Dust temperature evolution along the trajectory of a parcel of gas infalling toward the central protostar during the free-fall collapse phase for Sgr B2(N2-N5) (red line). In each panel the dashed lines show the dust temperature profiles computed using RADMC-3D with different luminosities (that is at different evolutionary stages). They show how the dust temperature at a given radius in the envelope of each source increases as the total luminosity of the central protostar rises with time (as indicated in the upper right corner). The observational constraint, T_d (r₀) = T₀, used to derive the current luminosity of the source is plotted with errorbars (1σ).