CO IN ABSORPTION AND EMISSION TOWARD COMPACT EXTRAGALACTIC RADIO-CONTINUUM SOURCES

We have observed galactic lambda 2.6mm and lambda 1.3mm 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 kin ematic components, and nearly every feature known from HCO+ spectra (L ucas and Liszt, 1996) has a direct CO counterpart: a few CO lines are missing or very weak in emission even when tau(CO) greater than or sim ilar to 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+ ton average 0.75 km s(-1) vs. 0.86 km s(-1 )). We derive (CO)-C-12 column densities 0.1 less than or similar to N ((CO)-C-12) less than or similar to 20 x 10(15) cm(-2) which are in al l cases very small compared to the column of carbon nuclei expected fo r 1 magnitude of visual extinction, even allowing for substantial depl etion. The partial thermal pressure of H-2 is inferred to be 1000 less than or equal to n(H-2)T-K less than or equal to 12,000 cm(-3) K, wit h a median p/k = 3.2 x 10(3) cm(-3) K. Thus the clouds are likely warm (T-K approximate to tens of K), somewhat diffuse (n(Ha) approximate to 50 - 300 cm(-3)), with the majority of the gas-phase carbon in the fo rm of C+ and perhaps even with a substantial fraction of H I in the th innest cases. The isotope ratios in the CO usually differ strongly fro m the local interstellar ratio which we have separately measured in th ese clouds to be C-12/C-13 = 60 (Lucas and Liszt, 1998); we find 15 le ss than or equal to N((CO)-C-12)/N((CO)-C-13)less than or equal to 54, declining with increasing N((CO)-C-12). The (CO)-C-13/(CO)-O-18 ratio seen in emission or absorption is typically 25 (instead of 8) and (CO )-O-18 is very difficult to detect in emission even when T-R((CO)-C-1 2)/T-R((CO)-C-13)<10. Apparently, the relative abundance of (CO)-C-13 is typically greatly enhanced, even at very low extinction, and never diminished by selective photodissociation. One effect of this enhance ment is that lines of (CO)-C-12 are substantially less optically thick than might otherwise have been inferred. There is little evidence for a general selective depletion of (CO)-O-18.