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

We derive the CO-to-H2 conversion factor, XCO = N(H2)/ICO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H2) data with the ICO data from the COMPLETE Survey. We estimate an average XCO ∼ 3 × 1019 cm−2 K−1 km−1 s and find a factor of ∼3 variations in XCO between the five sub-regions in Perseus. Within the individual regions, XCO varies by a factor of ∼100, suggesting that XCO strongly depends on local conditions in the interstellar medium. We find that XCO sharply decreases at AV ≲ 3 mag but gradually increases at AV ≳ 3 mag, with the transition occurring at AV where ICO becomes optically thick. We compare the N(H i), N(H2), ICO, and XCO 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 ICO distributions. 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 ICO, and a large scatter of ICO at small AV. These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.