Alternative input ²⁸SiO abundance profiles for Model 3 which produce similar modelled spectra. The solid black curve is the same two-step abundance profile as in Model 3, which is close to the minimum possible abundance to fit the ALMA spectra. The three other coloured curves are alternative abundance profiles also using two-step functions. Abundance profile 1 (blue, dashed) has an inner abundance of 1 × 10^-6 up to the radius of ~6 R_⋆ and an outer abundance of 1 × 10^-7; profile 2 (green, dash-dotted) has an inner abundance of 8 × 10^-7 up to ~10 R_⋆ and an outer abundance of 1 × 10^-8; and profile 3 (red, dotted) has an inner abundance of 1 × 10^-6 up to ~4 R_⋆ and an outer abundance of 1 × 10^-7.