H₂ volume density (n_H2) versus kinetic temperature (T_kin) traced by the N₂H⁺ (5–4)/(3–2) line flux ratio and based on the RADEX non-LTE code (van der Tak et al. 2007). The objects where this flux ratio was possible to measure are indicated in the figure by the labeled gray curves. The curves are dashed if T_kin > T_dust and full if T_kin < T_dust, where T_dust is the dust temperature measured with Herschel (see Table A.1).