Line and continuum fluxes of the sources, compared to grids of centrally heated models for typical sources parameters (D = 4 kpc, ΔV = 4 km s^-1, R_eff = 1 pc, n ∝ r^-1.5), convolved with the appropriate beam. The size of the symbol is proportional to the logarithm of MSX flux in band E (21 μm). The grids include a central object heating the gas, and differ for depletion factors identified with different colours, indicated above the panels (blue: f_D = 1, red: f_D = 2, green: f_D = 4). In each panel, the dashed lines connect models with constant luminosity of the central object (three lines, from 10²L_⊙ to 10⁵L_⊙) and varying mass. In a similar way, the dotted lines connect models with the same mass (six lines from ~ 200 M_⊙ to ~ 45 000 M_⊙) and varying luminosity of the central object. Panels a) to c) refer to subsample S1, panels d) to f) to subsample S2.