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

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.

Accepted for publication in A&A (20 September 2025). 19 pages including appendix

References in corpus (40)

• Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• Excess C/O and C/H in outer protoplanetary disk gas
• Atmosphere Expansion and Mass Loss of Close-Orbit Giant Exoplanets heated by Stellar XUV. II. Effects of Planetary Magnetic Field, Structuring of inner Magnetosphere
• Water vapor distribution in protoplanetary disks
• Absorption Line Survey of H3+ toward the Galactic Center Sources II. Eight Infrared Sources within 30 pc of the Galactic Center
• The penetration of FUV radiation into molecular clouds
• Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk
• Vibrational level population of H${}_2$ and H${}_2^{+}$ in the early Universe
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. I. Model and application to the HH 211 shock
• The diverse chemistry of protoplanetary disks as revealed by JWST
• Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star
• PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
• The kinematics and excitation of infrared water vapor emission from planet-forming disks: results from spectrally-resolved surveys and guidelines for JWST spectra
• The Interstellar Medium of IRAS 08572+3915 NW: H3+ and Warm High Velocity CO
• Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
• Survey of Orion Disks with ALMA (SODA) II: UV-driven disk mass loss in L1641 and L1647
• Planet formation via pebble accretion in externally photoevaporating discs
• The growth and migration of massive planets under the influence of external photoevaporation
• Formation of interstellar SH${}^{+}$ from vibrationally excited H${}_2$: Quantum study of S${}^{+}$ + H${}_2$ $\rightleftarrows$ SH${}^{+}$ + H reactions and inelastic collisions
• A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions
• Testing external photoevaporation in the $\sigma$-Orionis cluster with spectroscopy and disk mass measurements
• The Observational Anatomy of Externally Photoevaporating Planet-Forming Discs I: Atomic Carbon
• Chemical modeling of Orion Nebula Cluster disks: evidence for massive, compact gas disks with ISM-like gas-to-dust ratios
• ExoMol line lists -- L: High-resolution line lists of H${}_3^{+}$, H${}_2$D${}^{+}$, D${}_2$H${}^{+}$ and D${}_3^{+}$
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• PDRs4All XI. Detection of infrared CH${}^{+}$ and CH${}_3^{+}$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
• PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon
• The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
• Quantum study of reaction O(3P) + H2(v,j) $\to$ OH + H: OH formation in strongly UV-irradiated gas
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• On the suppression of giant planet formation around low-mass stars in clustered environments
• H${}_3^{+}$ absorption and emission in local U/LIRGs with JWST/NIRSpec: Evidence for high H${}_2$ ionization rates
• Near-resonant effects in the quantum dynamics of the H+H${}_2^{+}$ $\to$ H${}_2$+ H${}^{+}$ charge transfer reaction and isotopic variants
• A tell-tale tracer for externally irradiated protoplanetary disks: comparing the [CI] 8727 A line and ALMA observations in proplyds
• H2 ro-vibrational excitation in protoplanetary disks and its effects on the chemistry
• PDRs4All XV: CH radical and H${}_3^{+}$ molecular ion in the irradiated protoplanetary disk d203-506
• Vibrational effects in the quantum dynamics of the H + D_2^+ charge transfer reaction
• Inelastic H + H${}_3^{+}$ Collision rates and their impact in the determination of the excitation temperature of H${}_3^{+}$