Title: Understanding the relationship between CO and H2 abundances in

Giant Molecular Clouds


CO is often used as a tracer of molecular gas. However, its reliability has long been questioned in environments different from the Milky Way. We have studied the relationship between H2 and CO abundances in numerical models of GMCs using simulations of MHD turbulence coupled to a simplified chemical network. In this talk, I show that the abundance of H2 is primarily determined by the time available for its formation, and is insensitive to photodissociation. On the other hand, CO forms quickly, but is highly sensitive to photodissociation, with only a weak dependence on H2 abundance. As a result, there is a sharp cutoff in CO abundance at mean visual extinctions A_V < 3. At lower A_V, the ratio of H2 column density to CO emissivity scales as A_V-2.8 This explains the discrepancy observed in low metallicity systems between cloud masses derived from CO observations and other techniques.