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 C ^ { 18 } OÂ with the APEX telescope , and the ( 1,1 ) and ( 2,2 ) inversion transitions of NH _ { 3 } Â with the Australia Telescope Compact Array in 21 infrared-dark clouds already mapped in the 1.2Â 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 C ^ { 18 } OÂ ( 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 NH _ { 3 } Â and C ^ { 18 } OÂ in all targets .
The clumps have a median mass of \sim 244 M _ { \odot } , are gravitationally bound , have an average kinetic temperature of 17Â K and possess mass , H _ { 2 } 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 characterised 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 anti-correlation with the gas kinetic temperature .
Thirteen out of 21 clumps are undetected in the 24 \mu m Spitzer images , and have slightly lower kinetic temperatures , masses and H _ { 2 } 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 .