THE CO-TO-H2 CONVERSION FACTOR ACROSS THE PERSEUS MOLECULAR CLOUD

We derive the CO-to-H-2 conversion factor, XCO=N(H-2)/I-CO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H-2) data with the I-CO data from the COMPLETE Survey. We estimate an average X-CO similar to 3 x 10(19) cm(-2) K-1 km(-1) s and find a factor of similar to 3 variations in X-CO between the five sub-regions in Perseus. Within the individual regions, X-CO varies by a factor of similar to 100, suggesting that X-CO strongly depends on local conditions in the interstellar medium. We find that X-CO sharply decreases at A(V) less than or similar to 3 mag but gradually increases at A(V) greater than or similar to 3 mag, with the transition occurring at A(V) where I-CO becomes optically thick. We compare the N(H I), N(H-2), I-CO, and X-CO distributions with two models of the formation of molecular gas, a one-dimensional photodissociation region (PDR) model and a three-dimensional magnetohydrodynamic (MHD) model, tracking both the dynamical and chemical evolution of gas. The PDR model based on the steady state and equilibrium chemistry reproduces our data very well but requires a diffuse halo to match the observed N(H I) and I-CO distributions. The MHD model matches our data reasonably well, suggesting that time-dependent effects on H-2 and CO formation are insignificant for an evolved molecular cloud like Perseus. However, we find interesting discrepancies, including a broader range of N(H I), likely underestimated I-CO, and a large scatter of I-CO at small A(V). These discrepancies