Left panel: comparison between N_tot (HCNH⁺) and N_tot (HCN). The latter was derived from the H13CN J = 1− 0 line (Colzi et al. 2018b), assuming the same T_ex, by correcting it for the ¹²C/¹³C ratio obtained at the Galactocentric distance of the sources from the trend of Yan et al. (2019). N_tot (HCNH⁺) has been rescaled to the (larger) beam of the H13CN observations, ~ 28′′. Red stars are HMPOs; blue stars are UCHIIs; black symbols are HMSCs (filled circles for quiescent,stars for warm; see Table 1). Triangles indicate upper limits on N_tot (HCNH⁺). The dashed line is the locus where N_tot (HCNH⁺) = 0.01 N_tot (HCN). Middle panel: same as left panel, but for the fractional abundances with respect to H₂ calculated on a beam of ~28′′. The purple rectangle indicates the range of abundances that reproduces the observed [HCN], and [HCNH⁺] in the predictions of the WM (see Sect. 5 and right panel of Fig. 7). Similarly, the cyan square indicates the prediction of the CM (see Sect. 5 and left panel of Fig. 7). Right panel: comparison between the line FWHM of HCNH⁺, derived in this work from the J = 3−2 line, and H13 CN, derived by Colzi et al. (2018a) from the J = 1−0 line at ~87 GHz. The symbols have the same meaning as those in the left and middle panels.