The Physics and Chemistry of Small Translucent Molecular Clouds. X. SiO

Abstract

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 search- ed in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and nDr~a structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperÐne 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 D2.5 if these species are spatially centrally peaked and D6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2È2.7 mag range (edge to center), thus displaying the same characteristic transition between di†use 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 istry models have been carried out: a full model using the Standard Model rate Ðle and comprising 409 Av0D10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chem- species (by Lee and Herbst), and a simpliÐed model comprising 21 nitrogen-bearing species for condi- tions 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 ly favored over high ones; (3) the reaction C`]NH3 N]CH2 dominate the chemistry of HCN; (2) low ion-polar reaction rates are strong- uncoupling the CwN and NwH chemistries; (4) the ratio HCN/HNC is not a particularly important ]H2NC`]HNC is unimportant, thus largely diagnostic of the CwN chemistry; (5) model NH abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N]H3 3 `]NH2 ` is permitted at Langevin rate.