Observed inversion lines of NH₃ and ¹⁵NH₃. The species and the quantum numbers are indicated at the top of each panel. None of these lines is affected by RFI. The abscissa is the rest frequency in MHz adopting a v_{LS R} of 5.83 km s⁻¹ (Cernicharo et al. 2020a). The ordinate is the antenna temperature in milli-Kelvin. For the (2,2) inversion line the green scale corresponds to the intensity in milli-Kelvin but with a zoom of a factor 10 for the data and the synthetic model. The red line corresponds to the computed theoretical spectrum described in the text. Two velocity components have been considered for all ammonia lines (5.8 km s⁻¹ and 6.5 km s⁻¹).