2017MNRAS.470.3194Q

The L1544 pre-stellar core was observed as part of the ASAI (Astrochemical Surveys At IRAM) Large Program. We report the first detection in a pre-stellar core of the HCNH⁺ and HC₃NH⁺ ions. The high spectral resolution of the observations allows us to resolve the hyperfine structure of HCNH⁺. Local thermodynamic equilibrium (LTE) analysis leads to derive a column density equal to (2.0 ± 0.2) x 10¹³ cm^–2 for HCNH⁺ and (1.5 ± 0.5) x 10¹¹ cm^–2 for HC₃NH⁺. We also present non-LTE analysis of five transitions of HC₃N, three transitions of H¹³CN and one transition of HN¹³C, all of them linked to the chemistry of HCNH⁺ and HC₃NH⁺. We computed for HC₃N, HCN and HNC a column density of (2.0 ± 0.4) x 10¹³ cm^–2, (3.6 ± 0.9) x 10¹⁴ cm^–2 and (3.0 ± 1.0) x 10¹⁴ cm^–2, respectively. We used the gas-grain chemical code NAUTILUS to predict the abundances of all these species across the pre-stellar core. Comparison of the observations with the model predictions suggests that the emission from HCNH⁺ and HC₃NH⁺ originates in the external layer where non-thermal desorption of other species was previously observed. The observed abundance of both ionic species ([HCNH⁺] ≃ 3 x 10^–10 and [HC₃NH⁺] ≃ [1.5 - 3.0] x 10^–12, with respect to H₂) cannot be reproduced at the same time by the chemical modelling within the error bars of the observations only. We discuss the possible reasons for the discrepancy and suggest that the current chemical models are not fully accurate or complete. However, the modelled abundances are within a factor of 3, consistent with the observations, considering a late stage of the evolution of the pre-stellar core, compatible with previous observations.