Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

Abstract

We modeled and CO formation incorporating the fractionation and selective photodissociation affecting CO when  ≲ 2 mag. UV absorption measurements typically have N ( )/ N ( ) ≈ 65 that are reproduced with the standard UV radiation and little density dependence at n (H) ≈ 32–1024 : densities n (H) ≲ 256 avoid overproducing CO. Sightlines observed in millimeter wave absorption and a few in UV show enhanced by factors of two to four and are explained by higher n (H) ≳ 256 and/or weaker radiation. The most difficult observations to understand are UV absorptions having N ( )/ N ( ) > 100 and N (CO) ≳ 10 15 . Plots of versus N (CO) show that remains linearly proportional to N (CO) even at high opacity owing to sub-thermal excitation. and have nearly the same curve of growth so their ratios of column density/integrated intensity are comparable even when different from the isotopic abundance ratio. For n (H) ≳ 128 , plots of versus N(CO) are insensitive to n (H), and / N (CO) ≈ 1 /(10 15 CO ); this compensates for small CO/ to make more readily detectable. Rapid increases of N (CO) with n (H), N (H), and N ( ) often render the CO bright, i.e., a small CO- conversion factor. For n (H) ≲ 64 , CO enters the regime of truly weak excitation, where  ∝  n (H) N (CO). is a strong function of the average fraction and models with  = 1 fall in the narrow range of 0.65–0.8 or 0.4–0.5 at 0.1 . The insensitivity of easily detected CO emission to gas with small implies that even deep CO surveys using broad beams may not discover substantially more emission.