{"id":"ec8c322a-74d0-4ebf-8c8c-8c53ed30417e","arxiv_id":"2607.18683","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":10,"one_line_summary":"AB Aur's outer disk is chemically split in azimuth: SO peaks on the infall-hit north side, C2H on the south, pointing to a carbon-to-oxygen gradient.","lead":"Using NOEMA millimeter observations of the AB Aur planet-forming disk, the authors map six molecules and find sulfur monoxide (SO) concentrated in the north, near where a stream of infalling gas hits the disk, while the hydrocarbon C2H peaks on the opposite side. The pattern suggests the two sides of the disk carry different carbon-to-oxygen ratios, shaped by infalling material on one side and a forming planet on the other.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"C/O-gradient interpretation rests on a chemical model that cannot simultaneously reproduce CS, SO, and C2H; missing sulfur chemistry or the fixed T_rot=40 K could change the inferred C/O and its azimuthal sign.","rationale":"The reader's conditional verdict is well supported. The paper's main observational finding — a resolved SO/C2H anti-correlation with axisymmetric CS — stands on careful NOEMA imaging and multiple hyperfine components. However, the central interpretation (azimuthal gas-phase C/O gradient) is defended mainly by the ALCHEMIC comparison, and that comparison is internally acknowledged to fail for the very molecules it is supposed to interpret. Without a single C/O (or a smoothly varying C/O) that fits all three tracers, the inference that the north is O-rich and the south C-rich is an extrapolation. The fixed T_rot=40 K comparison adds a second uncontrolled degree of freedom. A controlled rerun of the model grid with updated sulfur chemistry and a temperature scan would settle whether the claimed sector C/O difference is required by the data or an artifact of the network and temperature assumption. I therefore agree with the reader's weakest_assumption and recommend no change to the conditional verdict.","tokens_in":30889,"tokens_out":3914,"duration_ms":38844,"concrete_test":"Run a controlled model experiment: take the ALCHEMIC grid of §4, add the missing sulfur chemistry (e.g., S/S2/H2S/organosulfides and S+OH/SH+O reactions, following Ferrari et al. 2024) and recompute the Fig. 11 comparison using the full posterior distributions of N(SO), N(CS) from the rotational fits and N(C2H) evaluated at T_rot=25, 40, and 100 K. If a single C/O per sector can reproduce all three species within 3σ under the updated network and any T_rot, the gradient survives; if the implied C/O values still disagree by >0.3 or the SO/C2H fit requires T_rot>100 K, the chemical-model support for the claimed azimuthal C/O gradient fails.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 4.2 concedes the key interpretive link: the models 'cannot simultaneously reproduce the abundances of CS, SO, and C2H ... with a single C/O ratio' — CS prefers C/O slightly below 1, SO slightly above 1, C2H requires C/O≫1. Yet the paper concludes C/O≥1 and a sector C/O gradient. The supporting mapping (SO at low C/O, C2H at high C/O, CS insensitive) is not validated for AB Aur's outer disk; the authors attribute the failure to missing sulfur chemistry. If missing sulfur pathways (or S-bearing ice chemistry) boost SO independently of C/O, the SO-bright north need not be O-rich, and the C2H-bright south could reflect local UV/temperature rather than C-rich gas. The comparison is also made at a fixed T_rot=40 K (Fig. 11); C2H column densities vary by ~2–3× over the plausible 15–160 K range, enough to shift the inferred C/O. Secondary: SO fluxes used for the rotational diagram are 2–3× higher than Dutrey et al. (2024) for the same transitions (Section 3.2.1), anchoring the 'warm north' interpretation; the authors decline to probe this. The morphological anti-correlation is robust; the C/O-gradient claim is not.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents new NOEMA 1.2 mm observations of AB Aur and analyses maps of CS, SO, C2H, HCN, HCO+, and H13CO+. The central observational result is a chemically selective azimuthal asymmetry: SO is enhanced in the northern sector near the inferred streamer–disk interaction region, C2H peaks on the opposite southern side, CS forms a nearly axisymmetric ring, and HCN/HCO+ are brightest near the dust continuum overdensity. The authors perform LTE rotational-diagram analyses for SO and CS, derive C2H column densities under assumed rotational temperatures, and compare the derived columns with ALCHEMIC gas-grain chemical models over a grid of C/O ratios. They conclude that the data favor gas-phase C/O near or above unity, with a higher effective C/O in the C2H-bright sector, and discuss two scenarios: infall-induced O-rich chemistry at the streamer impact site and planet-driven carbon enhancement on the opposite side. The paper includes extensive appendices with continuum maps, channel maps, and archival data.","tokens_in":31200,"tokens_out":6566,"duration_ms":65249,"significance":"If the azimuthal C/O interpretation is accepted, this would be an important step: it would demonstrate that infall and planet formation jointly produce resolvable chemical inhomogeneity in a single disk, with consequences for planet atmospheric composition. The strictly observational message is strong and valuable independent of the chemical modeling: multiple SO transitions, four C2H hyperfine components stacked in visibility, and an independent CS control all point to a robust spatial anti-correlation. The authors also make sensible methodological choices in moment extraction and provide the channel-map material needed to judge the maps. However, the quantitative bridge from the observed anti-correlation to the headline C/O gradient is currently built on a chemical model that the authors themselves state cannot reproduce the three key molecules simultaneously, and it is sensitive to a fixed T_rot assumption and to a factor 2–3 SO flux discrepancy with previously published work. The paper’s cautious wording in Section 4.2 is not carried through to the abstract and summary, which assert the C/O inference more strongly than the model validation supports.","major_comments":[{"comment":"The central interpretive claim, C/O ≥ 1 with a higher effective C/O in the C2H-bright sector, is not supported by a model that simultaneously fits the data. The text states that the models 'cannot simultaneously reproduce the abundances of CS, SO, and C2H ... with a single C/O ratio', that CS prefers C/O slightly below unity, SO slightly above unity, and C2H requires C/O ≫ 1, and that the sulfur network is likely incomplete. Yet the abstract and Section 6 conclude that the comparison favors C/O near or above unity. Because the SO–C2H anti-correlation is the basis for the sector C/O difference, the conclusion depends on the reliability of the tracer–C/O mapping. The authors should either provide sensitivity tests showing the mapping is robust against the known missing sulfur chemistry (e.g., varying sulfur depletion, adding simple organosulfide pathways, changing the gas-to-dust ratio), o","section":"§4.2, Figs. 11–12"},{"comment":"The model comparison for C2H is made at a fixed T_rot = 40 K, but the C2H column density depends sensitively on the assumed excitation. Eq. (3) and Fig. 8 show that N(C2H) varies by roughly a factor 2–3 over the plausible range 15–160 K, with the minimum near E_u = 25 K. Since C2H is the molecule that drives the C/O ≫ 1 conclusion, the inferred C/O could shift by a full grid step if the emitting layer is warmer or cooler than 40 K. The authors should show the C2H model comparison over the allowed T_rot range, or better, compute model line intensities at the model gas temperature rather than quoting a single fixed-temperature column. This is a load-bearing point because the abstract’s claim of C/O near or above unity rests substantially on the C2H comparison.","section":"§3.2.2, §4.2, Eq. (3), Fig. 8"},{"comment":"The SO rotational-temperature and column-density results, which anchor the 'warm, O-rich north' interpretation, are derived from fluxes that the paper reports as a factor 2–3 higher than Dutrey et al. (2024) for the same transitions. The authors state that the absolute flux calibration uncertainty is ~10% and that investigating the discrepancy is beyond their scope. This discrepancy is not a minor calibration detail: it moves the disk-averaged SO T_rot from ~20 K to 37 K and directly affects the NE/SW temperature and column-density contrast used to infer the sector C/O difference. I am not asking for a full re-analysis, but the manuscript should at least (i) quantify how the derived T_rot and N_SO profiles change if the Dutrey fluxes are adopted, and (ii) explicitly state that the quantitative sector C/O inference is uncertain at this level. The morphological anti-correlation is unaffect","section":"§3.2.1"},{"comment":"The two physical scenarios (infall-driven O-rich chemistry in the north, planet-driven carbon enhancement in the south) are presented as the likely origin of the C/O gradient, but neither scenario is modeled with local conditions; the case is made by qualitative mapping and by spatial coincidence with previously reported features. Given that the chemical network already fails to reproduce CS, SO, and C2H simultaneously in the axisymmetric model, the sector-specific scenarios would benefit from a concrete test: for example, a local C/O perturbation, a temperature perturbation, or an S-ice desorption prescription applied to the same grid, with predicted SO/C2H/CS maps or visibilities. The current text lists testable predictions, which is good, but the connection between the observed asymmetry and a specific physical mechanism remains model-unsupported. This should be acknowledged in the ab","section":"§5.1.1, §5.1.2"}],"minor_comments":[{"comment":"Typo: 'a small enhancement in appears in the quadratic fit' should read 'a small enhancement appears in ...'. Also, 'the only specie' should be 'the only species'.","section":"§3.1"},{"comment":"Fig. 11 caption says 'chemical productions'; this should be 'chemical model predictions' or 'model abundances'.","section":"§4.2, Fig. 11"},{"comment":"The sentence 'Together, these processes redistributes carbon- and oxygen-bearing volatiles' has a subject-verb agreement error: 'redistribute'.","section":"§4"},{"comment":"The statement 'we refrain from drawing conclusions regarding the differences with Dutrey et al. (2024)' is contradicted a little by the immediately preceding discussion; it would be helpful to state clearly which of the paper’s quantitative results would survive if the lower fluxes were used.","section":"§3.2.1"},{"comment":"The footnote for the CS 7–6 flux is verbose; a concise statement of the integration method and the comparison value would be easier to read.","section":"Table D.1"},{"comment":"The notation 'r=0.5' is used for the robust parameter in the caption text but is not defined there; since the main text uses 'Briggs robust parameter', the caption should be unambiguous.","section":"Fig. 3/4 captions"}],"recommendation":"major_revision","confidential_remarks":"The stress-test concern is largely on target: the model mismatch in Section 4.2 is acknowledged in the text but the abstract and summary do not carry the same caveat. The robust observational anti-correlation is the paper’s main asset; I would ask the authors to either validate the C/O mapping with sensitivity tests and a temperature treatment, or soften the headline claim to a model-dependent interpretation. The SO flux discrepancy with Dutrey et al. should be treated as a quantitative uncertainty rather than deferred."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague — quick take on Jiang et al.: the morphological result is the strong part. They've shown a resolved azimuthal anti-correlation between SO (north, at the streamer-impact site) and C2H (south), with CS plausibly axisymmetric, and they've got the first C2H and H13CO+ detections in AB Aur. Multiple SO transitions, stacked hyperfine components, and independent maps point the same way; I trust the morphology. The excitation-versus-abundance argument is also careful, and it holds: a temperature contrast can reduce the C2H asymmetry but it would also make CS asymmetric, which is not seen.\n\nWhat's new is the joint infall/planet narrative: late infall resets the north to O-rich chemistry while the south stays C-rich, perhaps due to a planet. That's a nice story and it makes testable predictions. But the story leans hard on the ALCHEMIC C/O mapping, and Section 4.2 admits the models cannot simultaneously reproduce CS, SO, and C2H at any single C/O. So the \"C/O >= 1\" and \"higher C/O in the C2H-bright sector\" conclusions are on shakier ground than the abstract implies. Missing sulfur chemistry or the fixed T_rot=40 K comparison could shift the inferred sector difference or its sign. The authors are transparent about this, which is good, but it means the chemical interpretation is conditional on a network they know is incomplete.\n\nThe second soft spot is the SO flux discrepancy with Dutrey et al. (2024): factor 2-3 higher than published values for the same lines, beyond calibration uncertainty, and they decline to investigate. That discrepancy anchors the warm-north/rich-SO rotational-temperature contrast. It's not fatal for the morphology, but it's a loose thread that a referee will want pulled.\n\nNet: this deserves a serious referee. The observational content is solid, the data handling is detailed, and the interpretation is honestly labelled. A good referee should push on the flux systematics and ask the authors to state more carefully what the C/O sector difference depends on. I'd cite this for the resolved chemical asymmetry; the C/O gradient needs more support before it becomes a reference result.","headline":"Resolved SO–C2H anti-correlation in AB Aur is a real observational result; the azimuthal C/O-gradient interpretation is plausible but conditional on a chemical model the authors admit cannot fit all three tracers.","tokens_in":31803,"tokens_out":2979,"would_cite":true,"duration_ms":28142,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"AB Aur's planet-forming disk is azimuthally chemically differentiated: SO peaks in the north at the streamer impact site, C2H in the south, which the paper attributes to a gas-phase C/O gradient jointly maintained by late infall and planet","keywords":["protoplanetary disks","disk chemistry","AB Aur","azimuthal chemical asymmetry","gas-phase C/O ratio","late infall","streamers","molecular line survey"],"falsifier":"Settle it with a calculation: run a chemical network that includes revised sulfur depletion or shock chemistry to see whether a single disk-wide C/O can already produce the observed SO and C2H contrasts; if so, the azimuthal C/O gradient is not needed. Observationally, detect a second C2H transition to break the excitation degeneracy—if C2H column densities become symmetric once temperature is modeled properly, the southern C-rich reading fails. Also image SO2 and OCS: if they do not peak near the northern streamer impact site, the infall-desorption scenario for the O-rich sector is contradict","tokens_in":30713,"feed_emoji":"🔭","tokens_out":5099,"duration_ms":46273,"temperature":0.7,"pith_summary":"Using new 1.2 mm interferometric observations, this paper tries to establish that the AB Aur protoplanetary disk is not chemically uniform around its ring. It finds sulfur monoxide (SO) emission concentrated in the northern sector at the inferred impact site of a late-infall streamer, while the ethynyl radical C2H peaks on the opposite southern side and carbon monosulfide (CS) remains an almost symmetric ring. Multi-transition analysis shows the SO enhancement is a real abundance and temperature contrast, not just an excitation effect. Comparison with gas-grain chemical models favors an azimuthal gradient in effective gas-phase C/O: lower (more oxygen-rich) in the north, higher (carbon-rich) in the south, with the global C/O at or above unity. If right, the result shows that environmental accretion and planet formation can jointly create chemical heterogeneity that affects the compositions of forming planets.","feed_headline":"AB Aur disk is chemically split: SO north, C2H south","feed_subtitle":"Millimeter-wave maps tie the split to an azimuthal C/O gradient shaped by late infall and planet formation.","key_machinery":"The analytical engine is a set of molecular line diagnostics applied to a 1.2 mm spectral survey: multi-transition LTE rotational diagram fits for SO and CS to separate column density from excitation; a stacked C2H hyperfine map; an HCO+/H13CO+ ratio to constrain optical depth; and a grid of time-dependent gas-grain chemical models varying elemental C/O. The load-bearing identity is the tracer–C/O mapping—SO favored at low C/O, C2H favored at high C/O, CS nearly insensitive—which converts the observed SO–C2H anti-correlation into an inferred azimuthal C/O gradient.","core_discovery":"The central discovery is the chemically selective azimuthal differentiation of the AB Aur disk at roughly 150–220 au: all detected SO transitions peak in the north, spatially coincident with the streamer–disk interaction region; C2H peaks on the opposite southern side; CS forms a nearly axisymmetric ring; HCN and HCO+ peak near a dust continuum overdensity. The paper further claims that this pattern cannot be fully explained by excitation or temperature differences, and that chemical modeling points to a relative difference in effective gas-phase C/O between sectors—higher in the C2H-bright south, with a global C/O at or above unity. It proposes two non-exclusive causes: infall-induced heati","pith_inferences":["If late infall episodically resets local C/O toward ISM-like values, disk-averaged C/O measurements in similar disks could hide strong azimuthal variation; averaging O-rich and C-rich sectors might produce a misleadingly solar value.","The SO/C2H anti-correlation could serve as a two-color azimuthal C/O diagnostic; applying it to other disks with known streamers would test whether infall impacts generically create SO-bright, O-rich sectors.","Because the sulfur network cannot fit all three species simultaneously, current models likely miss shock or organosulfur pathways; incorporating them might shift the inferred C/O values, so the quantitative gradient should be treated as provisional.","A direct test would be deep searches for SO2, OCS, warm H2CO, and CH3OH in the north and CN or c-C3H2 in the south; their spatial coincidence with the respective sectors would support the two-mechanism picture."],"forward_implications":["AB Aur's outer molecular ring is chemically heterogeneous: SO and C2H are anti-correlated at ~200 au while CS stays symmetric.","The global gas-phase C/O in AB Aur's outer disk is near or above unity rather than the solar/ISM value of ~0.4.","The northern sector is O-rich and the southern sector C-rich in effective gas-phase composition, implying that planets forming in different sectors can inherit different carbon-to-oxygen ratios.","HCO+ is optically thick across the molecular ring, so its azimuthal brightness maps excitation temperature and the tau~1 surface, not column density.","The elevated HCO+/H13CO+ ratio inside the cavity indicates an enhanced 12C/13C ratio from isotope-selective photodissociation, so giant planets with sub-ISM 12C/13C likely accreted their atmospheres from the outer disk."],"fun_headline_variants":["SO north, C2H south: AB Aur disk chemistry split by infall","Streamer-driven chemistry: SO enhanced where infall hits","C2H-rich south, SO-rich north: AB Aur's chemical map","AB Aur disk C/O gradient traced by SO and C2H","Infall and planets imprint chemical asymmetry on AB Aur"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The inference from chemical asymmetries to an azimuthal C/O gradient rests on the assumption that current sulfur chemistry networks map SO to low C/O, C2H to high C/O, and CS to insensitivity correctly in AB Aur's outer disk; the paper itself concedes that no single C/O ratio can simultaneously reproduce the observed CS, SO, and C2H abundances, so if that tracer–C/O mapping is warped by missing sulfur chemistry or by the fixed 40 K temperature used for comparison, the inferre","fun_headline_variants_meta":{"raw":{"variants":["SO north, C2H south: AB Aur disk chemistry split by infall","Streamer-driven chemistry: SO enhanced where infall hits","C2H-rich south, SO-rich north: AB Aur's chemical map","AB Aur disk C/O gradient traced by SO and C2H","Infall and planets imprint chemical asymmetry on AB Aur"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000761,"raw_usage":{"total_tokens":3290,"prompt_tokens":892,"completion_tokens":2398,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":636,"completion_tokens_details":{"reasoning_tokens":2321}},"tokens_in":636,"tokens_out":2398,"duration_ms":18508,"temperature":1.0,"reasoning_tokens":2321,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-01T14:39:24.707750+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Settle it with a calculation: run a chemical network that includes revised sulfur depletion or shock chemistry to see whether a single disk-wide C/O can already produce the observed SO and C2H contrasts; if so, the azimuthal C/O gradient is not needed. Observationally, detect a second C2H transition to break the excitation degeneracy—if C2H column densities become symmetric once temperature is modeled properly, the southern C-rich reading fails. Also image SO2 and OCS: if they do not peak near the northern streamer impact site, the infall-desorption scenario for the O-rich sector is contradict","supporting_citations":[],"review_version":1}