Observed C⁺(1900 GHz) line characteristics (Table B.1) vs. object parameters given in Table 1. The line is in absorption toward the low-mass objects and in emission toward the high-mass objects. The bottom row shows the luminosity of the transition normalized for the case of point-source emission. The symbols indicate 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).