Observed line parameters of OH⁺ (Table B.1) vs. observed line parameters of CH⁺. The symbols indicate different objects: Red diamonds for Class 0, yellow triangle up for Class I (L 1489), brown circle for intermediate mass, light blue triangle down for high-mass mid-IR quiet (NGC 6334 I(N)), green square for high-mass mid-IR bright and hot molecular core, and dark blue triangle left for high-mass ultra-compact Hii (NGC 7538 IRS1). In this and subsequent figures uncertainties of individual points are typically the size of the symbols and up to a factor of two for column densities. The Pearson correlation coefficient ρ is given only for statistically significant relations.