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Title: The CO-to-H{sub 2} conversion factor across the Perseus molecular cloud

We derive the CO-to-H{sub 2} conversion factor, X {sub CO} = N(H{sub 2})/I {sub CO}, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H{sub 2}) data with the I {sub CO} data from the COMPLETE Survey. We estimate an average X {sub CO} ∼ 3 × 10{sup 19} cm{sup –2} K{sup –1} km{sup –1} s and find a factor of ∼3 variations in X {sub CO} between the five sub-regions in Perseus. Within the individual regions, X {sub CO} varies by a factor of ∼100, suggesting that X {sub CO} strongly depends on local conditions in the interstellar medium. We find that X {sub CO} sharply decreases at A{sub V} ≲ 3 mag but gradually increases at A{sub V} ≳ 3 mag, with the transition occurring at A{sub V} where I {sub CO} becomes optically thick. We compare the N(H I), N(H{sub 2}), I {sub CO}, and X {sub 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 verymore » well but requires a diffuse halo to match the observed N(H I) and I {sub CO} distributions. The MHD model matches our data reasonably well, suggesting that time-dependent effects on H{sub 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 {sub CO}, and a large scatter of I {sub CO} at small A{sub V} . These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.« less
Authors:
;  [1] ;  [2] ; ; ; ;  [3]
  1. Department of Astronomy, University of Wisconsin, Madison, WI 53706 (United States)
  2. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
  3. Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Strasse 2, D-69120 Heidelberg (Germany)
Publication Date:
OSTI Identifier:
22351463
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 784; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CARBON MONOXIDE; COMPARATIVE EVALUATIONS; COMPRESSION; CONVERSION; COSMIC GASES; DENSITY; DISSOCIATION; DISTRIBUTION; DUSTS; EQUILIBRIUM; EVOLUTION; FLUCTUATIONS; HYDROGEN; MAGNETOHYDRODYNAMICS; MOLECULES; PHOTOLYSIS; SIMULATION; STEADY-STATE CONDITIONS; THREE-DIMENSIONAL CALCULATIONS; TIME DEPENDENCE