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

Journal Article · Thu Mar 20 00:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/784/1/80· OSTI ID:22351463

Lee, Min-Young; Stanimirović, Snežana ^[1]; Wolfire, Mark G. ^[2]; Shetty, Rahul; Glover, Simon C. O.; Molina, Faviola Z.

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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)

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 very 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.

• OSTI ID: 22351463
• Journal Information: Astrophysical Journal, Vol. 784, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

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