Schematic representation of the model source. The extremely compact and expanding core (H_C) is shown in blue (the 6 μm photosphere is its external surface), with gas flowing outward (the surrounding shell outflowing at ∼ − 400 km s⁻¹) in green. The starburst torus shown in magenta represents the W_C. The torus also appears to be expanding, because the far-IR lines are blueshifted (Appendix E, Fig. E.1). Both the H_C and W_C are embedded in an extended cold component (C_C), in orange. The model is axisymmetric about the line of sight.