The CO-to-H2 Conversion Factor across the Perseus Molecular Cloud
  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_CO data from the COMPLETE Survey. We estimate an average X_CO ~
3 x 10^19 cm^-2 K^-1 km^-1 s and find a factor of ~3 variations in X_CO between
the five sub-regions in Perseus. Within the individual regions, X_CO varies by
a factor of ~100, suggesting that X_CO strongly depends on local conditions in
the interstellar medium. We find that X_CO sharply decreases at Av < 3 mag but
gradually increases at Av > 3 mag, with the transition occurring at Av where
I_CO becomes optically thick. We compare the N(HI), N(H2), 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(HI) and I_CO distributions. The MHD model generally matches our
data 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(HI), likely
underestimated I_CO, and a large scatter of I_CO at small Av. These
discrepancies likely result from strong compressions/rarefactions and density
fluctuations in the MHD model.
