📝 SummarXiv

Abstract

The likely JWST detection of vibrationally excited H3+ emission in Orion's irradiated disk system d203-506, reported by Schroetter et al., raises an important question: is cosmic-ray ionization enhanced in disks within clustered star-forming regions, or do 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+) ~ 1E-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 (~1E13 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+, drive the vibrational excitation of H3+ in these regions. We expect this photochemistry to be highly active in disks where G_0 > 1E3. 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.