{"id":"851d48e1-593f-4e13-b750-7dba4f80a8ce","arxiv_id":"2505.20427","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Modeling irreversible thermal decomposition of refractory organics into C2H2 lets carbon-rich gas diffuse to about 7 au and raises inner-disk gas-phase C/H and C/O ratios.","lead":"Refractory carbon dust that heats up near a young star is assumed to break apart irreversibly into carbon gas, which can then diffuse outward through the disk instead of being trapped. This modeled mechanism raises the gas-phase C/O ratio over several astronomical units and offers a way to interpret carbon-rich inner disks seen by Spitzer and JWST.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"At Sc=0.7, a value within the range of published MHD estimates, the C2H2 reaching 7 au drops about fivefold and gas-phase C/O outside the water iceline falls below unity; the abstract's quantitative claim is therefore hostage to the poorly constrained Schmidt number.","rationale":"The reader's weakest assumption identifies the Schmidt number as the main source of uncertainty, and the manuscript's own sensitivity study in Section 4.4 supports that identification. At Sc = 0.7, a value consistent with published MHD simulations cited in the paper, the C2H2 reaching 7 au is reduced by roughly a factor of 5 and the gas-phase C/O outside the water iceline drops below unity. This directly qualifies the abstract's claims of survival to 7 au and a significantly enhanced C/O ratio. The order-of-magnitude increase in the carbon-rich gas reservoir may nevertheless survive at Sc = 0.7, because the baseline 'no thermal decomposition' model has essentially no C2H2 outside the organics line; the fragility is specifically in the C/O greater than unity outside the water iceline. The paper's mitigation, reducing the initial CO2 fraction from 10% to 1%, is a plausible but unconstrained adjustment. Therefore the paper is correctly assessed as CONDITIONAL: the mechanism is physically reasonable and clearly presented, but the headline quantitative outcome depends on a parameter whose value is not settled. My stress-test agrees with the reader's conditional verdict and does not find grounds to move it to accept or reject.","tokens_in":30321,"tokens_out":5929,"duration_ms":61580,"concrete_test":"Reproduce Fig. 8 with the authors' trace-gas implementation (Eq. 4) at Sc = 0.7 and extend the simulation to 5 Myr, computing the time- and radius-resolved gas-phase C/O and the total C2H2 mass; if C/O above unity outside the water iceline never develops at Sc = 0.7, then the observable implication is conditional on Sc < 0.7, and the abstract should be revised to state that condition explicitly.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The abstract's central claim bundles two results: (i) a carbon-rich gas reservoir increased by an order of magnitude and surviving out to 7 au, and (ii) a significant enhancement of the gas-phase C/O ratio, including C/O above unity outside the water iceline. Both are obtained with the standard chemcomp gas-evolution equation (Eq. 3), which fixes Sc = 1/3. The authors' own Section 4.4 shows that at Sc = 0.7—close to the value 0.85 from Johansen & Klahr (2005), which they cite—the C2H2 surface density at 7 au drops by a factor of about 5 and the gas-phase C/O outside the water iceline becomes smaller than unity. The mitigation shown in Fig. 8 (lowering CO2 from 10% to 1% of carbon) is a parameter adjustment, not an empirical constraint on Sc. Thus the headline quantitative claim, as written, is not robust to the plausible range of Sc. The paper itself flags this sensitivity, which is transparent, but the abstract does not qualify it. If Sc ≥ 0.7 in real disks, the order-of-magnitude C2H2 reservoir may still exist, but the specific C/O greater than unity outside the water iceline—a key observable implication for JWST/Spitzer—does not. This is the single most load-bearing concern because it controls the paper's interpretive reach, and it is internally conceded in Section 4.4.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a 1D dust/gas evolution model (based on chemcomp) that treats the thermal decomposition of refractory organics as an irreversible process producing gaseous C2H2 at the organics line (~0.3 au for a solar-mass star). Unlike the standard treatment in which sublimated vapor recondenses at the iceline, the decomposition product C2H2 has a much lower sublimation temperature (70 K), so the carbon-rich gas diffuses outward to the C2H2 iceline at ~7 au, where it eventually freezes out. The authors find that this outward transport increases the gas-phase C/H by factors of 2–5 and can raise the gas-phase C/O above unity outside the water iceline, depending on turbulence, stellar mass, and the CO2 abundance. They argue this process could explain JWST/Spitzer observations of hydrocarbon-rich inner disks and affect the heavy-element budgets of giant planets.","tokens_in":30703,"tokens_out":4801,"duration_ms":44300,"significance":"The mechanism proposed is physically plausible and distinct from previous treatments: irreversible decomposition turns a large solid carbon reservoir into a mobile gas-phase reservoir that is not trapped at the organics line. The paper is transparent about its assumptions and provides a thorough parameter study (turbulence, stellar mass, fragmentation velocity, viscous heating, carbon partitioning, product species, and Schmidt number). The authors are also candid about the limitations of their fiducial choices, especially in §4.4. If the fiducial parameters hold, the work identifies a new route to sustain C/O > 1 in the outer inner disk (several au), with observable consequences for C2H2 emission. However, the headline quantitative result is conditional on Sc = 1/3 and on C2H2 being the sole decomposition product; both assumptions are acknowledged in the text but not fully reflected in the abstract and conclusions.","major_comments":[{"comment":"The quantitative headline claim in the Abstract and Conclusions — that carbon-rich gas survives out to 7 au with an order-of-magnitude abundance increase and that C/O exceeds unity outside the water iceline — is obtained with Sc = 1/3 as fixed in Eq. (3). Figure 8 shows that at Sc = 0.7, a value within the published MHD range (Johansen & Klahr 2005; Carballido et al. 2005), the C2H2 surface density at 7 au drops by a factor of ~5 and the gas-phase C/O outside the water iceline falls below unity. The abstract presents the 7 au/order-of-magnitude result without qualification, while the paper's own §4.4 shows this result is not robust to plausible Sc values. The suggested mitigation of lowering CO2 from 10% to 1% of carbon is a parameter adjustment, not an empirical constraint. This is load-bearing because the abstract and conclusion 3 explicitly claim C/O > 1 outside the water iceline for the fiducial model. I recommend revising the abstract and conclusions to state the Sc dependence, or providing a more empirically grounded argument for Sc = 1/3.","section":"§4.4, Fig. 8"},{"comment":"The choice that 100% of the decomposed refractory carbon is released as C2H2 (T_sub = 70 K) sets the extent of outward diffusion (to the 7 au C2H2 iceline) and therefore the entire radial scale of the claimed effect. The paper acknowledges this in §4.3 and shows the CH4 end-member case, but the fiducial result is not bracketed by a realistic product distribution: the decomposition of macromolecular organics likely yields a mixture of species, some of which (e.g., benzene, T_sub comparable to ammonia as noted in §4.3) have higher binding energies and would freeze out closer in. Such a mixture would reduce the effective outward transport distance. Because the 7 au distance is essentially set by the assumed product's sublimation temperature, I ask the authors to either treat the product speciation more explicitly, or explicitly reframe the result as an upper limit on the outward extent.","section":"§2.1 and §4.3"}],"minor_comments":[{"comment":"The phrase 'allowing gaseous carbon to diffuse outward without returning to the solid phase' is imprecise because C2H2 does recondense at its own iceline at ~7 au; suggest 'without returning to the solid phase at the organics line' or similar.","section":"Abstract"},{"comment":"The sentence 'We set up chemcomp over a discretised grid of 500 radial cells log-spaced between either 0.01 au or 0.1 au to 1000 au' is ambiguous; please specify which inner boundary is used for which stellar mass (0.01 au for the 0.1 M_sun case, 0.1 au for the solar-mass case, as elsewhere in the text).","section":"§2.2"},{"comment":"The phrase 'to show that it allows to obtain C/O ratios greater than unity' is awkward; suggest 'to show that C/O ratios greater than unity can be obtained'.","section":"Fig. 8 caption"},{"comment":"The chemcomp code is cited as Schneider & Bitsch (2023, arXiv:2401.15686); if the code has since been published or a DOI is available, please update the reference.","section":"References"},{"comment":"The text quantifies the drop at Sc = 0.7 but not at Sc = 1 or Sc = 3, although these cases are shown in Fig. 8; adding a sentence quantifying the reduction at Sc = 1 would help the reader.","section":"§4.4"},{"comment":"The notation for sublimation temperature is inconsistent (Tsub vs. T_sub); please unify, and likewise use a consistent subscript style for the Schmidt number (Sc vs. Sc_g).","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The paper is well within the scope of A&A and addresses a timely topic. The main concern is the sensitivity of the headline quantitative claims to the Schmidt number and product speciation, both of which are acknowledged in the text but not reflected in the abstract. This is a fixable issue that requires reframing the claims or adding a more thorough empirical justification. The other exploratory results (parameter study) are solid and the code description is reproducible."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a well-posed 1D disk evolution study with a genuinely new mechanism — irreversible thermal decomposition of refractory organics into C2H2 turns the soot line from a trap into a one-way valve for carbon. The modeling is transparent, the sensitivity runs are honest, and they modified chemcomp to free the Schmidt number and tested the new implementation against the standard one. Credit where due: the paper clearly shows that if decomposition is irreversible and the product is as volatile as C2H2, carbon-rich gas can diffuse outward and survive far longer than in models with reversible sublimation, with order-of-magnitude increases in the carbon-rich gas mass. That reframes how inner-disk C/O and C/H are set.\n\nThe soft spots are real and mostly internal. The headline claim that C/O exceeds unity outside the water iceline out to ~7 au depends on Sc=1/3. Their own Section 4.4 shows that at Sc=0.7 — within the published MHD range, including their cited Johansen & Klahr 0.85 — the C2H2 reaching 7 au drops by a factor ~5 and C/O outside the water iceline falls below unity. The abstract does not qualify this. To recover C/O>1 at Sc=0.7 they lower CO2 from 10% to 1% of carbon, which is a parameter adjustment, not an empirical constraint. So the single most load-bearing quantitative implication is hostage to a poorly constrained number. The alternative decomposition product CH4 pushes the effect further out, which helps, but the default assumption of pure C2H2 is itself a guess, acknowledged in the text.\n\nIt is also worth saying what is not broken: the qualitative behavior — irreversible decomposition plus outward diffusion redistributes carbon over several au — is not an artifact of one parameter choice, and the paper's own discussion is unusually candid about the sensitivities. They flag the Schmidt number issue in Section 4.4 and in the conclusions. But candor does not remove the uncertainty; the abstract as written overstates robustness.\n\nWho is this for? Disk chemical evolution modelers, planet formation people interested in C/O ratios and giant planet enrichment, and anyone interpreting Spitzer or JWST C2H2 observations. It deserves a serious referee. I would send it out with a request to qualify the abstract and make the Sc dependence front-and-center, not buried in Section 4.4. If the authors reframe the claims as conditional on Sc<~0.5 and treat the beyond-water-iceline C/O>1 as illustrative rather than robust, this is a solid contribution.","headline":"A clean numerical experiment showing irreversible decomposition of refractory organics can redistribute carbon outward, but the headline C/O>1 result hangs on Sc=1/3 and C2H2 as sole product.","tokens_in":31226,"tokens_out":2451,"would_cite":true,"duration_ms":25700,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Irreversible thermal decomposition of refractory organics in the inner protoplanetary disc creates a long-lived, outward-diffusing reservoir of carbon-rich gas that raises the gas-phase C/O ratio to super-stellar and even super-unity…","keywords":["protoplanetary discs","C/O ratio","refractory organics","thermal decomposition","C2H2","dust evolution","icelines","Schmidt number"],"falsifier":"Run the paper's model with a Schmidt number of 1 instead of 1/3, or measure the turbulent Schmidt number in the inner disc with non-ideal MHD simulations; either would show whether the outward C2H2 flux survives. A direct observational test: in a solar-mass disc still delivering pebbles, spatially resolve C2H2 (or the gas-phase C/O ratio) between the water iceline and about 7 au; absence of the predicted order-of-magnitude carbon-rich reservoir there would falsify the central claim in its fiducial setup.","tokens_in":30163,"feed_emoji":"🔥","tokens_out":8344,"duration_ms":80323,"temperature":0.7,"pith_summary":"The paper argues that the irreversibility of thermal decomposition changes how the largest carbon reservoir in protoplanetary discs behaves. Refractory organics, which carry roughly 60% of the carbon, are usually destroyed so close to the star that the released gas is assumed to be quickly accreted and irrelevant. The authors show that if, as laboratory experiments indicate, these organics break apart into volatile molecules such as C2H2 rather than sublimating and recondensing, the organics line becomes permeable: carbon-rich gas can diffuse outward to the C2H2 iceline at about 7 au around a solar-mass star and survive for much longer. In their 1D evolution model this raises the gas-phase carbon reservoir by about an order of magnitude and markedly changes the C/O ratio, with values above unity outside the water iceline for most of the disc lifetime. If correct, this gives a new path for carbon enrichment of the inner disc that does not rely on large CH4 abundances, and a new way to read Spitzer and JWST observations of C2H2-rich discs.","feed_headline":"Burned organics push carbon-rich gas 7 au from the star","feed_subtitle":"Irreversible decomposition lets C2H2 escape the inner disc, boosting gas-phase C/O by an order of magnitude.","key_machinery":"The mechanism is the permeable organics line, produced by irreversible thermal decomposition. In ordinary iceline chemistry, vapor that diffuses outward across its sublimation line recondenses onto grains (the cold-finger effect), trapping the volatile inside the iceline; for refractory organics, decomposition into simpler molecules such as C2H2 makes that return path impossible, so the line does not hold carbon back. The argument is carried quantitatively by the 1D evolution of gas and dust: a two-population grain-growth scheme sets pebble drift, and each gas species is advected and diffused according to a viscous disc model in which the Schmidt number (ratio of viscosity to gas diffusivity) is $Sc = 1/3$ in the standard setup, giving diffusion three times the reach of inward advection. With that transport, C2H2 released at roughly 0.3 au spreads to its own iceline at about 7 au, and the longer viscous timescale there keeps the carbon-rich gas in the disc for millions of years instead of accreting onto the star.","core_discovery":"The central claim is that thermally decomposing refractory organics release carbon into the gas phase in a form that cannot return to the solid phase, and that this one-way valve redistributes carbon through the inner disc. The paper's model takes refractory organics to decompose at 350 K into gaseous C2H2 (sublimation temperature 70 K). Because C2H2 does not recondense at the organics line, the usual cold-finger trapping that holds vapor just inside an iceline is absent; instead the organics line lets carbon-rich vapor diffuse outward against the accretion flow. The result is a reservoir of C2H2-rich gas extending out to the C2H2 iceline at roughly 7 au, where the viscous accretion timescale is about 1.5 Myr compared with 0.06 Myr at the organics line, and a total carbon-rich gas mass about an order of magnitude larger than in models without irreversible decomposition. The gas-phase C/O ratio consequently becomes super-stellar inside the water iceline and exceeds unity outside it in the fiducial model, with the detailed outcome depending on turbulence, pebble flux lifetime, and the partitioning of carbon.","pith_inferences":["Because the paper's own $Sc = 0.7$ run reduces C2H2 at 7 au by about a factor of 5 and drops gas-phase C/O below unity outside the water iceline, the headline result is conditional on turbulent diffusion being genuinely stronger than advection; pinning down the Schmidt number observationally or with non-ideal MHD simulations would decide how far the effect extends.","The same one-way-valve logic applies to any refractory carrier whose decomposition products are volatile: if organics decompose into CH4 (30 K) rather than C2H2, the carbon-rich gas would reach roughly 40 au, making the C/O footprint much more extended.","Coupling this mechanism with FUor-type outbursts, which push the organics line outward to several au, suggests that the gas-phase C/O enhancement could outlast the outburst by hundreds of kyr and serve as a chemical fossil of past accretion events.","A testable separation: in discs where pebble drift is still ongoing, spatially resolved C2H2 emission (or C/O mapping) beyond the water iceline would distinguish this decomposition-driven reservoir from the alternative late-stage CH4 inflow scenario, which operates only after the pebble flux has died."],"forward_implications":["The carbon-rich gas reservoir produced by refractory organics is roughly ten times more massive and survives far longer in the disc than in models where the released carbon is trapped inside the organics line.","Gas-phase C/O is significantly enhanced: super-stellar values appear just inside the water iceline, and values above unity appear outside it, with the pattern persisting through most of the disc lifetime for alpha >= 1e-3.","The usual anti-correlation between gas and solid composition breaks down: carbon-rich gas and carbon-rich solids can coexist at the same radius, changing how planetesimals and planets sample carbon relative to oxygen.","The model can produce hydrocarbon-rich inner discs and a colder, more extended C2H2 component without requiring a large volatile CH4 reservoir, offering an alternative explanation for Spitzer and JWST spectra.","Giant planets migrating in the inner few au have a wider window to accrete carbon-rich gas, which alters the heavy-element content they inherit."],"supporting_citations":[{"why":"Laboratory experiments establishing that refractory organics decompose irreversibly at 300–500 K; this sets the organics line temperature used in the model.","marker":"Nakano et al. 2003"},{"why":"Supplies the 1D dust and gas evolution code (grain growth, advection, diffusion, sublimation, condensation) that the paper adapts to include irreversible decomposition.","marker":"Schneider & Bitsch 2023"},{"why":"The standard model of inner-disc C/O evolution that treats refractory organics without irreversible decomposition; provides the baseline for comparison and the late-stage high-C/O scenario.","marker":"Mah et al. 2023"},{"why":"Two-population algorithm for dust coagulation and radial drift used to compute pebble sizes and fluxes in the model.","marker":"Birnstiel et al. 2012"},{"why":"Argues the Schmidt number can be as low as 1/3, the value that maximizes outward diffusion of the carbon-rich gas in the fiducial setup.","marker":"Pavlyuchenkov & Dullemond 2007"},{"why":"Gives the sublimation temperature of C2H2 (70 K), which fixes the C2H2 iceline at about 7 au, the outer reach of the carbon-rich reservoir.","marker":"Penteado et al. 2017"},{"why":"Solar elemental abundances used to set the stellar C/O = 0.55 and the initial partitioning of elements among the model species.","marker":"Asplund et al. 2009"},{"why":"Measurements of interstellar ices and comets used to set initial abundances of volatile carbon carriers (CH4 about 1%, CO, CO2) and to argue against a large CH4 fraction.","marker":"Gibb et al. 2004"}],"fun_headline_variants":["Organics burn opens one-way door for carbon","C2H2 from burned organics drifts out to 7 au","Refractory organics boost gas C/O by tenfold","Carbon-rich gas escapes organics line one-way"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The model's outspread of carbon-rich gas assumes a Schmidt number of 1/3, meaning gas diffuses three times more efficiently than it is dragged inward; if the real value is 0.7 or higher, the paper itself finds the C2H2 reaching 7 au drops by about a factor of 5 and the C/O rise outside the water iceline disappears.","fun_headline_variants_meta":{"raw":{"variants":["Organics burn opens one-way door for carbon","C2H2 from burned organics drifts out to 7 au","Refractory organics boost gas C/O by tenfold","Carbon-rich gas escapes organics line one-way"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000668,"raw_usage":{"total_tokens":3121,"prompt_tokens":1096,"completion_tokens":2025,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":712,"completion_tokens_details":{"reasoning_tokens":1957}},"tokens_in":712,"tokens_out":2025,"duration_ms":17822,"temperature":1.0,"reasoning_tokens":1957,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T13:55:27.165473+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the paper's model with a Schmidt number of 1 instead of 1/3, or measure the turbulent Schmidt number in the inner disc with non-ideal MHD simulations; either would show whether the outward C2H2 flux survives. A direct observational test: in a solar-mass disc still delivering pebbles, spatially resolve C2H2 (or the gas-phase C/O ratio) between the water iceline and about 7 au; absence of the predicted order-of-magnitude carbon-rich reservoir there would falsify the central claim in its fiducial setup.","supporting_citations":[{"cited_title":"2003, , 592, 1252","cited_arxiv_id":null,"evidence_quote":"Laboratory experiments establishing that refractory organics decompose irreversibly at 300–500 K; this sets the organics line temperature used in the model."},{"cited_title":"chemcomp: Modeling the chemical composition of planets formed in protoplanetary disks","cited_arxiv_id":"2401.15686","evidence_quote":"Supplies the 1D dust and gas evolution code (grain growth, advection, diffusion, sublimation, condensation) that the paper adapts to include irreversible decomposition."},{"cited_title":"& Dullemond , C","cited_arxiv_id":null,"evidence_quote":"Argues the Schmidt number can be as low as 1/3, the value that maximizes outward diffusion of the carbon-rich gas in the fiducial setup."},{"cited_title":"M., Walsh , C., & Cuppen , H","cited_arxiv_id":null,"evidence_quote":"Gives the sublimation temperature of C2H2 (70 K), which fixes the C2H2 iceline at about 7 au, the outer reach of the carbon-rich reservoir."}],"review_version":1}