2025A&A...696A..99Z

Context. The methylidyne cation (CH⁺) and the methyl cation (CH₃⁺) are building blocks of organic molecules in the ultraviolet (UV) irradiated gas, yet their coupled formation and excitation mechanisms mostly remain unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules.
Aims. Our goal is to use the first detection of CH⁺ and CH₃⁺ infrared rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506 to probe their formation and excitation mechanisms and constrain the physico-chemical conditions of the environment.
Methods. We used spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the infrared CH⁺ and CH₃⁺ spatial distribution at very small scales (down to 0.1'') and compared it to excited H₂ emission. We studied their excitation in detail, and in the case of CH⁺, we compared the observed line intensities with chemical formation pumping models based on recent quantum dynamical calculations. Throughout this study, we compare the emission of these molecules in two environments: the Bar a photodissociation region - and a protoplanetary disk (d203-506), both of which are irradiated by the Trapezium cluster.
Results. We detected CH⁺ and CH₃⁺ vibrationally excited emission both in the Bar and d203-506. These emissions originate from the same region as highly excited H₂ (high rotational and rovibrational levels) and correlate less with the lower rotational levels of H₂ (J' < 5) or the emission of aromatic and aliphatic infrared bands. Our comparison between the Bar and d203-506 revealed that both CH⁺ and CH₃⁺ excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH⁺ and CH₃⁺ rovibrational lines is around T ∼ 1500 K in the Bar and T ∼ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1% of the total known (CH⁺) and expected (CH₃⁺) column densities. These different results show that CH⁺ and CH₃⁺ level populations strongly deviate from local thermodynamical equilibrium. The CH⁺ rovibrational supra-thermal emission (v = 1 and v = 2) can be explained by chemical formation pumping with excited H₂ via C⁺ + H₂^* = CH⁺ + H. The difference in the population distribution of the H₂^* energy levels between the Orion Bar and d203-506 then result in different excitation temperatures. These results support a gas phase formation pathway of CH⁺ and CH₃⁺ via successive hydrogen abstraction reactions. However, we do not find any evidence of CH₃⁺ emission in the JWST spectrum, which may be explained by the fact its spectroscopic signatures could be spread in the JWST spectra. Finally, the observed CH⁺ intensities coupled with a chemical formation pumping model provide a diagnostic tool to trace the local density.
Conclusions. Line emission from vibrationally excited CH⁺ and CH₃⁺ provides new insight into the first steps of hydrocarbon gas-phase chemistry in action. This study highlights the need for extended molecular data of detectable molecules in the interstellar medium in order to analyze the JWST observations.