a) SiO column density estimated from the LTE assumption and the (2−1) transition versus L_bol/M. b) SiO abundance for the same sample versus L_bol/M. Triangles show abundance estimates for a few sources where the H¹³CO⁺ 1–0 line could be used as an alternative estimate for H₂ column density. These points follow the same trend as the abundance estimate for the entire sample based on dust.