The interstellar chemistry of C3H and C3H2 isomers

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 varies from 110 ± 30 for molecular clouds with a total density of about 1 × 10
⁴
molecules cm
⁻³
to 30 ± 10 for molecular clouds with a total density of about 4 × 10
⁵
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-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.