1998A&A...339..561L

We have observed galactic λ2.6mm and λ1.3mm CO absorption and emission along nine lines of sight toward compact extragalactic mm-wave continuum sources, using the IRAM Plateau de Bure Interferometer and NRAO 12m telescopes. In absorption we detected some two dozen kinematic components, and nearly every feature known from HCO⁺ spectra (Lucas & Liszt, 1996A&A...307..237L) has a direct CO counterpart: a few CO lines are missing or very weak in emission even when τ(CO)>0.5. The column densities of CO and HCO⁺ are well correlated, but not linearly related. The widths of the CO lines are typically 15% smaller than those of HCO⁺ (on average 0.75km/s vs. 0.86km/s). We derive ¹²CO column densities 0.1≲N(¹²CO)≲20x10¹⁵cm^–2 which are in all cases very small compared to the column of carbon nuclei expected for 1 magnitude of visual extinction, even allowing for substantial depletion. The partial thermal pressure of H₂ is inferred to be 1000≤n(H₂)T_K≤12,000cm^–3.K, with a median p/k=3.2x10³cm^–3.K. Thus the clouds are likely warm (T_K≃ tens of K), somewhat diffuse (n(H₂)≃50-300cm^–3), with the majority of the gas-phase carbon in the form of C⁺ and perhaps even with a substantial fraction of H I in the thinnest cases. The isotope ratios in the CO usually differ strongly from the local interstellar ratio which we have separately measured in these clouds to be ¹²C/¹³C=60 (Lucas & Liszt, 1998A&A...337..246L); we find 15≤N(¹²CO)/N(¹³CO)≤54, declining with increasing N(¹²CO). The ¹³CO/C¹⁸O ratio seen in emission or absorption is typically 25 (instead of 8) and C¹⁸O is very difficult to detect in emission even when T_R^*(¹²CO)/T_R^*(¹³CO)<10. Apparently, the relative abundance of ¹³CO is typically greatly enhanced, even at very low extinction, and never diminished by selective photodissociation. One effect of this enhancement is that lines of ¹²CO are substantially less optically thick than might otherwise have been inferred. There is little evidence for a general selective depletion of C¹⁸O.