2019A&A...622A.135N

Context. High-mass stars are formed within massive molecular clumps, where a large number of stars form close together. The evolution of the clumps with different masses and luminosities is mainly regulated by their high-mass stellar content and the formation of such objects is still not well understood.
Aims. In this work, we characterise the mid-J CO emission in a statistical sample of 99 clumps (TOP100) selected from the ATLASGAL survey that are representative of the Galactic proto-cluster population.
Methods. High-spatial resolution APEX-CHAMP⁺ maps of the CO (6-5) and CO (7-6) transitions were obtained and combined with additional single-pointing APEX-FLASH⁺ spectra of the CO (4-3) line. The data were convolved to a common angular resolution of 13.4". We analysed the line profiles by fitting the spectra with up to three Gaussian components, classified as narrow or broad, and computed CO line luminosities for each transition. Additionally, we defined a distance-limited sample of 72 sources within 5kpc to check the robustness of our analysis against beam dilution effects. We have studied the correlations of the line luminosities and profiles for the three CO transitions with the clump properties and investigate if and how they change as a function of the evolution.
Results. All sources were detected above 3-σ in all three CO transitions and most of the sources exhibit broad CO emission likely associated with molecular outflows. We find that the extension of the mid-J CO emission is correlated with the size of the dust emission traced by the Herschel-PACS 70 µm maps. The CO line luminosity (L_CO) is correlated with the luminosity and mass of the clumps. However, it does not correlate with the luminosity-to-mass ratio.
Conclusions. The dependency of the CO luminosity with the properties of the clumps is steeper for higher-J transitions. Our data seem to exclude that this trend is biased by self-absorption features in the CO emission, but rather suggest that different J transitions arise from different regions of the inner envelope. Moreover, high-mass clumps show similar trends in CO luminosity as lower mass clumps, but are systematically offset towards larger values, suggesting that higher column density and (or) temperature (of unresolved) CO emitters are found inside high-mass clumps.