Spatially associated clump populations in Rosette from CO and dust maps

Abstract

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 $M_{\rm cl}\propto R_{\rm cl}^γ$ with $γ\simeq3$ (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 ${\rm d}N_{\rm cl}/{\rm d}\,\log M_{\rm cl}\propto M_{\rm cl}^Γ$ and display a nearly Salpeter slope $Γ\sim-1.3$. On the other hand, clumps extracted from the dust-emission map exhibit a shallower mass-size relation with $γ=2.5$ and mass functions with very steep slopes $Γ\sim-2.3$ 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.