2025A&A...699L..13S

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present a detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATMOL, which we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and the likely detection of the H$ _{3}^{+} $ molecular ion, with estimated abundances of a few times 10^–7 and approximately 10^–8, respectively. The presence of CH is predicted by gas-phase models and is well explained by hydrocarbon photochemistry. Interstellar H$ _{3}^{+} $ is usually formed through reactions of H₂ with H$ _{2}^{+} $ originating from cosmic ray ionization of H₂. However, recent theoretical studies suggest that H$ _{3}^{+} $ also forms through far-UV (FUV)-driven chemistry in strongly irradiated (G₀ > 10³), dense (n_H > 10⁶ cm^–3) gas. The latter is favored as an explanation for the presence of hot H$ _{3}^{+} $ (T_ex ≳ 1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H₂ and other photodissociation region (PDR) species, such as CH⁺, CH$ _{3}^{+} $, and OH. Our detection of infrared emission from vibrationally excited H$ _{3}^{+} $ and CH raises questions about their excitation mechanisms and underscores that FUV radiation can have a profound impact on the chemistry of planet-forming disks. They also demonstrate the power of JWST to push the limit of the detection of elusive species in protoplanetary disks.