Abstract
We report the detection of linear and cyclic isomers of C(3)H and C(3)H(2) towards various starless cores and review the corresponding chemical pathways involving neutral (C(3)H(x) with x=1,2) and ionic (C(3)H(x)(+) with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C(3)H(2)(+) and C(3)H(3)(+) isomers showing that the statistical treatment of the relaxation of C(3)H(*) and C(3)H(2)(*) produced in these DR reactions may explain the relative c,l-C(3)H and c,l-C(3)H(2) abundances. We have also introduced in the model the third isomer of C(3)H(2) (HCCCH). The observed cyclic-to-linear C(3)H(2) ratio vary from 110 ± 30 for molecular clouds with a total density around 1×10(4) molecules.cm(-3) to 30 ± 10 for molecular clouds with a total density around 4×10(5) 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-C(3)H(2) → H + c-C(3)H(2) and H + t-C(3)H(2) → H + c-C(3)H(2) isomerization reactions.