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Description
In the low-mass regime, molecular cores have spatially resolved temperature and density profiles allowing a detailed study of their chemical properties. It is found that the gas-phase abundances of C-bearing molecules in cold starless cores rapidly decrease with increasing density. Here the molecules tend to stick to the grains, forming ice mantles. We study CO depletion in a large sample of massive clumps, and investigate its correlation with evolutionary stage and with the physical parameters of the sources. Moreover, we study the gradients in [^12^C]/[^13^C] and [^18^O]/[^17^O] isotopic ratios across the inner Galaxy, and the virial stability of the clumps. From the ATLASGAL 870{mu}m survey we selected 102 clumps, which have masses in the range ~10^2^-3x10^4^M_{sun}_, sampling different evolutionary stages. We use low-J emission lines of CO isotopologues and the dust continuum emission to infer the depletion factor f_D_. RATRAN one-dimensional models were also used to determine f_D_ and to investigate the presence of depletion above a density threshold. The isotopic ratios and optical depth were derived with a Bayesian approach. We find a significant number of clumps with a high degree of CO depletion, up to ~20. Larger values are found for colder clumps, thus for earlier evolutionary phases. For massive clumps in the earliest stages of evolution we estimate the radius of the region where CO depletion is important to be a few tenths of a pc. The value of the [^12^C]/[^13^C] ratio is found to increase with distance from the Galactic centre, with a value of ~66+/-12 for the solar neighbourhood. The [^18^O]/[^17^O] ratio is approximately constant (~4) across the inner Galaxy between 2kpc and 8kpc, albeit with a large range (~2-6). Clumps are found with total masses derived from dust continuum emission up to ~20 times higher than M_vir_, especially among the less evolved sources. These large values may in part be explained by the presence of depletion: if the CO emission comes mainly from the low-density outer layers, the molecules may be subthermally excited, leading to an overestimate of the dust masses. CO depletion in high-mass clumps seems to behave as in the low-mass regime, with less evolved clumps showing larger values for the depletion than their more evolved counterparts, and increasing for denser sources. The ratios [^12^C]/[^13^C] and [^18^O]/[^17^O] are consistent with previous determinations, and show a large intrinsic scatter.
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