Detection of the HC3NH+ and HCNH+ ions in the L1544 pre-stellar core

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 10(13) cm(-2) for HCNH+ and (1.5 +/- 0.5) x 10(11) 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 10(13) cm-2, (3.6 +/- 0.9) x 10(14) cm(-2) and (3.0 +/- 1.0) x 10(14) 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+] similar or equal to 3 x 10(-10) and [HC3NH+] similar or equal to [1.5 -3.0] x 10(-12), with respect to H-2) 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.