Grid results for SiO RT models for four of the sample stars. The 1, 2, and 3σ contours in the χ² space are shown for ²⁸SiO (red), ²⁹SiO (green), and ³⁰SiO (blue). The thick lines mark the 1σ contours, while the thin lines indicate the 2σ and 3σ contours. The hatched region represents the parameter space where the 1σ contours of the three isotopologues overlap. The squares mark the ‘best-fit’ models for the three isotopologues. The x-axis ticks show ²⁸SiO abundances. The ²⁹SiO and ³⁰SiO abundances, at each point of the grid, are given by the corresponding ²⁸SiO abundance divided by 20 and 30, respectively (see Sect. 3.2).