The interstellar chemistry of C₃H and C₃H₂ isomers

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 vary from 110 ± 30 for molecular clouds with a total density around 1×10⁴ molecules.cm^-3 to 30 ± 10 for molecular clouds with a total density around 4×10⁵ molecules.cm^-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C₃H₂ → H + c-C₃H₂ and H + t-C₃H₂ → H + c-C₃H₂ isomerization reactions.