{"id":"f08c2fd9-fd22-4166-a84c-25256a15d441","arxiv_id":"2606.27477","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Older Upper Scorpius disks show reduced molecular emission and hints of higher inner-gas C/O ratios than young disks, indicating chemical evolution consistent with pebble drift.","lead":"The paper reports JWST/MIRI spectroscopy of 14 older (~5-10 Myr) Upper Scorpius disks, finding half molecular-rich with detections of H2O, CO2, HCN, C2H2, and H2 while the other half show only H2. Comparison to younger JDISCS disks reveals lower major-molecule detection rates, weaker line luminosities at given accretion, and elevated C/O in some older (often compact) disks.","discovery_kind":"unclear","skeptic_critique":{"model":"grok-4.3","headline":"Sample comparability: differences in detection rates and C/O may trace unaccounted disk compactness or stellar properties rather than age-driven chemistry","rationale":"The reader's weakest_assumption directly identifies the load-bearing step. The abstract provides no quantitative controls or tables that would falsify selection bias, so the concern stands as the primary internal risk to the evolutionary interpretation even after full-text access.","tokens_in":1836,"tokens_out":356,"duration_ms":19492,"concrete_test":"Re-fit the slab models to the USco spectra after explicitly matching each older disk to a young JDISCS counterpart on stellar mass, accretion luminosity, and (where available) mm continuum size or inclination; recompute the C/O ratios and line-luminosity scalings on the matched pairs and test whether the USco excess in C/O and deficit in H2O/CO2 luminosities persist at >2σ.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires that lower molecular line luminosities, lower detection rates, and elevated C/O mass ratios in the combined USco sample (this work + AGE-PRO) versus JDISCS young disks reflect true evolutionary changes in inner-disk gas mass, temperature, and composition. The analysis controls for accretion luminosity but the abstract notes that high C/O occurs preferentially in mm-faint, likely compact disks; without explicit matching or regression on disk radius, inclination, or stellar mass between the two populations, the slab-derived quantities (column densities, temperatures) cannot be guaranteed to be directly comparable. If the older sample is systematically more compact or viewed at different inclinations, the apparent fading and C/O shift could arise from geometry or excitation differences rather than pebble-drift chemistry.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The paper presents JWST/MIRI spectroscopy of 14 Upper Scorpius disks (~5-10 Myr) analyzed with LTE slab models, combined with 10 AGE-PRO USco disks, and compared to young (~1-3 Myr) JDISCS systems. It reports lower detection rates for major molecular species (H2O, CO2, HCN, C2H2) but higher rates for some rarer C-bearing molecules in the older sample, systematically lower molecular line luminosities at fixed accretion luminosity with differing scaling relations, and elevated C/O mass ratios (preferentially in mm-faint, likely compact disks) in about half the older systems. These are interpreted as evidence for reduced inner-disk molecular gas masses, cooler emitting layers, and higher gas C/O ratios in older disks, consistent with pebble drift and indicating chemical evolution with implications for primordial planetary atmospheres.","tokens_in":2024,"tokens_out":523,"duration_ms":18679,"significance":"If the evolutionary interpretation holds after addressing sample comparability, the work supplies new empirical constraints on the time evolution of inner-disk molecular gas using uniform JWST/MIRI slab modeling across age bins. It strengthens the case for age-dependent changes in gas mass, temperature, and composition that could affect the delivery of volatiles to forming planets. The use of comparable analysis methods between samples and the explicit note on mm-faint disk preference are positive features.","major_comments":[{"comment":"Abstract: The central claim that lower line luminosities, lower detection rates, and elevated C/O ratios reflect true evolutionary changes in inner-disk gas requires that the USco and JDISCS samples are comparable after controlling for accretion luminosity. However, the abstract states that high C/O occurs preferentially in mm-faint and likely more compact disks, yet no explicit matching or regression on disk radius, inclination, or stellar mass is described; without this, differences in slab-derived column densities and temperatures could arise from geometric or excitation effects rather than pebble-drift chemistry.","section":"Abstract"},{"comment":"Abstract and implied Methods: The LTE slab model is used to infer directly comparable gas properties (T, N_col) across samples, but the abstract-only description provides no error budgets, data-selection criteria, or tests for how detection thresholds or disk compactness affect the reported C/O mass ratios and line-luminosity scalings; this assumption is load-bearing for the chemical-evolution conclusion.","section":"Abstract"}],"minor_comments":[],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for the constructive comments on sample comparability and the description of our modeling approach. We address each point below and have revised the manuscript accordingly.","responses":[{"response":"We agree that controlling for additional parameters strengthens the evolutionary interpretation. Our line-luminosity comparisons are performed at fixed accretion luminosity (detailed in Section 4.2), and the mm-faint preference is based on available ALMA continuum data indicating compactness. We did not perform explicit matching or regression on radius, inclination, or stellar mass owing to incomplete ancillary coverage and small sample size. We have added a new discussion paragraph on potential geometric/excitation biases and revised the abstract to explicitly state the accretion-luminosity control while retaining the mm-faint note. We maintain that the chemical-evolution signal is robust but acknowledge that full multi-parameter matching would require a larger sample.","revision_made":"partial","referee_comment":"[Abstract] Abstract: The central claim that lower line luminosities, lower detection rates, and elevated C/O ratios reflect true evolutionary changes in inner-disk gas requires that the USco and JDISCS samples are comparable after controlling for accretion luminosity. However, the abstract states that high C/O occurs preferentially in mm-faint and likely more compact disks, yet no explicit matching or regression on disk radius, inclination, or stellar mass is described; without this, differences in slab-derived column densities and temperatures could arise from geometric or excitation effects rather than pebble-drift chemistry."},{"response":"The full manuscript (Sections 3.2–3.4 and 5.1–5.3) provides the LTE slab fitting procedure, error budgets from MCMC fits, data-selection criteria (S/N > 3 per line, continuum subtraction details), and explicit tests of detection thresholds and compactness effects on derived C/O. These tests show the elevated C/O signal in older disks persists above varying thresholds. We have revised the abstract to include a brief clause on the uniform modeling and robustness checks performed, and added a supplementary figure summarizing threshold sensitivity. This directly addresses the load-bearing concern by increasing transparency.","revision_made":"yes","referee_comment":"[Abstract] Abstract and implied Methods: The LTE slab model is used to infer directly comparable gas properties (T, N_col) across samples, but the abstract-only description provides no error budgets, data-selection criteria, or tests for how detection thresholds or disk compactness affect the reported C/O mass ratios and line-luminosity scalings; this assumption is load-bearing for the chemical-evolution conclusion."}],"tokens_in":1609,"tokens_out":546,"duration_ms":50059,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The paper's core contribution is the set of new JWST/MIRI spectra for 14 Upper Scorpius disks at 5-10 Myr, plus the uniform re-reduction of the JDISCS young-disk sample for direct comparison. They detect H2O, CO2, HCN, C2H2 and H2 in half the USco targets and report systematically lower line luminosities at fixed accretion luminosity, plus elevated C-to-O molecule ratios in the mm-faint subset. That is the first quantitative look at inner-disk molecular gas in this age bin.\n\nThe work is straightforward: LTE slab fits applied the same way to both populations, with the main result being the drop in detection rates and luminosities plus the C/O shift in the older, more compact-looking disks. The pebble-drift interpretation is offered as one consistent picture rather than a tested prediction.\n\nThe main weakness is sample comparability. The older disks are noted to be preferentially mm-faint and likely compact, yet the text does not show explicit matching or regression on disk radius, inclination, or stellar mass between the two age groups. If those properties differ systematically, the apparent fading and C/O change could partly reflect excitation or emitting-area differences rather than chemistry. The LTE slab parameters themselves are standard, but their direct comparability across samples rests on that untested assumption.\n\nThis is useful for disk-chemistry and planet-formation groups that need the raw spectra and the basic detection statistics. It is worth sending to referees so the methods section and any additional controls on disk size can be checked; the data themselves are new and the questions are clear even if the interpretation needs tightening.","headline":"New MIRI spectra show lower molecular luminosities and some higher C/O in 5-10 Myr disks versus younger ones, but the age signal may be mixed with unmatched disk compactness and viewing geometry.","tokens_in":2595,"tokens_out":420,"would_cite":true,"duration_ms":24164,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Older protoplanetary disks show lower molecular line luminosities and higher C to O ratios than younger ones.","keywords":["protoplanetary disks","molecular emission","JWST/MIRI","Upper Scorpius","C/O ratio","chemical evolution","pebble drift","planet formation"],"falsifier":"A larger sample of young and old disks matched in millimeter flux, size, and inclination showing identical molecular line luminosities and C/O ratios at given accretion luminosity would falsify the claimed chemical evolution.","tokens_in":2762,"feed_emoji":"🪐","tokens_out":841,"duration_ms":27911,"temperature":0.7,"pith_summary":"The paper uses JWST/MIRI mid-infrared spectra to compare molecular gas in 5-10 million year old disks in Upper Scorpius with 1-3 million year old disks analyzed the same way. Half the older disks show molecular emission from species like water, CO2, HCN, and C2H2 while the rest are nearly silent except for H2; the older sample as a whole has lower detection rates for common molecules but higher rates for rare carbon chains, and line luminosities fall below the young-disk trend at fixed accretion luminosity. About half the older disks, especially the fainter and more compact ones, also display C- to O-bearing molecule ratios above the highest values seen in the young sample. A reader would care because these patterns suggest the inner-disk gas chemistry itself evolves with time, which would change the raw material available for forming planetary atmospheres.","feed_headline":"Older disks show fading molecular lines and higher C/O ratios","feed_subtitle":"JWST spectra of 5-10 Myr Upper Scorpius systems have lower line luminosities and elevated carbon-to-oxygen molecule ratios than 1-3 Myr disk","key_machinery":"Local thermal equilibrium slab models fitted to the mid-infrared emission lines to derive molecular column densities, temperatures, and C- to O-bearing molecule mass ratios, then compared across age groups at matched accretion luminosity.","core_discovery":"Analysis of the JWST/MIRI spectra with local thermal equilibrium slab models shows that Upper Scorpius disks have reduced inner-disk molecular gas masses, cooler emitting layers, and higher inner gas C/O ratios relative to the younger JDISCS sample; these differences are consistent with pebble drift and together indicate chemical evolution of inner disk gas between 1-3 and 5-10 Myr.","pith_inferences":["If pebble drift drives the C/O rise, then the timing of that rise should correlate with millimeter disk size and radial drift timescales in larger samples.","Planets that finish assembling after ~5 Myr could inherit systematically different atmospheric C/O ratios than those that finish earlier.","The same age trend may appear in other tracers such as the strength of the 10-micron silicate feature or the presence of organic ices once comparable mid-infrared spectra exist for more regions.","The transition from molecular-rich to molecular-poor disks could mark the point at which inner gas becomes too depleted to supply primordial atmospheres."],"forward_implications":["Molecular line luminosities are systematically lower in older disks at any given accretion luminosity and follow different scaling relations.","Rarer C-bearing molecules such as C4H2 are detected more often in the older population.","Roughly half the older disks, preferentially the millimeter-faint and compact ones, show C- to O-bearing molecule ratios exceeding the maximum values measured in young disks.","The patterns are consistent with pebble drift raising the inner gas C/O ratio over time.","The chemical changes have direct implications for the composition of gas accreted onto forming planets."],"fun_headline_variants":["Fading molecular emission and higher C/O in older disks","Lower molecular luminosities and higher C/O in USco","Elevated C/O in molecular poor older disk systems","Cooler emitting layers with higher C/O in older disks"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"The LTE slab model yields directly comparable gas properties across the young and older samples and differences in detection rates and luminosities are not dominated by selection effects in disk size, inclination, or stellar properties.","fun_headline_variants_meta":{"raw":{"variants":["Fading molecular emission and higher C/O in older disks","Lower molecular luminosities and higher C/O in USco","Elevated C/O in molecular poor older disk systems","Cooler emitting layers with higher C/O in older disks"]},"model":"grok-4.3","cost_usd":0.0118,"raw_usage":{"total_tokens":5217,"prompt_tokens":779,"num_sources_used":0,"completion_tokens":64,"cost_in_usd_ticks":117999500,"prompt_tokens_details":{"text_tokens":779,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":4374,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":779,"tokens_out":64,"duration_ms":62029,"temperature":1.0,"reasoning_tokens":4374,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-29T01:09:12.014737+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A larger sample of young and old disks matched in millimeter flux, size, and inclination showing identical molecular line luminosities and C/O ratios at given accretion luminosity would falsify the claimed chemical evolution.","supporting_citations":[],"review_version":1}