Astrophys. J., 784, 80 (2014/March-3)
The CO-to-H2 conversion factor across the Perseus molecular cloud.
LEE M.-Y., STANIMIROVIC S.Z., WOLFIRE M.G., SHETTY R., GLOVER S.C.O., MOLINA F.Z. and KLESSEN R.S.
Abstract (from CDS):
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 ICOdata from the COMPLETE Survey. We estimate an average XCO∼ 3x1019/cm2/K/kms and find a factor of ∼3 variations in XCObetween the five sub-regions in Perseus. Within the individual regions, XCOvaries by a factor of ∼100, suggesting that XCOstrongly depends on local conditions in the interstellar medium. We find that XCOsharply decreases at AV ≲ 3 mag but gradually increases at AV ≳ 3 mag, with the transition occurring at AVwhere ICObecomes optically thick. We compare the N(H I), N(H2), ICO, and XCOdistributions 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 ICOdistributions. 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 ICOat small AV. These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.
dust, extinction - infrared: ISM - ISM: individual: Perseus - ISM: molecules - radio lines: ISM
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