THE PHYSICS AND CHEMISTRY OF SMALL TRANSLUCENT MOLECULAR CLOUDS - IX - ACETYLENIC CHEMISTRY

We have conducted a survey of C3H2, HC3N, and C2S (two rotational tran sitions in each) in our standard sample of 11 cirrus cores and 27 Clem ens-Barvainis translucent cores whose structures and chemistry have be en studied in this series. C3H2 is seen in 31 objects, HC3N in six, an d C2S in 14. These results are modeled in terms of our previous hydros tatic equilibrium and n similar to r(-alpha) structures together with other chemical and physical properties derived earlier. The complex ra diative transfer and excitation of C3H2 and HC3N is discussed, includi ng weak population inversions in the lower states. Radiative transfer of C2S is less complicated. Abundances of each species increase monoto nically with increasing extinction in the 1.2-2.7 mag range (edge-to-c enter), thus displaying the same characteristic transition between dif fuse and dense cloud chemistry as do most other species we have studie d. The chemistry has been modeled by solving the complete set of simil ar to 4000 reactions in the New Standard Chemistry Model, adapted to t ranslucent cloud conditions of density, elemental abundances, extincti ons, and certain ion-polar rates. C3H2 abundances are underestimated b y the Standard chemistry model by a factor of similar to 10, resulting , we believe, from a single uncertain reaction rate and perhaps one om itted neutral-neutral process. HC3N and C2S abundances fit chemistry m odel predictions well. They appear not to be intimately connected chem ically with C3H2. Aside from the overall success of the gas-phase chem istry models, we emphasize that (1) the observed abundances are consis tent with steady state chemistry, not early-time chemistry, and (2) ne utral-neutral reactions are fundamentally important in forming HC3N, p ossibly important to C3H2, and not important to C2S.