2012MNRAS.423.2342F

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 (f_D), the gas kinetic temperature and the gas mass. In particular, the C¹⁸O (3–2) line allows us to derive f_D 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 f_D 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 f_D larger than in low-mass pre-stellar cores. On the other hand, f_D 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-{img}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. Herschel and JCMT observations of the early-type dwarf galaxy NGC 205