Radius vs. envelope mass of our protostellar (blue dots), prestellar (red dots) sources at the bar. In the small windows, the same trend for the rest of the sample is also shown. The green solid and the purple dashed areas are the loci with medium densities higher than 1 g/cm² and 0.2 g/cm², respectively, where massive star formation starts. The yellow shaded area indicates the region of young massive clusters progenitors. To have a broad picture of the loci occupied by objects of a different nature, we also show the GMC (light blue stars) located in the analyzed regions, as calculated and reported in Sect. 4 and Table 4.