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

TL;DR

This study models how strong external FUV radiation drives H3+ formation in irradiated protoplanetary disks, linking it to water vapor and vibrational excitation, independent of cosmic rays, with implications for disk chemistry and observations.

Contribution

It introduces detailed state-to-state reaction calculations and considers photoionization of vibrationally excited H2, revealing FUV-driven H3+ formation dominates in strongly irradiated disks.

Findings

H3+ abundance peaks at ~1E-8 in the PDR

H3+ column density matches observations (~1E13 cm^-2)

FUV-driven reactions dominate H3+ formation, independent of cosmic rays

Abstract

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+)~1E-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 (~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+, drives 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.