CO J=3-]2 OBSERVATIONS OF TRANSLUCENT AND HIGH-LATITUDE MOLECULAR CLOUDS

Measurements of the CO J = 3 → 2 emission line at 345 GHz in translucent and high-latitude clouds are reported. Together with data on the J = 2 → 1 and J = 1 → 0 lines of both 12CO and 13CO, they are used to constrain the densities in the clouds. The physical conditions in the high-latitude clouds are found to be very similar to those in the translucent clouds. The CO emission-line data complement optical absorption-line observations of the C2 molecule in the same region, which provide independent constraints on temperature and density. The densities derived from the 12CO 1 → 0/3 → 2 ratios are often higher than those inferred from the C2 excitation, whereas densities derived from the 3 → 2/13CO 1 → 0 ratios are found to be very similar to those based on C2. Several explanations for these differences are discussed, including the possibility that the CO and C2 molecules are distributed differently throughout a cloud. Various "clumpy" cloud models have been developed to explore this question. No CO J = 3 → 2 emission has been detected from the classical diffuse cloud toward ζ Oph. The very low limit, T*A(3 → 2) ≤ 0.08 K, excludes high-density (nH > 700 cm-3) models for this line of sight. Evidence is found for a slight decrease in the J = 1 → 0/3 → 2 and 2 → 1/3 → 2 line intensity ratios from the edge to the center of one cloud, which can be explained by a corresponding increase in the density into the center. The CO lines often show complex profiles consisting of several narrow (ΔV ≤ 1 km s-1) components; in some cases the profiles show centrally reversed line cores that can be interpreted in terms of gradients in density, temperature, and molecular abundance within a single kinematic component.