores and review the corresponding chemical pathways involving neutral (C₃H_x with x = 1,2) and ionic (C₃H_x⁺ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic dissociative recombination (DR) reactions of C₃H₂⁺ and C₃H₃⁺ isomers, showing that the statistical treatment of the relaxation of C₃H* and C₃H₂* produced in these DR reactions may explain the relative c,l-C₃H and c,l-C₃H₂ abundances. We have also introduced in the model the third isomer of C₃H₂ (HCCCH). The observed cyclic-to-linear C₃H₂ ratio varies from 110 ± 30 for molecular clouds with a total density of about 1 x 10⁴ molecules cm^–3 to 30 ± 10 for molecular clouds with a total density of about 4 x 10⁵ molecules cm^–3, a trend well reproduced with our updated model. The higher ratio for molecular clouds with low densities is determined mainly by the importance of the H + l-C₃H₂ - H + c-C₃H₂ and H + t-C₃H₂ - H + c-C₃H₂ isomerization reactions.