H3+in irradiated protoplanetary disks: Linking far-ultraviolet radiation and water vapor

Journal article, 2025

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 (G 0). 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 G 0 > 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.