THE PHYSICS AND CHEMISTRY OF SMALL MOLECULAR CLOUDS IN THE GALACTIC PLANE .1. PHYSICAL CONDITIONS FROM (CO)-O-18 AND (CO)-C-13 OBSERVATIONS

We have observed the J=1-0 and 2-1 transitions of (CO)-O-18 and (CO)-C -13 in, 27 small Galactic plane clouds taken from the 1988 catalog of Clemens and Barvainis. The observations are analyzed by means of polyt ropic hydrostatic equilibrium models, the same as used by the author i n a series of papers on cirrus cloud cores. The wide range of (CO)-C-1 3 to (CO)-O-18 intensity ratios is successfully modeled by including v ariations in the UV radiation field intensity from source to source, a lthough 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 galaxie s. The ISM of the Milky Way is not well mixed as regards the nucleosyn thesis 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 air respects. The apparent dynamical equ ilibrium 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 co ld quiescent clouds, which correspondingly exhibit early-times chemist ry.