SIMBAD references

We modeled H₂ and CO formation incorporating the fractionation and selective photodissociation affecting CO when A_V <= 2 mag. UV absorption measurements typically have N(¹²CO)/N(¹³CO) ≃ 65 that are reproduced with the standard UV radiation and little density dependence at n(H) ≃ 32-1024 cm^–3: densities n(H) <= 256 cm^–3 avoid overproducing CO. Sightlines observed in millimeter wave absorption and a few in UV show enhanced ¹³CO by factors of two to four and are explained by higher n(H) >= 256 cm^–3 and/or weaker radiation. The most difficult observations to understand are UV absorptions having N(¹²CO)/N(¹³CO) > 100 and N(CO) >= 10¹⁵ cm^–2. Plots of W_CO versus N(CO) show that W_CO remains linearly proportional to N(CO) even at high opacity owing to sub-thermal excitation. ¹²CO and ¹³CO 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 cm^–3, plots of W_CO versus N(CO) are insensitive to n(H), and W_CO/N(CO) ≃ 1  K km s^–1/(10¹⁵ CO  cm^–2); this compensates for small CO/H₂ to make W_CO more readily detectable. Rapid increases of N(CO) with n(H), N(H), and N(H₂) often render the CO bright, i.e., a small CO-H₂ conversion factor. For n(H) <= 64  cm^–3, CO enters the regime of truly weak excitation, where W_CO ∝ n(H)N(CO). W_CO is a strong function of the average H₂ fraction and models with W_CO = 1  K km s^–1 fall in the narrow range of <f_H2_>0.65-0.8 or <f_H2_>0.4-0.5 at W_CO 0.1  K km s^–1. The insensitivity of easily detected CO emission to gas with small <f_H2_>implies that even deep CO surveys using broad beams may not discover substantially more emission.