Journal Article

High CO depletion in southern infrared dark clouds

F. Fontani, A. Giannetti, M. T. Beltrán, R. Dodson, M. Rioja, J. Brand, P. Caselli and R. Cesaroni

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 423, issue 3, pages 2342-2358
Published in print July 2012 | ISSN: 0035-8711
Published online June 2012 | e-ISSN: 1365-2966 | DOI:
High CO depletion in southern infrared dark clouds

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Infrared dark high-mass clumps are among the most promising objects to study the initial conditions of the formation process of high-mass stars and rich stellar clusters. In this work, we have observed the (3–2) rotational transition of C18O with the Atacama Pathfinder EXperiment telescope and the (1,1) and (2,2) inversion transitions of NH3 with the Australia Telescope Compact Array in 21 infrared dark clouds already mapped in the 1.2-mm continuum, with the aim of measuring basic chemical and physical parameters such as the CO depletion factor (fD), the gas kinetic temperature and the gas mass. In particular, the C18O (3–2) line allows us to derive fD in gas at densities higher (and hence potentially more depleted) than that traced by the (1–0) and (2–1) lines, typically used in previous works. We have detected NH3 and C18O in all targets. The clumps have a median mass of ∼244 M, are gravitationally bound, have an average kinetic temperature of 17 K and possess mass, H2 column and surface densities consistent with being potentially the birthplace of high-mass stars. We have measured fD in between 5 and 78, with a mean value of 32 and a median of 29. These values are, to our knowledge, larger than the typical CO depletion factors measured towards infrared dark clouds and high-mass dense cores, and are comparable to or larger than the values measured in low-mass pre-stellar cores close to the onset of the gravitational collapse. This result suggests that the earliest phases of the high-mass star and stellar cluster formation process are characterized by fD larger than in low-mass pre-stellar cores. On the other hand, fD does not seem to be correlated to any other physical parameter, except for a faint anticorrelation with the gas kinetic temperature. 13 out of 21 clumps are undetected in the 24-m Spitzer images, and have slightly lower kinetic temperatures, masses and H2 column densities with respect to the eight Spitzer-bright sources. This could indicate that the Spitzer-dark clumps are either less evolved or are going to form less massive objects.

Keywords: molecular data; stars: formation; ISM: molecules; radio lines: ISM; submillimetre: ISM

Journal Article.  8829 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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