The physics and chemistry of small molecular clouds in the galactic plane. 1: Physical conditions form C(18)O and (13)CO observations

We have observed the J = 1 goes to 0 and 2 goes to 1 transitions of C(18)O and (13)CO in 27 small Galactic plane clouds taken from the 1988 catalog of Clemens & Barvainis. The observations are analyzed by means of polytropic hydrostatic equilibrium models, the same as used by the author in a series of papers on cirrus cloud cores. The wide range of (13)CO to C(18)O intensity ratios is successfully modeled by including variations in the UV radiation field intensity from source to source, although differences in the C-12/C-13 ratio are also required. Most of the objects (20) have C-12/C-13 in the range 45-90, but seven objects require considerably larger values, as found recently in other galaxies. The ISM of the Milky Way is not well mixed as regards the nucleosynthesis products of massive stars (a C-12 excess) and those of evolved stars (a C-13 excess). The properties of the CB objects are similar to those of the cirrus cores in all respects. The apparent dynamical equilibrium of these objects is consistent with their chemical abundances, which favor steady state chemistry, and is distinct from the highly nonequilibrium density distributions required to model more massive cold quiescent clouds, which correspondingly exhibit early-times chemistry.