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Column-density maps of molecular clouds are one of the most importantobservables in the context of molecular cloud- and star-formation (SF) studies.With the Herschel satellite it is now possible to determine the column densityfrom dust emission. We use observations and simulations to demonstrate how LOScontamination affects the column density probability distribution function(PDF). We apply a first-order approximation (removing a constant level) to themolecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreementwith the simulations, we find that the PDFs become broader, the peak shifts tolower column densities, and the power-law tail of the PDF flattens aftercorrection. All PDFs have a lognormal part for low column densities with a peakat Av~2, a deviation point (DP) from the lognormal at Av(DP)~4-5, and apower-law tail for higher column densities. Assuming a density distributionrho~r^-alpha, the slopes of the power-law tails correspond to alpha(PDF)=1.8,1.75, and 2.5 for Auriga, Carina, and NGC3603 (alpha~1.5-2 is consistentgravitational collapse). We find that low-mass and high-mass SF clouds displaydifferences in the overall column density structure. Massive clouds assemblemore gas in smaller cloud volumes than low-mass SF ones. However, for bothcloud types, the transition of the PDF from lognormal shape into power-law tailis found at the same column density (at Av~4-5 mag). Low-mass and high-mass SFclouds then have the same low column density distribution, most likelydominated by supersonic turbulence. At higher column densities, collapse andexternal pressure can form the power-law tail. The relative importance of thetwo processes can vary between clouds and thus lead to the observed differencesin PDF and column density structure.

Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure