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

TL;DR

The paper investigates H$_3^+$ formation and excitation in the externally irradiated outer disk and photoevaporative wind of the protoplanetary disk d203-506, using state-to-state quantum-dynamical rate constants for H$_2(v'')$ + HOC$^+$ and H$_2(v'')$ + H$^+$, as well as FUV photoionization of vibrationally excited H$_2$ (H$_2^*$, $v''\geq 4$). It demonstrates that FUV-driven chemistry in the PDR dominates H$_3^+$ production, largely independent of the cosmic-ray ionization rate, with H$_3^+$ peaking at $x({\rm H_3^+})\sim 10^{-8}$ and $N({\rm H_3^+})\sim 10^{13}$ cm$^{-2}$, particularly in a molecular PDR zone near the H/H$_2$ transition. Non-LTE excitation modeling shows that formation pumping from exoergic reactions can reproduce the JWST-detected H$_3^+$ lines, implying a formation temperature around $T_{\rm form} \approx 3000$ K and a ro-vibrational temperature $T_{\rm rot} \sim 1100$ K. The results imply H$_3^+$ is a diagnostic of external FUV fields ($G_0$) and disk photochemistry in strongly irradiated systems, with relevance to inner disks, exoplanet ionospheres, and early-Universe contexts.

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.

Figures (18)

• Figure 1: Physical (approximately vertical) structure of d203-506. Upper panels: Model profiles of gas density, gas temperature, and $a^{-3.5}$ grain-size distribution, showing the highest and lowest dust temperatures (corresponding to $a_{\rm min}$ and $a_{\rm max}$ grain radii, respectively) as a function of depth into the PDR. The green curve shows the density of H$_{2}^{*}$($v$$\geq$4). Lower panel: Abundance profiles of selected species. Bottom sketch: Simplified sketch illustrating the PDR component of an externally irradiated protoplanetary disk. The different colored regions correspond to the chemical zones discussed in Sect. \ref{['sec:chemistry']}.
• Figure 2: Predicted $\lambda$-dependent FUV radiation field at different depth positions from the PDR surface into the disk. We show the local FUV energy density in units of erg cm$^{-3}$ Å$^{-1}$, from the Lyman cut to 12.6 eV.
• Figure 3: Dominant H$_3^+$ formation and destruction pathways in different zones of a strongly irradiated protoplanetary disk (see Fig. \ref{['fig:chemical_model_d203-506']}). A thicker arrow represents a dominant chemical pathway. Red arrows show endoergic reactions. These reactions become fast at high $T_{\rm gas}$ and where significant H$_{2}^{*}$ exists. “CR” refers to ionization caused by cosmic rays and “$\gamma$” refers to FUV photons. Orange arrows represent ion-molecule and charge exchange reactions. Green arrows represent dissociative recombinations with electrons. Incomplete circles indicate that these are not the starting points of the chemistries.
• Figure 4: Calculated vibrational-state-specific rate constants of reaction H$_2$($v$") + H$^+$$\rightarrow$ H$_2^+$ + H (reaction~\ref{['reaction_hp']}). The dashed curve represents the thermal$^6$ rate constant determined from thermal averages of these constants.
• Figure 5: Calculated H$_2$ vibrational-state-specific rate constants of reaction H$_2$($v$") + HOC$^+$ producing H$_3^+$ + CO (left box) and HCO$^+$ + H$_2$ (right box). The dashed curves represent the thermal$^6$ rate constant determined from thermal averages of the state-dependent rates.