The structure of molecular clouds and the universality of the clump mass function

Using a smoothed particle hydrodynamics (SPH) simulation of a star-forming region in a molecular cloud, we show that the emergence of a clump mass function (CMF) resembling the stellar initial mass function (IMF) is a ubiquitous feature of molecular cloud structure, but caution against its overinterpretation. We employ three different techniques to extract the clumps in this study. In the first two, we interpolate the SPH particle data to two- and three-dimensional grids before performing the clump-find, using position-position (PP) and position-position-velocity (PPV) information, respectively. In the last technique, the clump-finding is performed on the SPH data directly, making use of the full three-dimensional position information. Although the CMF is typically similar to that observed in regions of nearby star formation, the individual clumps and their masses are found to be unreliable since they depend strongly on the parameters and the method of the clump-finding. In particular, we find that the resolution and orientation of the data make a significant difference to the resulting properties of the identified clumps in the PP and PPV cases. We conclude that making comparisons between a CMF and the stellar IMF should be done with caution, since the definition of a clump boundary, and hence the number of clumps and their properties, is arbitrary in the extraction method. This is especially true if molecular clouds are truly scale free.