Observed H₃O⁺(1031 GHz) line characteristics (Table B.1) vs. object parameters given in Table 1. The line is in emission. The bottom row shows the line luminosity multiplied with distance squared (normalized to 1 pc) to correct in case of point-source emission. The symbols mark different types of 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).