THE PERILS OF CLUMPFIND: THE MASS SPECTRUM OF SUBSTRUCTURES IN MOLECULAR CLOUDS

We study the mass spectrum of substructures in the Perseus Molecular Cloud Complex traced by (13)CO(1-0), finding that dN/dM proportional to M(-2.4) for the standard Clumpfind parameters. This result does not agree with the classical dN/dM proportional to M(-1.6). To understand this discrepancy, we study the robustness of the mass spectrum derived using the Clumpfind algorithm. Both two- and three-dimensional Clumpfind versions are tested, using 850 mu m dust emission and (13)CO spectral-line observations of Perseus, respectively. The effect of varying threshold is not important, but varying stepsize produces a different effect for two- and three-dimensional cases. In the two- dimensional case, where emission is relatively isolated (associated with only the densest peaks in the cloud), the mass spectrum variability is negligible compared to the mass function fit uncertainties. In the three-dimensional case, however, where the (13)CO emission traces the bulk of the molecular cloud (MC), the number of clumps and the derived mass spectrum are highly correlated with the stepsize used. The distinction between "two dimension" and "three dimension" here is more importantly also a distinction between "sparse" and "crowded" emission. In any "crowded" case, Clumpfind should not be used blindly to derive mass functions. Clumpfind's output in the "crowded" case can still offer a statistical description of emission useful in intercomparisons, but the clump-list should not be treated as a robust region decomposition suitable to generate a physically meaningful mass function. We conclude that the (13)CO mass spectrum depends on the observations resolution, due to the hierarchical structure of the MC.