The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH⁺ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r^-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at A_v0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH₂ dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C⁺ + NH₃ → H₂NC⁺ → HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH₃ abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H⁺₃→NH⁺₂ is permitted at Langevin rate.