H3+ In Irradiated protoplanetary disks: Linking Far-ultraviolet radiation And Hot Water Vapor
The likely JWST detection of vibrationally excited H3+ emission in the irradiated disk d203-506, reported by Schroetter et al., raises the question of whether cosmic-ray ionization is enhanced in disks within clustered star-forming regions, or if 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 + HOC+ -> H3+ + CO and H2 + H+ -> H2+ + H.
We also consider the effects of photoionization of vibrationally excited H2(v>=4), a process that has not previously been included in 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 peaks at x(H3+)~10-8, coinciding with regions where HOC+ is also abundant. The high abundances of H3+ and HOC+ are ultimately linked to the strength of the external FUV field (G0), the presence of C+, and an enhanced reservoir of hot H2O. The predicted H3+ column densities (~1013 cm-2) are consistent with the presence of H3+ in the PDR of irradiated disks.
We also find that formation pumping, resulting from exoergic reactions between excited H2 and HOC+, drive the vibrational excitation of H3+. 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.
Javier R. Goicoechea, Octavio Roncero, Evelyne Roueff, John H. Black, Ilane Schroetter, Olivier Berné
Comments: Submitted to A&A. Comments are welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2506.05189 [astro-ph.GA] (or arXiv:2506.05189v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2506.05189
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Submission history
From: J. R. Goicoechea
[v1] Thu, 5 Jun 2025 16:02:11 UTC (1,823 KB)
https://arxiv.org/abs/2506.05189
Astrobiology, Astrochemistry,
