The physics and chemistry of small translucent molecular clouds. XIII. The basic hydrocarbon chemistry

We have observed 10 hydrocarbon species at 3 mm wavelength in three translucent clouds and in TMC-1 and L183. The 10 species are C2H, c-C3H, l-C3H, c-C3H2, l-C3H2, CH3CCH, C4H, C4H2, C5H, and C6H, where c- and l- designate the cyclical and linear isomers. Abundances for these, as well as the previously observed species CH3C4H and C6H2, have been derived from statistical equilibrium analyses. These species are key members of the hydrocarbon network-the backbone of gas-phase interstellar chemistry, from which most species containing N, O, and C atoms stem directly. All of the observed hydrocarbon species except c-C3H and c-C3H2 are chain-type molecules or have a linear-chain isomer. The latter species are known to have stable cyclic isomers as well. We observe large c-/l-abundance ratios for C3H2, especially in the dark clouds, that can be explained under steady state conditions by existing knowledge of the chemistry. We also observe ortho/para ratios for c- and l-C3H2, and for C4H2, that exceed the high-temperature limit in nearly every case, often by large factors. O/P ratios as large as similar to 5 can be explained by protonation via H-3(+). By contrast, E/A ratios are observed to be less than the high-ir limit for all three C-3 nu species (CH3C2H, CH3C4H, CH3CN), because H-3(+) protonation is not possible. The New Standard Chemistry model (Lee et al.) has been modified to accommodate several cyclical and chain-type hydrocarbon species. In addition to the large c-/l- and O/P ratios, we find very good agreement between model predictions and observed total abundances for all 12 species under steady state conditions. Early-time chemistry is not required but cannot be ruled out. It easily produces the very large c-C3H2/l-C3H2 ratio observed in the dark clouds, consistent with the possibility that they are chemically young, while translucent clouds are in chemical and dynamical steady state. For a consistent set of C/O ratios, over 80% of all model abundances investigated are within a factor of 5 of the observed abundances. Low-metal abundances and C/O ratios of 0.7 or higher are required to explain all 12 species. Distinctions between translucent and dark clouds are at most nominal. The success of the New Standard Model in explaining hydrocarbons exceeds its success in explaining all other species studied to date. In particular, hydrocarbons are better explained than the equally complex nonhydrocarbon species studied in the twelfth paper in this series.