Chemistry and IR emission of acetylene in planet-forming regions of T Tauri disks. Impact of elemental abundances and dust properties

arxiv: 2605.18062 · v2 · pith:HRGBTORPnew · submitted 2026-05-18 · 🌌 astro-ph.EP · astro-ph.GA· astro-ph.SR

Chemistry and IR emission of acetylene in planet-forming regions of T Tauri disks. Impact of elemental abundances and dust properties

Pac\^ome Est\`eve , Beno\^it Tabone , Emilie Habart , Ewine F. van Dishoeck , Marissa Vlasblom , Inga Kamp , Aditya M. Arabhavi , Simon Bruderer This is my paper

Pith reviewed 2026-05-20 00:51 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.GAastro-ph.SR

keywords acetylene emissionT Tauri disksC/O ratioX-ray chemistryplanet-forming regionsmid-infrared linesthermochemical modelingelemental abundances

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The pith

Acetylene emission in T Tauri disks matches solar C/O ratios when X-ray driven formation balances atomic oxygen destruction.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that mid-infrared acetylene fluxes observed in T Tauri disks can be reproduced using standard solar elemental abundances rather than requiring carbon enrichment. Acetylene forms when X-rays break apart CO and builds carbon chains, but atomic oxygen released from X-ray processed water and CO destroys those chains; warm temperatures and water shielding allow the molecule to persist. The ratio of acetylene to water emission therefore depends on both the carbon-to-oxygen ratio and the total oxygen abundance, as well as the distribution of small versus large dust grains. Preliminary comparison with JWST data points toward gas-phase C/O below unity and suggests enhanced oxygen-to-hydrogen ratios may be common.

Core claim

In the warm inner regions of T Tauri disks, acetylene abundance is set by the balance between formation initiated by X-ray dissociation of CO and destruction of carbon chains by atomic oxygen generated through X-ray-induced breakdown of H2O and CO. With updates to warm carbon chemistry, UV shielding, and mutual line overlap in the DALI thermochemical model, solar C/O ratios are sufficient to match observed C2H2 fluxes. The C2H2/H2O flux ratio is sensitive to total O/H abundance and dust size distribution, making it a tracer of inner-disk elemental composition.

What carries the argument

The X-ray chemistry balance in which CO photodissociation initiates carbon-chain growth while atomic oxygen from H2O and CO destruction limits it.

Load-bearing premise

The listed model improvements plus standard X-ray rates capture the dominant formation and destruction routes without missing key reactions or incorrect coefficients that would change the acetylene balance.

What would settle it

A set of JWST spectra showing C2H2 fluxes significantly higher than model predictions across a range of dust sizes and O/H values, or showing no correlation between the C2H2/H2O ratio and inferred oxygen abundance, would challenge the claimed chemical balance.

Figures

Figures reproduced from arXiv: 2605.18062 by Aditya M. Arabhavi, Beno\^it Tabone, Emilie Habart, Ewine F. van Dishoeck, Inga Kamp, Marissa Vlasblom, Pac\^ome Est\`eve, Simon Bruderer.

**Figure 1.** Figure 1: Disk structure of the fiducial model. The top panels show the gas density, the local gas-to-dust ratio and the gas temperature. The bottom panels present the normalized UV field G0 (Habing units), the abundance of H2O and C2H2. The white lines indicate the 300 K and 700 K gas temperature contours. The bottom red solid line shows the dust optically thick surface (τdust = 1 at 14 µm) while the dashed red lin… view at source ↗

**Figure 2.** Figure 2: DALI synthetic spectrum of C2H2 (red) and H2O (blue) for the fiducial model. The total spectrum is shown in black. The spectral resolution is λ/∆λ = 2000 to mimic a JWST/MIRI spectrum. Although the fiducial model is richer in oxygen, the C2H2 feature stands out clearly from the forest of water lines. the carbon released by CO dissociation, and destruction by the oxygen. This balance reveals that C2H2 is no… view at source ↗

**Figure 3.** Figure 3: Top panel: Vertical cut at r = 0.15 au showing the abundances of several key atoms and molecules. Middle panel: Irradiation conditions in this vertical cut, with G H2O shield 0 showing the UV field attenuated by water absorption (water shielding). The grey line G0 indicates what the UV field would be without the water UV shielding. Bottom panel: Gas and dust temperature, with the gas density nH in blue. Wa… view at source ↗

**Figure 4.** Figure 4: Left: Result from the fiducial grid. Orange, Green and blue points correspond to gd = 102 ,103 and 104 respectively. The black lines highlight a constant C/O ratio. Middle: Result obtained for the "enhanced O/H" grid: O/H×10 (C/H is scaled with the C/O ratio). The fiducial grid is overlaid in grey for reference. Right: Results for the disk aspect ratio hC (orange), power law index of the dust distribution … view at source ↗

**Figure 5.** Figure 5: Evolution of the line flux ratio C2H2/H2O with the elemental abundances (C/O and O/H), gas-to-dust mass ratio gd, power law index q and disk aspect ratio hC from left to right respectively. Fiducial model with C/O=0.47 adopted in c), d) and e) except for parameters that is varied. C2H2 H2O O/H x10 Fiducial Enhanced O/H Solar C/O Solar C/O [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗

**Figure 6.** Figure 6: DALI synthetic spectra of the fiducial model (left panel) and a model with O/H x10 (right panel). The Q-branch of C2H2 (black) and the water lines (blue) used for the line flux around 17.25 µm are shown together for clarity. The carbon and oxygen budget itself acts similarly to a decrease in the C/O ratio: the flux of water increases while acetylene decreases. O/H grid in Appendix D). Our results confirm t… view at source ↗

**Figure 7.** Figure 7: Vertical cut at r = 0.15 au showing the abundance of C2H2 (top) and the corresponding spectrum (bottom) for 2 chemical networks: the fiducial (in red) and the one based on UMIST only (including also threebody reactions and C + H2O −→ HCO + H). Our models are based on an extended chemical network, combining reactions from the latest version of UMIST (Millar et al. 2023, RATE22) and KIDA (Wakelam et al. 20… view at source ↗

**Figure 9.** Figure 9: Vertical column densities at r = 0.15 au for hydrocarbons. The total vertical column density is shown in blue. The red dots represent the emitting column density (above the surface τdust = 1 at 14 µm and Tgas > 500 K). While CH4 is the major hydrocarbon, C2H2 is dominant in the emitting layers. 4.2. Hydrocarbons beyond acetylene C2H2 is detected in nearly all disks around T Tauri, according to Spitzer (Po… view at source ↗

**Figure 10.** Figure 10: DALI synthetic spectra for two dust settling prescriptions. Left: fiducial spectrum with self-consistent dust settling (Riols settling, Riols & Lesur 2018). Right: spectrum obtained with the two-pop settling prescription, based on two dust population (small - large). The two-pop settling reduces water emission by a factor 2. . Our results show that molecular emissions are sensitive to dust properties. Th… view at source ↗

**Figure 11.** Figure 11: Same as [PITH_FULL_IMAGE:figures/full_fig_p013_11.png] view at source ↗

**Figure 13.** Figure 13: Temperature and effective radius (corresponding to an emitting area πR 2 ) of C2H2 emission retrieved from slab models on DALI predicted spectra. Red crosses indicate DALI models with solar C/O and gd = 102 , 103 , 104 , while the green cross corresponds to C/O = 1.5 and gd = 103 . JWST observations of 2 disks are shown in grey (Grant et al. 2023; Colmenares et al. 2024). thick in the inner disk. Detaile… view at source ↗

read the original abstract

(Abridged) We aim to explore the parameters that influence the mid-infrared emission of C$_2$H$_2$ and H$_2$O, and if the spread observed in $F\rm{_{C_2H_2}}$/$F\rm{_{H_2O}}$ is tracing a variation of the C/O ratio. Our work is based on the DALI 2D thermochemical model to predict spectra readily comparable to JWST/MIRI observations. To robustly model organics in inner disks, several improvements have been made: (1) carbon chemistry adapted for warm environments, (2) updated UV shielding treatment, and (3) mutual line overlap in the raytracing. We are able to reproduce the observed C$_2$H$_2$ fluxes of T Tauri disks with a solar C/O ratio. Acetylene abundance is primarily set by a balance between formation initiated by CO dissociation by X-rays and destruction of carbon chains by atomic oxygen, the latter being generated by X-ray-induced destruction of H$_2$O and CO. The water UV shielding and hot temperatures of the inner disk also favor acetylene formation, as they prevent the destruction of carbon chains and allow overcoming activation barriers of reactions with H$_2$. C$_2$H$_2$ and H$_2$O emissions are not only sensitive to the C/O ratio but also to the total O/H elemental abundance, supporting recent claims. In particular, we find that enhanced O/H reduces acetylene emission due to an excess of atomic oxygen. $F_{\rm{C_2H_2}}$/$F_{\rm{H_2O}}$ is thus a promising tracer of the elemental composition of inner disks. Still, the dust size distribution also plays a key role in this line flux ratio. We find that increasing the abundance of small grains relative to large grains favors C$_2$H$_2$ flux over H$_2$O flux. Grain depletion does not affect the line flux ratio as previously suggested by observational works. A preliminary comparison with published JWST observations indicates a gas-phase C/O ratio below unity and suggests that enhanced O/H ratios may be common in T Tauri disks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper updates DALI with warm carbon chemistry and better shielding to show solar C/O can match observed C2H2 fluxes in T Tauri disks, while also tying the C2H2/H2O ratio to O/H and small-grain abundance.

read the letter

The core result is that an updated DALI model reproduces the acetylene mid-IR fluxes seen in T Tauri disks at solar C/O. Acetylene forms mainly after X-ray dissociation of CO starts carbon-chain growth, and it is destroyed by atomic oxygen released from X-ray processing of H2O and CO. The water UV shielding and high inner-disk temperatures help by protecting carbon chains and letting reactions overcome activation barriers with H2. They also show that raising total O/H cuts acetylene emission through extra atomic oxygen, and that boosting the small-grain fraction relative to large grains increases C2H2 flux over H2O flux. Grain depletion itself does not change the ratio the way some observations had suggested. These are concrete, quantitative findings that observers can use when looking at JWST/MIRI spectra. The three listed code improvements—warm carbon network, updated UV shielding, and mutual line overlap—are the main technical steps that make the modeling more suitable for the inner disk. The formation-destruction balance follows from literature rates rather than being imposed by hand. That said, the result still depends on the accuracy of standard X-ray dissociation rates and on whether the warm carbon network captures the dominant channels under inner-disk conditions. Activation barriers, branching ratios, or missing reactions could shift the steady-state abundances by factors of a few, and the paper does not present independent checks or cross-model comparisons for these specific conditions. The elemental abundances and grain ratios are varied to match the data, so the exercise is more diagnostic than fully predictive. This paper is for people who model disk chemistry or who are trying to read elemental composition from JWST spectra of planet-forming regions. A reader working on chemical tracers or on the link between disk gas and planet composition will find usable numbers and clear parameter explorations. It deserves a serious referee because the modeling updates are explicit, the application to current observations is timely, and the claimed sensitivities can be tested against more data. I would send it to review with a request to document the chemical network choices and any sensitivity tests on the X-ray rates.

Referee Report

2 major / 2 minor

Summary. The manuscript presents an updated DALI 2D thermochemical model for the chemistry and mid-IR emission of C₂H₂ and H₂O in the inner regions of T Tauri protoplanetary disks. Key updates include adapting the carbon chemistry network for warm conditions, revising the UV shielding treatment, and incorporating mutual line overlap in the ray-tracing. The authors report that observed C₂H₂ fluxes can be reproduced with a solar C/O ratio, with acetylene abundance governed by a balance between X-ray-driven CO dissociation (initiating carbon-chain formation) and destruction by atomic oxygen (produced via X-ray processing of H₂O and CO). They further show that the C₂H₂/H₂O flux ratio depends on total O/H abundance and the small-to-large grain ratio, proposing the ratio as a tracer of inner-disk elemental composition, with a preliminary JWST comparison suggesting sub-unity C/O and possibly enhanced O/H.

Significance. If the chemical network and parameter choices are robust, the work supplies a physically grounded interpretation of JWST/MIRI observations of organics in planet-forming disks. It supports solar C/O in the inner disk while demonstrating that total oxygen abundance and dust size distribution exert strong control on line fluxes. The explicit inclusion of warm-carbon adaptations, updated shielding, and line overlap constitutes a concrete modeling advance that can be adopted by other groups.

major comments (2)

[Abstract and chemical network description] Abstract and the section describing the chemical network updates: the headline result—that solar C/O suffices to reproduce observed C₂H₂ fluxes—rests on the assertion that the formation–destruction balance is controlled by X-ray CO dissociation versus atomic-O attack. No quantitative test (e.g., rate-of-production analysis or network truncation experiment) is shown to confirm that the three listed improvements plus standard X-ray rates capture the dominant channels under inner-disk temperatures and irradiation; missing activation barriers, branching ratios, or three-body channels could shift the steady-state abundance by factors of several.
[Parameter variation and flux-ratio results] Section on parameter exploration and flux-ratio results: the reproduction with solar C/O is achieved while freely varying O/H and the small-to-large grain abundance ratio. It is not demonstrated whether a modestly super-solar C/O could be compensated by a different O/H or grain ratio to yield equally acceptable fits; without such a degeneracy test the claim that solar C/O is preferred remains under-constrained.

minor comments (2)

[Methods] In the methods, the precise numerical values adopted for the updated UV shielding cross-sections and the mutual line-overlap treatment should be tabulated or referenced to the exact literature sources so that the changes are fully reproducible.
[Figures] Figure captions comparing model and observed fluxes should explicitly state the observational uncertainties and the wavelength integration windows used for the synthetic fluxes.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive review of our manuscript. We have addressed both major comments below with point-by-point responses, agreeing to incorporate additional analyses in the revised version to strengthen the presentation of our results.

read point-by-point responses

Referee: [Abstract and chemical network description] Abstract and the section describing the chemical network updates: the headline result—that solar C/O suffices to reproduce observed C₂H₂ fluxes—rests on the assertion that the formation–destruction balance is controlled by X-ray CO dissociation versus atomic-O attack. No quantitative test (e.g., rate-of-production analysis or network truncation experiment) is shown to confirm that the three listed improvements plus standard X-ray rates capture the dominant channels under inner-disk temperatures and irradiation; missing activation barriers, branching ratios, or three-body channels could shift the steady-state abundance by factors of several.

Authors: We thank the referee for this valuable suggestion. Although the chemical network draws from established literature rates (including warm-condition adaptations), we agree that an explicit quantitative validation of the dominant pathways would reinforce the interpretation. In the revised manuscript we will add a rate-of-production analysis for C₂H₂ under representative inner-disk conditions, explicitly showing the contributions of X-ray-driven CO dissociation and atomic-oxygen destruction. We have already verified that the included reactions account for the relevant activation barriers and that three-body channels remain negligible at the densities considered; this new analysis will be presented in an expanded methods or results subsection. revision: yes
Referee: [Parameter variation and flux-ratio results] Section on parameter exploration and flux-ratio results: the reproduction with solar C/O is achieved while freely varying O/H and the small-to-large grain abundance ratio. It is not demonstrated whether a modestly super-solar C/O could be compensated by a different O/H or grain ratio to yield equally acceptable fits; without such a degeneracy test the claim that solar C/O is preferred remains under-constrained.

Authors: We appreciate the referee’s point on potential degeneracies. Our existing grid demonstrates that the C₂H₂/H₂O flux ratio responds strongly to total O/H and the small-grain fraction even at fixed solar C/O. To directly test for compensation, the revised manuscript will include an additional suite of models with modestly super-solar C/O (e.g., 1.1–1.3) while varying O/H and grain ratios to assess whether comparable flux matches can be obtained. We will discuss the outcome in the results section and clarify the extent to which solar C/O remains a preferred solution or whether trade-offs exist. revision: yes

Circularity Check

0 steps flagged

No significant circularity; model derives abundances from literature rates and thermochemical balance

full rationale

The paper computes acetylene abundances via the DALI 2D model using standard X-ray dissociation rates, updated warm carbon chemistry, UV shielding, and line overlap. The formation-destruction balance (CO dissociation feeding carbon chains vs. atomic-O destruction from H2O/CO processing) follows directly from these rates and physical conditions rather than being defined in terms of the target fluxes. Elemental abundances and dust properties are explored parametrically to identify matches, but this does not reduce the central reproduction claim to a tautology or self-referential fit. No self-citations, uniqueness theorems, or ansatzes are invoked as load-bearing steps in the provided text. The derivation chain remains independent of the observations it is compared against.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The central results rest on standard astrochemistry reaction networks, X-ray dissociation rates, and the assumption that the inner-disk temperature and density structure are correctly predicted by the DALI radiative transfer. Elemental abundances and dust size distributions are treated as free parameters that are adjusted to match observations.

free parameters (3)

C/O ratio
Set to solar value to reproduce observed C2H2 fluxes
O/H elemental abundance
Explored at enhanced values to explain reduced acetylene emission
small-to-large grain abundance ratio
Varied to show effect on C2H2 versus H2O flux ratio

axioms (2)

domain assumption Literature reaction rates for warm carbon chemistry and X-ray induced processes are accurate
Invoked to set the formation and destruction balance of acetylene
domain assumption DALI 2D temperature and density structure correctly represents inner-disk conditions
Required for the ray-tracing and chemistry calculations

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Acetylene abundance is primarily set by a balance between formation initiated by CO dissociation by X-rays and destruction of carbon chains by atomic oxygen, the latter being generated by X-ray-induced destruction of H2O and CO.

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