2014ApJ...784...80L

We derive the CO-to-H₂ conversion factor, X_CO= N(H₂)/I_CO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H₂) data with the I_COdata from the COMPLETE Survey. We estimate an average X_CO∼ 3x10¹⁹/cm2/K/kms and find a factor of ∼3 variations in X_CObetween the five sub-regions in Perseus. Within the individual regions, X_COvaries by a factor of ∼100, suggesting that X_COstrongly depends on local conditions in the interstellar medium. We find that X_COsharply decreases at A_V ≲ 3 mag but gradually increases at A_V ≳ 3 mag, with the transition occurring at A_Vwhere I_CObecomes optically thick. We compare the N(H I), N(H₂), I_CO, and X_COdistributions 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_COdistributions. The MHD model matches our data reasonably well, suggesting that time-dependent effects on H₂ 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_COat small A_V. These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.