Spatial association of clumps from different tracers turns out to be a valuable tool to determine the physical properties of molecular clouds .
It provides a reliable estimate for the X -factors , serves to trace the density of clumps seen in column densities only and allows to measure the velocity dispersion of clumps identified in dust emission .
We study the spatial association between clump populations , extracted by use of the Gaussclumps technique from ^ { 12 } CO ( 1 - 0 ) , ^ { 13 } CO ( 1 - 0 ) line maps and Herschel dust-emission maps of the star-forming region Rosette , and analyse their physical properties .
All CO clumps that overlap with another CO or dust counterpart are found to be gravitationally bound and located in the massive star-forming filaments of the molecular cloud .
They obey a single mass-size relation M _ { cl } \propto R _ { cl } ^ { \gamma } with \gamma \simeq 3 ( implying constant mean density ) and display virtually no velocity-size relation .
We interpret their population as low-density structures formed through compression by converging flows and still not evolved under the influence of self-gravity .
The high-mass parts of their clump mass functions are fitted by a power law { d } N _ { cl } / { d } \log M _ { cl } \propto M _ { cl } ^ { \Gamma } and display a nearly Salpeter slope \Gamma \sim - 1.3 .
On the other hand , clumps extracted from the dust-emission map exhibit a shallower mass-size relation with \gamma = 2.5 and mass functions with very steep slopes \Gamma \sim - 2.3 even if associated with CO clumps .
They trace density peaks of the associated CO clumps at scales of a few tenths of pc where no single density scaling law should be expected .