2021A&A...651A..94F

Context. Most stars in the Galaxy, including the Sun, were born in high-mass star-forming regions. It is hence important to study the chemical processes in these regions to better understand the chemical heritage of the Solar System and most of the stellar systems in the Galaxy.
Aims. The molecular ion HCNH⁺ is thought to be a crucial species in ion-neutral astrochemical reactions, but so far it has been detected only in a handful of star-forming regions, and hence its chemistry is poorly known.
Methods. We observed with the IRAM 30 m Telescope 26 high-mass star-forming cores in different evolutionary stages in the J=3-2 rotational transition of HCNH⁺.
Results. We report the detection of HCNH⁺ in 16 out of 26 targets. This represents the largest sample of sources detected in this molecular ion to date. The fractional abundances of HCNH⁺ with respect to H₂, [HCNH⁺], are in the range 0.9-14x10^–11, and the highest values are found towards cold starless cores, for which [HCNH⁺] is of the order of 10^–10. The abundance ratios [HCNH⁺]/[HCN] and [HCNH⁺]/[HCO⁺] are both ≤0.01 for all objects except for four starless cores, which are well above this threshold. These sources have the lowest gas temperatures and average H₂ volume density values in the sample. Based on this observational difference, we ran two chemical models, 'cold' and 'warm', which attempt to match the average physical properties of the cold(er) starless cores and the warm(er) targets as closely as possible. The reactions occurring in the latter case are investigated in this work for the first time. Our predictions indicate that in the warm model HCNH⁺ is mainly produced by reactions with HCN and HCO⁺, while in the cold model the main progenitor species of HCNH⁺ are HCN⁺ and HNC⁺.
Conclusions. The observational results indicate, and the model predictions confirm, that the chemistry of HCNH⁺ is different in cold-early and warm-evolved cores, and the abundance ratios [HCNH⁺]/[HCN] and [HCNH⁺]/[HCO⁺] can be useful astrochemical tools to discriminate between different evolutionary phases in the process of star formation.