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 (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.
