We derive CO column densities 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 is inferred to be K, with a median p/k = K. Thus the clouds are likely warm ( tens of K), somewhat diffuse ( ), 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 C/ C = 60 ; we find N N , declining with increasing N( CO). The CO/C O ratio seen in emission or absorption is typically 25 (instead of 8) and C O is very difficult to detect in emission even when . Apparently, the relative abundance of CO is typically greatly enhanced, even at very low extinction, and never diminished by selective photodissociation. One effect of this enhancement is that lines of CO are substantially less optically thick than might otherwise have been inferred. There is little evidence for a general selective depletion of C O.
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