The interstellar chemistry of C3H and C3H2 isomers

TL;DR

This study investigates the chemistry and observed abundance ratios of C3H and C3H2 isomers in starless cores, highlighting the role of dissociative recombination reactions and introducing a third isomer to explain the density-dependent ratios.

Contribution

The paper provides new observational data on C3H and C3H2 isomers in starless cores and updates chemical models to include a third isomer and detailed reaction pathways.

Findings

Cyclic-to-linear C3H2 ratio varies with cloud density.

Model reproduces observed density-dependent ratios.

Reactions involving isomerization significantly influence isomer abundances.

Abstract

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 (C3Hx with x=1,2) and ionic (C3Hx+ 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 + or - 30 for molecular clouds with a total density around 1e4 molecules.cm-3 to 30 + or - 10 for molecular clouds with a total density around 4e5 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.