Comparison among the observed profiles of H¹³CO⁺ J = 1− 0 (black histograms) and HCNH⁺ J = 3−2 (red histograms) towards, from left to right, the HMSCs, the HMPOs, and the UCHIIs detected in HCNH⁺ (Table 1). The red number in each plot indicates the multiplicative factor applied to the HCNH⁺ spectrum. The blue dashed vertical line indicates the systemic velocity used to centre the observed spectra (see e.g. Fontani et al. 2015a). The peak velocities of HCNH⁺ are listed in Table 1.