Two-stage physical model assumed for the time evolution of Sgr B2(N2-N5). The physical properties derived at the boundaries of each stage are shown in boxes (see also Table 2). The parameters that are common to all sources are highlighted in boldface. The evolution of the physical parameters during each stage is indicated in green with arrows. The evolution of T_d during the first stage depends on the adopted minimum dust temperature (see Sect. 3.5.1).