Column density of N₂H⁺ as a function of the estimated optical depth (Fig. 6) of its J = 1−0, F = 2−1 transition for different assumed excitation temperatures (black dashed lines). In colour we represent isotemperature curves of simulated line intensities in main beam temperature scale for the 26.4″ primary beam of the IRAM 30 m at the frequency of this transition. The curves are calculated for different source sizes: extended (continous line); two thirds of the primary beam (dashed); and one third of the primary beam (dotted). The simulation considers a line width Δv = 2 km s⁻¹, and T_bg = 2.73 K.