Spectrum obtained at the dust continuum emission peak of the G31 Main core (black) and best fit to the CH₃ CN (12–11) K = 0 to 6 transitions (red). The spectrum has been modeled with different components: i) a background layer with homogeneous intensity to describe the dust continuum core, ii) a core layer to describe the emission, and iii) a foreground layer located in front of the core component to describe the absorption (see Sect. 5.2.2). We note that the other emission and absorption lines not fitted correspond to other species.