2025A&A...703A.189G

The likely JWST detection of vibrationally excited H₃⁺ emission in Orion's irradiated disk system d203-506 raises the important question of whether cosmic-ray ionization is enhanced in disks within clustered star-forming regions, or whether alternative mechanisms contribute to H₃⁺ formation and excitation. We present a detailed model of the photodissociation region (PDR) component of a protoplanetary disk - comprising the outer disk surface and the photoevaporative wind - exposed to strong external far-ultraviolet (FUV) radiation. We investigate key gas-phase reactions involving excited H₂ that lead to the formation of H₃⁺ in the PDR, including detailed state-to-state dynamical calculations of reactions H₂(v ≥ 0) + HOC⁺ → H₃⁺ + CO and H₂(v ≥ 0) + H⁺ → H₂⁺ + H. We also consider the effects of photoionization of vibrationally excited H₂(v ≥ 4), a process not previously included in PDR or disk models. We find that these FUV-driven reactions dominate the formation of H₃⁺ in the PDR of strongly irradiated disks, largely independently of cosmic-ray ionization. The predicted H₃⁺ abundance in the disk PDR peaks at x(H₃⁺) ≳ 10^–8, coinciding with regions of enhanced HOC⁺ and water vapor abundances, and is linked to the strength of the external FUV field (G₀). The predicted H₃⁺ column density (≲10¹³ cm^–2) agrees with the presence of H₃⁺ in the PDR of d203-506. We also find that formation pumping, resulting from exoergic reactions between excited H₂ and HOC⁺, drives the vibrational excitation of H₃⁺ in these regions. We expect this photochemistry to be highly active in disks where G₀ > 10³. The H₃⁺ formation pathways studied here may also be relevant in the inner disk region (near the host star), in exoplanetary ionospheres, and in the early Universe.