CO depletion in infrared dark clouds
Astronomy and Astrophysics, vol. 705, pp. A72
Abstract
Context . Infrared dark clouds (IRDCs) are cold, dense structures that are likely representative of the initial conditions of star formation. Many studies of IRDCs employ CO to investigate cloud dynamics, but CO can be highly depleted from the gas phase in IRDCs, which affects its fidelity as tracer. The CO depletion process is also of great interest in astrochemistry because CO ice in dust grain mantles provides the raw material for the formation of complex organic molecules. Aims . We study CO depletion towards four IRDCs to investigate its correlation with the H 2 number density and dust temperature, calculated from Herschel far-infrared images. Methods . We used 13 CO J = 1 → 0 and 2 → 1 maps to measure the CO depletion factor, f D , across IRDCs G23.46-00.53, G24.49-00.70, G24.94-00.15, and G25.16-00.28. We also considered a normalised CO depletion factor, f′ D , which takes a value of unity, that is, no depletion, in the outer lower-density and warmer regions of the clouds. We then investigated the dependence of f D and f′ D on the gas density, n H , and dust temperature, T dust . Results . The CO depletion rises as the density increases and reaches maximum values of f′ D ∼ 10 in some regions with n H ≳ 3 × 10 5 cm −3 , although with significant scatter at a given density. We find a tighter, less scattered relation of f′ D with temperature that rapidly rise for temperatures ≲18 K. We propose a functional form f′ D = exp( T 0 /[ T dust − T 1 ]) with T 0 ≃ 4 K and T 1 ≃ 12 K to reproduce this behaviour. Conclusions . We conclude that CO is strongly depleted from the gas phase in cold, dense regions of IRDCs. This means that if it is not accounted for, CO depletion can lead to an underestimation of the total cloud masses based on CO line fluxes by factors up to ∼5. These results indicate a dominant role for thermal desorption in setting near equilibrium abundances of gas-phase CO in IRDCs and provide important constraints for astrochemical models and the chemodynamical history of gas in the early stages of star formation.
Authors 12
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European Southern Observatory · Institut de Radioastronomie Millimétrique · European Southern Observatory
Affiliation as printed
European Southern Observatory
Institut de Radioastronomie Millimétrique
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University of Virginia · Chalmers University of Technology
Affiliation as printed
Department of Astronomy, University of Virginia
Department of Space, Earth and Environment, Chalmers University of Technology
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Affiliation as printed
Department of Astronomy, Yale University
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Arcetri Astrophysical Observatory · National Institute for Astrophysics
Affiliation as printed
INAF Osservatorio Astronomico di Arcetri
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Affiliation as printed
Department of Astronomy, University of Virginia
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California Institute of Technology
Affiliation as printed
California Institute of Technology
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Affiliation as printed
Centro de Astrobiología (CSIC/INTA)
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European Southern Observatory · European Southern Observatory
Affiliation as printed
European Southern Observatory
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Arcetri Astrophysical Observatory · Max Planck Institute for Extraterrestrial Physics · Laboratoire d'Étude de l'Univers et des Phénomènes Extrêmes · National Institute for Astrophysics
Affiliation as printed
INAF Osservatorio Astronomico di Arcetri
Laboratory for the study of the Universe and eXtreme phenomena (LUX)
Max Planck Institute for Extraterrestrial Physics
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Liverpool John Moores University
Affiliation as printed
Astrophysics Research Institute, Liverpool John Moores University
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Max Planck Institute for Extraterrestrial Physics
Affiliation as printed
Max Planck Institute for Extraterrestrial Physics
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Leiden University · Leiden Observatory
Affiliation as printed
Leiden Observatory, Leiden University
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