{"id":"3adb6805-aa8c-4df7-80bf-7781b6581d30","arxiv_id":"2506.02748","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"All 10 very low-mass star and brown dwarf disks observed with JWST-MIRI show hydrocarbon-rich inner gas; detection rates anti-correlate between organic and inorganic molecules, with tentative evidence of carbon enrichment with disk age.","lead":"JWST spectra of 10 planet-forming disks around the smallest stars and brown dwarfs show they are all rich in carbon-based molecules, with water detected in only half. The findings are the first systematic evidence that these disks grow more carbon-rich with age, which shapes the chemistry of any planets that form around the galaxy's most common stars.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The F13CCH2/FC2H2 column-density proxy is not yet validated against the optical-depth and isotope-fractionation degeneracies that the paper itself concedes, so the cross-sample trends in Figs. 5, 6, and 8 remain qualitative.","rationale":"The reader's weakest_assumption matched nearly verbatim the concern I would raise: Sect. 4.1's isotopologue flux ratio is load-bearing for Figs. 5, 6, and 8, and the paper itself concedes the fractionation caveat. My stress-test does not find an internal inconsistency or a numerical error; the detection-rate results and the anti-correlation between organic and inorganic detections are supported by the displayed spectra and by prior Spitzer work. The central evolution claim (C/O enrichment with age) is additionally weakened by the use of canonical region ages, which the paper also acknowledges, but that is a secondary concern because Fig. 8 is explicitly labeled 'tentative'. The most efficient way to settle the matter is a synthetic-spectrum test of the proxy; this is a concrete, feasible check that does not require new observations. I therefore keep CONDITIONAL rather than accepting fully, and I agree with the reader that the manuscript's own listed limitations are the reason for conditionality.","tokens_in":28644,"tokens_out":1747,"duration_ms":15011,"concrete_test":"Run a small grid of 0D or 1D slab/thermochemical models varying column density, temperature, emitting area, and 12C/13C ratio (65-80) and compute synthetic MIRI/MRS spectra at R~1500-3000 over 13.65-13.75 µm, using line lists that include 13CCH2 hot bands or an estimated contribution, then compare predicted F13CCH2/FC2H2 against both J1605 (ratio ~0.33) and J1558 (low ratio, bright C2H2). If a plausible fractionation difference (e.g. 12C/13C = 40-60) alone can reproduce the full observed ratio spread without invoking column density, the proxy is not monotonic and the Figs. 5, 6, and 8 trends need re-binning. If the ratio remains monotonic in column even under 20-30% fractionation variation, the central interpretation survives.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The paper's evolution narrative and the age trend in Fig. 8 rest on ordering all sources by the flux ratio F13CCH2/FC2H2 and treating it as a monotonic proxy for the column of gas probed (Sect. 4.1). This requires: (1) uniform 12C/13C fractionation across all disks; (2) 13CCH2 co-spatial and co-thermal with C2H2; (3) negligible contamination of the 13.698-13.734 µm windows by hot-band or other hydrocarbon emission; and (4) that the ratio is insensitive to temperature and emitting area. The paper explicitly concedes that UV shielding and chemical fractionation could change the local 12C/13C ratio, and that the observed ratio range could be 'partly due to these fractionation processes' (Sect. 4.1). Since the ratio appears both as the independent variable in Figs. 5-6 and as the x-axis in Fig. 8, any isotopic fractionation correlated with environment or age would masquerade as the claimed C/O-age trend. J1558 is already an internal warning sign: it is C2H2-bright (comparable peak flux to J1605, ~22.2 mJy) yet shows a very low F13CCH2/FC2H2 ratio, forcing the paper to argue for an order-of-magnitude lower column at high temperature/area; without a radiative-transfer or slab-model calibration of the proxy, this degeneracy is unresolved. The reader's conditional verdict correctly identifies this as the weakest point.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents JWST MIRI/MRS spectra of 10 disks around very low-mass stars and brown dwarfs (0.02–0.14 M_sun) in Taurus, Chamaeleon, Upper Sco, and TW Hya, and derives molecular detection rates, flux-ratio trends, and a tentative evolutionary C/O-enhancement scenario. The central claims are that the inner disks are extremely molecular-rich, with C2H2 and HCN detected in all sources and larger hydrocarbons in nearly all; that hydrocarbon-rich sources have weaker 10 um dust features and lower millimeter-derived dust masses; and that the observed trends are consistent with rapid inward solid transport and grain growth. The analysis uses the F13CCH2/FC2H2 flux ratio as a proxy for the column of gas probed, relates this ratio to dust strength, dust mass, luminosity, and oxygen-bearing molecule fluxes, and interprets the source ordering in terms of disk age across star-forming regions.","tokens_in":28932,"tokens_out":4606,"duration_ms":47509,"significance":"If the results hold, this is the first systematic MIRI census of the inner-disk gas of very low-mass objects and would establish that hydrocarbon-dominated inner disks are common rather than rare, with implications for the C/O budget of planets forming around M dwarfs and brown dwarfs. The paper's strengths include a well-defined sample spanning several star-forming regions, cross-checks against Spitzer spectra, displayed continuum-subtracted spectra for many detections in Appendix C, and an unusually candid discussion of caveats, including the isotopic-fractionation limitation of the column proxy and the age diversity within star-forming regions. The detection-rate statistics and the qualitative consistency with the Mah et al. (2023) transport models are useful even if the evolutionary interpretation is tentative. The central limitations are that the main ordering quantity is an uncalibrated isotopologue ratio and that the age trend relies on canonical region ages; both are explicitly acknowledged in the text but remain load-bearing for the conclusions.","major_comments":[{"comment":"The monotonic interpretation of F13CCH2/FC2H2 as a column-depth proxy is not calibrated. The paper itself concedes that preferential UV shielding or chemical fractionation could change the local 12C/13C ratio and that the observed ratio range could be partly due to these processes. Since this ratio is the independent variable in the central cross-sample trends and the x-axis of the age plot, the authors should either validate the proxy with slab-model or radiative-transfer grids spanning plausible temperatures, columns, and emitting areas, or demonstrate that the trends survive when an independent column indicator is used. The J1558 case, which is C2H2-bright but shows a very low isotopologue ratio, illustrates that the ratio cannot be interpreted as a simple column tracer without additional modeling.","section":"Sect. 4.1, Figs. 5, 6, 8"},{"comment":"The claimed C/O enhancement with age is anchored to the canonical ages of the star-forming regions and moving groups, even though the paper cites Ratzenböck et al. (2023) showing substantial age diversity within regions and explicitly notes that environmental differences such as the elevated UV field in Upper Sco could influence the trend. Because conclusion item 6 states this as tentative evidence, the authors should show how the ordering changes under alternative age assignments (for example, ChaI at ~4 Myr instead of 1–2 Myr) or should downgrade the language from evidence to consistency with model predictions.","section":"Sect. 5.1, Fig. 8"},{"comment":"The detection inventory for NC1 and TW A27 is adopted from Morales-Calderón et al. (subm.) and Patapis et al. (subm.), and part of the J1605 detections from Kanwar et al. (subm.), none of which are publicly available at the time of writing. Because the detection rates are a central result, the manuscript should either include the relevant continuum-subtracted spectra and detection criteria for these sources in an appendix or clearly state that the companion papers are available as part of the peer-review process. Without this, the reader cannot independently verify the most novel claims, such as C2H2 and HCN in all sources.","section":"Sect. 3 and Appendix C"},{"comment":"All measured line fluxes, dust strengths, and upper limits depend on the two Savitzky-Golay continuum definitions, but no sensitivity test is shown for the choice of filter widths. Since several of the features are weak or flat-topped and the paper's trends are built from ratios of these fluxes, a short test varying the narrow and broad continuum widths (for example, by ±30%) should be added to demonstrate that the reported anti-correlations in Figs. 5 and 6 are not continuum artifacts.","section":"Sect. 2.3 and Appendix A"}],"minor_comments":[{"comment":"The integration windows for F13CCH2 and FC2H2 are defined, but no uncertainty from the window placement is provided; a brief statement on how sensitive the ratios are to the window boundaries would help.","section":"Sect. 4.1"},{"comment":"The statement that all errorbars are 1-sigma with an assumed SNR of 100 is very coarse, and the method for computing 1-sigma upper limits for non-detections is not described; please specify the upper-limit prescription.","section":"Fig. 5 caption"},{"comment":"The sentence beginning \"Moreover, switching the ratios on either axes no trend was observed\" is grammatically incomplete and should be rephrased.","section":"Sect. 5.1"},{"comment":"There is a typo in the phrase \"Spitzerspecrta\" near the start of Appendix B.","section":"Appendix B"},{"comment":"The extinction value for J0438 is described in the text as an upper limit because of the high inclination, but the table lists Av=2.8 without a symbol; please add an inequality or footnote.","section":"Table 1"},{"comment":"The caption says the spectra are ordered by increasing F13CCH2/FC2H2, but J0438 is excluded from the ratio-ordering in Sect. 4.2 and appears first in the figure; clarify the ordering convention.","section":"Fig. 1 caption"}],"recommendation":"major_revision","confidential_remarks":"The paper is a solid observational survey and well within the scope of A&A; the main reason for major revision is that the principal ordering quantity and the age interpretation, although explicitly caveated, are not yet demonstrated to be robust. The most constructive path would be a targeted validation of the isotopologue ratio proxy, or a clear reframing of the evolutionary narrative as model-consistent rather than directly evidenced. I see no issue with novelty or citation practice, and the reliance on companion papers is a standard practice if those papers are made available during review."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take: this is the first systematic sample-level statement from MINDS on VLMS/brown dwarf inner disks, and the core result — C2H2 and HCN detected in all 10 sources, larger hydrocarbons nearly everywhere, oxygen-bearing species less common — is credible. The displayed spectra and Spitzer comparisons back the detections, and the paper is unusually explicit about its own limitations. That honesty is real: they flag the isotope-fractionation degeneracy, the canonical-age caveat, and the Spitzer-selected sample.\n\nWhat is genuinely new: detection rates over 10 objects, the Yule-phi correlation structure among species, the F13CCH2/FC2H2 column-depth diagnostic, and the region-age ordering in Fig. 8. The anti-correlation between organic and inorganic detections is a clear pattern worth explaining, and the dust-strength/disk-mass trends are plausible consequences of grain growth and transport.\n\nThe soft spots are real but not disqualifying. The F13CCH2/FC2H2 ratio is the load-bearing proxy for column depth, and the paper concedes that fractionation could mimic the trends. J1558 is the internal stress test: a C2H2-bright source with a low ratio forces an order-of-magnitude column difference, which is not calibrated. So Figs. 5, 6, and 8 should be read as qualitative correlations, not measurements. The age trend in Fig. 8 is suggestive at best — canonical region ages with known internal diversity, a handful of sources per region, and one axis defined by the same proxy. I also note that some detections (NC1, TW A27, J1605 bits) live in companion papers; the authors say so, which helps, but the referee will need to check those.\n\nNet: this deserves a serious referee. The central detection claim is solid and new at the sample level. The trends are hypotheses, and the paper treats them as such. A good referee should push for a clearer separation between the firm detection-rate result and the qualitative proxy-driven trends, and ask for a slab-model or radiative-transfer sanity check of the 13CCH2/C2H2 ratio across the sample, even if only for a few sources. I would cite it for the detection rates and sample. Worth bringing to reading group.","headline":"A credible sample-level survey showing VLMS and brown dwarf inner disks are hydrocarbon-dominated, with trends that are real but qualitative.","tokens_in":29726,"tokens_out":1736,"would_cite":true,"duration_ms":16882,"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":"The inner disks of very low-mass stars and brown dwarfs are systematically hydrocarbon-dominated, and their carbon-to-oxygen ratio appears to rise with disk age.","keywords":["protoplanetary disks","very low-mass stars","brown dwarfs","JWST MIRI spectroscopy","disk chemistry","hydrocarbons","C/O ratio","disk evolution"],"falsifier":"A decisive test would be to fit full slab-model or retrieval column densities for a subset of these sources and measure the C-12/C-13 ratio independently, for example from millimeter isotopologue lines of CO or HCN; if F_13CCH2/F_C2H2 does not rise monotonically with the retrieved C2H2 column, or if the isotope ratio varies between sources, the reported column-depth trends and the implied C/O-age trend would be partly fractionation artifacts.","tokens_in":2173,"feed_emoji":"🪐","tokens_out":2244,"duration_ms":127854,"temperature":0.7,"pith_summary":"The paper sets out to establish that the inner regions of planet-forming disks around the smallest stars and brown dwarfs are regularly carbon-dominated. It analyzes ten JWST MIRI spectra of disks around objects of 0.02 to 0.14 solar masses spanning roughly 1 to 10 Myr, reporting detection rates rather than individual model fits. Acetylene and hydrogen cyanide appear in every source, larger hydrocarbons such as diacetylene and benzene in nearly all, while carbon dioxide, water, and carbon monoxide are firmly detected in 90%, 50%, and 20% of the sample. The authors show that hydrocarbon detections cluster together and anti-correlate with inorganic molecules, that hydrocarbon-rich sources have weaker 10 um silicate emission and lower outer-disk dust masses, and that the carbon-to-oxygen ratio appears to rise with disk age. If true, the composition of rocky planets forming around these common stars would start from carbon-rich, water-poor gas, and disk evolution would be governed by rapid inward solid transport and grain growth.","feed_headline":"JWST finds tiny-star inner disks are carbon-rich, not watery","feed_subtitle":"Rocky planets form most often around these stars, so their starting chemistry is set by carbon-rich gas.","key_machinery":"The load-bearing observable is the ratio F_13CCH2/F_C2H2, the integrated fluxes of the rare-isotope and main-isotope Q-branches of acetylene near 13.7 um. At high column density the ordinary C2H2 emission saturates while 13CCH2 remains optically thinner, so the ratio increases with the column of gas probed, provided the C-12/C-13 ratio is uniform. The paper uses this ratio as its main axis for nearly all trends and combines it with F_CO2/F_C2H2, F_H2O/F_C2H2, the 9.8 um dust strength, millimeter dust mass, stellar luminosity, and the 13CO2/13CCH2 ratio; the last of these, grouped by star-forming region, carries the age trend.","core_discovery":"On the paper's own terms, the discovery is that the carbon-rich inner disk seen earlier in individual objects is a systematic property of very low-mass star and brown dwarf disks, not a curiosity. In the ten-object sample, C2H2 and HCN are detected in all sources, C4H2 in nine and C6H6 in eight, and more than 80% of the detected molecules contain carbon. Oxygen carriers are present but subordinate: CO2 in nine sources, H2O in five, and CO in two. The paper introduces the F_13CCH2/F_C2H2 flux ratio as a qualitative measure of gas column depth; against this axis, sources with deeper probing show more hydrocarbon species, weaker 9.8 um dust features, lower outer-disk dust masses, lower stellar luminosities, and weaker CO2 and H2O relative to acetylene. Differences across star-forming regions line up with age in the sense that older regions appear more carbon-rich, which the paper interprets as tentative evidence that the C/O ratio in the emitting layers increases with disk age, consistent with rapid inward transport of solids and grain growth.","pith_inferences":["Editorial inference: applying the same F_13CCH2/F_C2H2 proxy to a matched sample of higher-mass T Tauri disks should give systematically lower ratios; a carbon-rich inner disk at T Tauri masses would require a different mechanism or a much longer timescale.","Editorial inference: a direct test would measure C-12/C-13 from millimeter isotopologue lines in the same sources; if the isotope ratio varies between disks, part of the reported age trend in the infrared ratio is chemical fractionation rather than deeper probing.","Editorial inference: if inward transport drives the carbon enrichment, the carbon-rich phase should coincide with measurable depletion of the innermost rocky solids, which future ALMA pebble-flux measurements or refractory-mineral observations could separate from carbon-grain-destruction scenarios."],"forward_implications":["The detection of acetylene and hydrogen cyanide in all ten disks, and larger hydrocarbons in nearly all, makes the carbon-rich inner disk the rule rather than the exception for stars below roughly 0.15 solar masses.","Because rocky planets form most frequently around such stars, planets assembled in these inner regions would inherit carbon-rich, water-poor gas, shifting predicted planet compositions toward carbon-rich refractory material.","The anti-correlation between hydrocarbon and inorganic molecule detections implies that water-bearing and hydrocarbon-dominated spectra represent two distinct disk states, with the hydrocarbon state becoming more common as disks age.","Disks with deeper probed hydrocarbon columns have weaker 10 um silicate features and lower millimeter dust masses, linking inner carbon enrichment to outer-disk dust depletion through grain growth and inward drift.","If the age trend holds, older disks such as those in Upper Sco and the TW Hya association should be systematically more carbon-rich than younger disks in Chamaeleon I and Taurus, making the C/O ratio a rough disk clock."],"supporting_citations":[{"why":"Supplies the earlier Spitzer census of very low-mass star disks whose detection rates and weaker silicate features this sample extends.","marker":"Pascucci et al. 2009"},{"why":"Extends the Spitzer census to more VLMS disks and provides tentative water and [Ne II] detections used as the pre-JWST baseline.","marker":"Pascucci et al. 2013"},{"why":"Provides the first JWST MIRI demonstration of an extremely carbon-rich VLMS disk, including the 13CCH2 isotopologue analysis and the carbon-grain-destruction scenario built on here.","marker":"Tabone et al. 2023"},{"why":"Establishes the carbon-rich MIRI spectrum of IC147 and the C/O>1 interpretation, supplying detection criteria and slab-model line lists used for the other sources.","marker":"Arabhavi et al. 2024"},{"why":"Gives the disk-evolution model predicting that the inner disk C/O ratio rises fastest for low-mass disks, the theoretical trend the age comparison is testing.","marker":"Mah et al. 2023"},{"why":"Provides the grain-growth and rapid inward-drift model used to explain weaker 10 um silicate features and lower outer-disk dust masses in hydrocarbon-rich sources.","marker":"Pinilla et al. 2013"},{"why":"Reports the water-rich VLMS disk Sz 114, the main contrasting object and an extra data point in the age-trend diagram.","marker":"Xie et al. 2023"},{"why":"Models the onion-like vertical layering of molecular emission in disk atmospheres, underlying the paper's explanation of why deeper dust opacity reveals hydrocarbon layers.","marker":"Woitke et al. 2018b"},{"why":"Shows that C2H2 has two reservoirs and that CO2 can persist at high C/O, used to interpret the continued CO2 detection in carbon-rich disks.","marker":"Kanwar et al. 2024"},{"why":"Provides the compiled stellar properties, accretion rates, and millimeter-derived dust masses plotted against the isotopologue ratio.","marker":"Manara et al. 2023"}],"fun_headline_variants":["JWST reveals carbon-rich inner disks are standard for tiny stars","Small star disks: carbon molecules dominate, oxygen is rare","Carbon-rich gas widespread in disks around very low-mass stars","Brown dwarf disks are carbon factories, JWST finds"],"cache_read_input_tokens":31488,"weakest_assumption_plain":"The central assumption is that the F_13CCH2/F_C2H2 flux ratio tracks how deep the mid-infrared sees into the disk gas, which holds only if the carbon isotope ratio is uniform across the disk; ultraviolet shielding or chemical fractionation could change the ratio without any change in column density.","fun_headline_variants_meta":{"raw":{"variants":["JWST reveals carbon-rich inner disks are standard for tiny stars","Small star disks: carbon molecules dominate, oxygen is rare","Carbon-rich gas widespread in disks around very low-mass stars","Brown dwarf disks are carbon factories, JWST finds"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000274,"raw_usage":{"total_tokens":1755,"prompt_tokens":1175,"completion_tokens":580,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":791,"completion_tokens_details":{"reasoning_tokens":513}},"tokens_in":791,"tokens_out":580,"duration_ms":6087,"temperature":1.0,"reasoning_tokens":513,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T11:19:24.852612+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A decisive test would be to fit full slab-model or retrieval column densities for a subset of these sources and measure the C-12/C-13 ratio independently, for example from millimeter isotopologue lines of CO or HCN; if F_13CCH2/F_C2H2 does not rise monotonically with the retrieved C2H2 column, or if the isotope ratio varies between sources, the reported column-depth trends and the implied C/O-age trend would be partly fractionation artifacts.","supporting_citations":[{"cited_title":"2009, , 696, 143","cited_arxiv_id":null,"evidence_quote":"Supplies the earlier Spitzer census of very low-mass star disks whose detection rates and weaker silicate features this sample extends."},{"cited_title":"S., & Bruderer , S","cited_arxiv_id":null,"evidence_quote":"Extends the Spitzer census to more VLMS disks and provides tentative water and [Ne II] detections used as the pre-JWST baseline."},{"cited_title":"F., et al","cited_arxiv_id":null,"evidence_quote":"Provides the first JWST MIRI demonstration of an extremely carbon-rich VLMS disk, including the 13CCH2 isotopologue analysis and the carbon-grain-destruction scenario built on here."},{"cited_title":"2023, , 677, L7","cited_arxiv_id":null,"evidence_quote":"Gives the disk-evolution model predicting that the inner disk C/O ratio rises fastest for low-mass disks, the theoretical trend the age comparison is testing."},{"cited_title":"2013, , 554, A95","cited_arxiv_id":null,"evidence_quote":"Provides the grain-growth and rapid inward-drift model used to explain weaker 10 um silicate features and lower outer-disk dust masses in hydrocarbon-rich sources."},{"cited_title":"2023, , 959, L25","cited_arxiv_id":null,"evidence_quote":"Reports the water-rich VLMS disk Sz 114, the main contrasting object and an extra data point in the age-trend diagram."},{"cited_title":"2024, , 689, A231","cited_arxiv_id":null,"evidence_quote":"Shows that C2H2 has two reservoirs and that CO2 can persist at high C/O, used to interpret the continued CO2 detection in carbon-rich disks."}],"review_version":1}