Top panel: time evolution of the volume density profile of the stellar component for the model of Sect. 2.2: M₀ = 10⁴   M_⊙, R = 6   pc, ϵ_ff = 0.1 along with p₀ = 1.7. Model time spans t are given in the key. Bottom panel: time evolution of the corresponding surface density profiles superimposed with the surface density limit, Σ_bck, imposed by the stellar background against which the star-forming region is projected (Carpenter et al. 2000). The shaded area shows how the surface density threshold conceals the “wings” of the stellar component, leading to observed radii (red diamonds) smaller than the actual one, i.e. s_bck < R.