The CO-to-H2 Conversion Factor across the Perseus Molecular Cloud

Type Journal Article
Names Min-Young Lee, Snežana Stanimirović, Mark G. Wolfire, Rahul Shetty, Simon C. O. Glover, Faviola Z. Molina, Ralf S. Klessen
Publication The Astrophysical Journal
Volume 784
Pages 80
Journal Abbreviation The Astrophysical Journal
Date March 1, 2014
DOI 10.1088/0004-637X/784/1/80
ISSN 0004-637X
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Abstract 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 × 1019 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 lsim 3 mag but gradually increases at AV gsim 3 mag, with the transition occurring at AV where I CO becomes optically thick. We compare the N(H I), 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(H I) and I CO 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 I CO, and a large scatter of I CO at small AV . These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.
Tags DUST, ISM: MOLECULES, ISM: individual objects: Perseus, Radio Lines: ISM, extinction, infrared: ISM
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