Clump mass versus source radius. The shaded area marks the regime where only low-mass stars form, as determined by Kauffmann et al. (2010). The black dashed line marks the lower limit for effective high-mass star formation as determined by Urquhart et al. (2014c), indicating a surface density threshold of 0.05 g cm⁻². The horizontal gray dotted line marks the threshold of 8 M_⊙, above whicha clump is considered massive. The colored dash-dotted lines are linear fits to the corresponding subsamples, and the cyan dash-dotted line is a fit to the full sample.