{"id":"79e09ea7-a58f-4ee0-be10-f1d294b881ae","arxiv_id":"2604.25742","paper_version":2,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":6.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"A multi-fluid extension of FARGO3D simulates polydisperse pebble accretion, reports efficiencies consistent with prior monodisperse work for static discs, and finds lower rates for high-Stokes particles when the gas disc is perturbed by the planet.","lead":"The paper develops and validates a multi-fluid hydrodynamic framework in modified FARGO3D to simulate pebble accretion from a continuous range of particle sizes rather than a single size. This enables study of how evolving gas discs and size distributions alter accretion rates onto growing planets.","discovery_kind":"new_method","skeptic_critique":{"model":"grok-4.3","headline":"Discretization of continuous MRN pebble distribution into modest number of St bins may not be converged for accretion rate and gas response","rationale":"The reader's weakest_assumption is precisely the load-bearing point. Full-text details on bin choice and any internal checks would be needed to move beyond UNVERDICTED, but the absence of explicit convergence evidence keeps the central accuracy claim conditional rather than established.","tokens_in":1860,"tokens_out":367,"duration_ms":20756,"concrete_test":"Rerun the fiducial MRN evolving-disc simulation (same planet mass, same FARGO3D setup) with twice as many logarithmically spaced St bins while keeping the same total solid mass; if the time-averaged total accretion rate or the gas surface-density perturbation at the planet's orbit changes by >10 %, the modest discretization is not yet converged.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim that the multi-fluid framework accurately represents polydisperse pebble accretion (and yields a higher polydisperse-to-monodisperse ratio than prior work) depends on the discrete bins (St ∈ [0.01,1]) introducing negligible error relative to a true continuous distribution. This must hold both for the static-disc validation and, more critically, for the evolving-disc case where the planet perturbs the gas and back-reaction matters. The abstract states consistency with earlier monodisperse studies but provides no bin-convergence tests, no sensitivity to bin placement or number, and no direct comparison against a finer or moment-based continuous treatment. Because accretion is stated to be dominated by the largest St and the MRN effect on gas perturbation increases with planet mass, even modest binning errors could alter the reported efficiencies and the claimed advantage over single-fluid models.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The paper develops a multi-fluid extension of FARGO3D to simulate polydisperse pebble accretion by representing a continuous MRN size distribution with a modest number of discrete fluid species at selected Stokes numbers (St ∈ [0.01,1]). It validates the approach by recovering accretion efficiencies consistent with prior monodisperse studies in static discs, then explores the effects of an evolving gas disc with solid-to-gas back-reaction, reporting lower efficiencies for St ≳ 0.3, higher efficiencies for St ≲ 0.3 (with the difference growing at higher planet masses), dominance of accretion by the largest St, and a higher polydisperse-to-monodisperse accretion-rate ratio than earlier estimates.","tokens_in":2063,"tokens_out":543,"duration_ms":23775,"significance":"If the discretization into a small number of St bins introduces negligible error relative to a true continuous distribution, the framework would enable simulations at higher planet masses and with multiple coupled pebble species, clarifying how protoplanet-induced gas perturbations alter accretion under realistic polydisperse conditions.","major_comments":[{"comment":"Abstract: the claim of consistency with earlier monodisperse studies is stated only qualitatively, with no reported quantitative metrics (relative errors, direct efficiency comparisons, or resolution details) to substantiate that the multi-fluid representation reproduces prior results within stated tolerances.","section":"Abstract"},{"comment":"Validation and methods sections: no bin-convergence tests, sensitivity studies on number of St bins, or bin-placement variations are presented for the range St ∈ [0.01,1]. Because accretion is stated to be dominated by the highest St and the MRN back-reaction effect strengthens with planet mass, this omission directly affects the reliability of both the static-disc validation and the reported changes in the evolving-disc case.","section":"Methods / Validation"},{"comment":"Results on evolving disc: the reported directional changes in efficiency (lower for St ≳ 0.3, higher for St ≲ 0.3) and the elevated polydisperse/monodisperse ratio rest on the assumption that the discrete-bin representation introduces negligible error in the gas response; without convergence data this assumption remains untested and load-bearing for the central claim.","section":"Results"}],"minor_comments":[{"comment":"Notation for the Stokes-number bins and the precise mapping from MRN distribution to fluid species should be defined explicitly with an equation or table.","section":"Methods"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their constructive and detailed report. We address each major comment below and commit to revisions that directly strengthen the validation and robustness of the multi-fluid framework.","responses":[{"response":"We agree that quantitative support is needed. In the revised manuscript we will expand the abstract and add a dedicated validation paragraph (or table) reporting relative differences in accretion efficiency, direct numerical comparisons to the cited monodisperse benchmarks, and the exact number of St bins and grid resolution employed.","revision_made":"yes","referee_comment":"[Abstract] Abstract: the claim of consistency with earlier monodisperse studies is stated only qualitatively, with no reported quantitative metrics (relative errors, direct efficiency comparisons, or resolution details) to substantiate that the multi-fluid representation reproduces prior results within stated tolerances."},{"response":"This is a fair criticism. While the bin selection was motivated by the MRN distribution and prior single-fluid work, explicit convergence tests were not included. We will add a new subsection presenting results for 3, 5, and 7 bins, together with shifts in bin placement, to demonstrate that the key accretion efficiencies and back-reaction trends converge for the adopted parameter range.","revision_made":"yes","referee_comment":"[Methods / Validation] Validation and methods sections: no bin-convergence tests, sensitivity studies on number of St bins, or bin-placement variations are presented for the range St ∈ [0.01,1]. Because accretion is stated to be dominated by the highest St and the MRN back-reaction effect strengthens with planet mass, this omission directly affects the reliability of both the static-disc validation and the reported changes in the evolving-disc case."},{"response":"We accept that the evolving-disc conclusions depend on the discretization error being small. The bin-convergence tests described above will directly test this assumption for both the static and perturbed cases. If any sensitivity is found, we will increase the number of bins and re-run the relevant simulations before final submission.","revision_made":"partial","referee_comment":"[Results] Results on evolving disc: the reported directional changes in efficiency (lower for St ≳ 0.3, higher for St ≲ 0.3) and the elevated polydisperse/monodisperse ratio rest on the assumption that the discrete-bin representation introduces negligible error in the gas response; without convergence data this assumption remains untested and load-bearing for the central claim."}],"tokens_in":1608,"tokens_out":536,"duration_ms":28855,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The core advance is a working multi-fluid extension inside FARGO3D that tracks several discrete Stokes-number bins at once while letting them exchange momentum with the gas. For a static disc the efficiencies line up with earlier monodisperse calculations, which at least shows the implementation is not obviously broken. When the gas is allowed to respond, the planet’s perturbation reduces accretion for St ≳ 0.3 and increases it for smaller St, and the net polydisperse-to-monodisperse ratio ends up larger than previous estimates. That directional change with planet mass is the quantitative result that was not already in the literature.\n\nThe code handles multiple species coupled to the gas, which is useful for runs at higher planet masses where single-fluid or test-particle approximations start to fail. The finding that the largest St bins still dominate the total rate is consistent with what one would expect from the MRN slope.\n\nThe main weakness is that the paper never tests whether the modest number of bins (St from 0.01 to 1) is sufficient. Accretion is stated to be dominated by the high-St end, and the gas back-reaction grows with planet mass, so even modest shifts in bin placement or number could change the reported efficiencies. No resolution study, no sensitivity run with more bins, and no comparison to a moment-based or finer discretization appear in the abstract or the summary. Without those, the claim that the framework “accurately” represents a continuous distribution rests only on matching static-disc literature.\n\nThis is the sort of methods paper that a small group working on global pebble-accretion simulations would want to read for the implementation details. A population-synthesis modeller could use the directional trends as a rough guide but would need the convergence numbers before adopting the efficiencies. The work is coherent on its own terms and shows honest engagement with the existing literature, so it deserves referee time once the authors supply the missing numerical checks.","headline":"The multi-fluid polydisperse setup in FARGO3D recovers prior static-disc results and shows gas perturbation effects on accretion, but the binning of the MRN distribution lacks any convergence checks.","tokens_in":2563,"tokens_out":475,"would_cite":false,"duration_ms":26990,"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":"Multi-fluid model shows a protoplanet's gas perturbation lowers polydisperse pebble accretion rates for MRN distributions.","keywords":["pebble accretion","polydisperse pebbles","multi-fluid hydrodynamics","planet formation","protoplanetary discs","accretion efficiency","MRN distribution"],"falsifier":"A side-by-side comparison of the multi-fluid accretion rate against a high-resolution particle simulation using the identical continuous size distribution would reveal whether the chosen discretization produces a measurable error.","tokens_in":2770,"feed_emoji":"🪐","tokens_out":454,"duration_ms":31698,"temperature":0.7,"pith_summary":"The paper develops and validates a multi-fluid hydrodynamic framework that treats a continuous pebble size distribution as several discrete fluid species, each with its own Stokes number, all coupled to the gas in 2D global disc simulations. When the gas disc remains static the resulting accretion efficiencies match those found in earlier single-size studies. Allowing the planet to perturb the gas flow reduces efficiency for larger pebbles while raising it for smaller ones, producing an overall lower accretion rate under an MRN size distribution. The framework is presented as a practical route to higher planet masses and to runs that include multiple pebble species simultaneously interacting with the gas.","feed_headline":"Planet gas perturbation cuts polydisperse pebble accretion","feed_subtitle":"Multi-fluid runs show lower rates for MRN solids once the disc evolves under the planet's gravity","key_machinery":"Multi-fluid treatment of polydisperse pebbles as separate fluid species with distinct Stokes numbers evolved inside modified FARGO3D global disc simulations.","core_discovery":"We constructed a multi-fluid model framework capable of accurately simulating polydisperse pebble accretion consistent with previous studies. This framework offers advantages for simulating higher planet masses and for modelling multiple pebble species coupled to the gas. We find that the protoplanet's perturbation of the gas-disc lowers the accretion rate when assuming an MRN-distribution of solids.","pith_inferences":[],"forward_implications":[],"fun_headline_variants":["Planet perturbs gas to reduce polydisperse pebble accretion","Multi-fluid model shows MRN solids accrete slower","Gas evolution under planet lowers polydisperse accretion","Protoplanet perturbation drops pebble accretion rates"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"Representing a continuous pebble size distribution by a modest number of discrete fluid species introduces negligible error in the total accretion rate and in the response of the gas disc.","fun_headline_variants_meta":{"raw":{"variants":["Planet perturbs gas to reduce polydisperse pebble accretion","Multi-fluid model shows MRN solids accrete slower","Gas evolution under planet lowers polydisperse accretion","Protoplanet perturbation drops pebble accretion rates"]},"model":"grok-4.3","cost_usd":0.006252,"raw_usage":{"total_tokens":2994,"prompt_tokens":771,"num_sources_used":0,"completion_tokens":60,"cost_in_usd_ticks":62524500,"prompt_tokens_details":{"text_tokens":771,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2163,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":771,"tokens_out":60,"duration_ms":22561,"temperature":1.0,"reasoning_tokens":2163,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-01T08:23:48.224881+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A side-by-side comparison of the multi-fluid accretion rate against a high-resolution particle simulation using the identical continuous size distribution would reveal whether the chosen discretization produces a measurable error.","supporting_citations":[],"review_version":2}