2024A&A...681L..19B

The protonated form of hydrogen cyanide, HCNH⁺, holds significant importance in astrochemistry, serving as an intermediate species in ion-neutral reactions occurring in the cold molecular clouds. Although it plays a crucial role in the chemistry of HCN and HNC, the excitation rate coefficients of this molecular cation by the dominant interstellar colliders have not been thoroughly investigated, leading to limitations in the radiative transfer models used to derive its abundance. In this work, we present the first hyperfine-resolved excitation rate coefficients for HCNH⁺ induced by collisions with both He and H₂ at low temperatures, addressing a crucial requirement for precise modeling of HCNH⁺ abundance in typical cold dense molecular clouds. Using non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations, we reproduced the 1 → 0 and 2 → 1 observational spectra of HCNH⁺ fairly well and derived updated molecular column densities. For the TMC-1 molecular cloud, the new HCNH⁺ abundance is twice as large as suggested by previous LTE modeling, whereas the column density of this molecular cation is improved only by 10% in the case of the L483 proto-star. The factor of two in the case of TMC-1 most likely arises from an error in the early analysis of observational spectra rather than an effect of the LTE assumption, given that the HCNH⁺ lines are predominantly thermalized at densities higher than 2 × 10⁴ cm^–3. For multiline studies of clouds of moderate densities, we strongly recommend using the collisional rate coefficients reported in this work.