{"id":"0ab857fc-40e2-4d86-a636-c586ad222e6c","arxiv_id":"2608.02204","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Cloud-fed ideal-MHD zoom-in simulations of nine young stars show discs are replenished on ~10,000-year timescales via surface-layer accretion and can be truncated by massive streamers.","lead":"This paper uses 3D magnetohydrodynamic simulations to follow the first 100,000 years of nine newborn star–disc systems forming inside a turbulent molecular cloud. It reports that young discs are continuously restocked, that clumpy streamers can temporarily shrink discs, and that accretion happens mainly through disc surface layers once a disc matures.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Universal rapid-replenishment claim rests on a single quiet system (180); surface-layer mode not shown for other eight systems.","rationale":"The reader's weakest assumption concerned ideal MHD, barotropic EOS, and resolution-dependent sink accretion, which are external validity threats. My concern is internal to the presented analysis: the key quantitative evidence for the universal replenishment and surface-layer claim comes from a single, deliberately quiet system (180) and is not demonstrated for the other eight systems. This is a load-bearing gap because the abstract and conclusion generalize the finding—e.g., 'the full disc mass reservoir is replenished on 10 kyr-timescales' and 'Common to all systems... predominantly along the disc surface'—without showing the corresponding mass-flux decomposition for the rest of the sample. I also note the explicit 50% vs. 65% disc-size reduction inconsistency between the abstract and Sec 5.5/conclusion, which warrants correction. My recommended verdict remains CONDITIONAL (unchanged from the reader), but the condition should include verifying the generality of the surface-layer analysis across the sample. I do not find an internal logical contradiction in the central argument; the simulations are state-of-the-art for this regime and the authors are transparent about resolution caveats (Appendix A.4). The concern is about overgeneralization from a single case, which is testable with the existing output data by applying the published analysis pipeline to other systems.","tokens_in":33493,"tokens_out":9913,"duration_ms":86949,"concrete_test":"Apply the same mass-flux decomposition (Eq. 5, Fig. 6) and replenishment-time calculation (Eqs. 12–13, Fig. 10) to at least three other systems from the sample, e.g., 13 (with a streamer), 82 (still growing), and 162 (quiet but formed from a different core). If the surface-layer inward mass flux and τ_rep ≈ 10 kyr are not recovered in these systems, the paper must soften the universal claim to a case study of system 180.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that during later quiescent stages (t≳50 kyr) accretion is predominantly through the midplane and disc surface layers, and that the full disc mass reservoir is replenished on 10 kyr timescales, is supported only by the detailed analysis of system 180. Section 4.5 explicitly presents the replenishment-time calculation (Eq. 12–13) and Fig. 10 for system 180, which was selected (Sec. 4) because it did not experience a major disruptive streamer event, has a low magnetic field strength, low mass budget, and low rotational energy. The conclusion (Sec. 8) nevertheless asserts this is 'common to all systems', and the abstract generalizes to 'young discs' at large. Section 5 shows other systems behave diversely—e.g., 82 and 122 do not approach a quasi-steady state (Sec. 5.5)—but no equivalent mass-flux or replenishment analysis is presented for them. Additionally, the abstract states streamers reduce disc size 'by half' while Sec 5.5 and the conclusion quote 65%, an internal inconsistency that should be reconciled. If the surface-layer mode and 10 kyr replenishment are not recovered in the other systems, the universal, planet-formation-relevant conclusion is not justified.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents three-dimensional ideal-MHD zoom-in simulations of nine young protoplanetary-disc systems formed in a (4 pc)^3 molecular-cloud simulation, resolving the inner discs to 0.8 au. The authors characterize disc formation, accretion modes, magnetic-field structure, streamer infall, and disc-size/mass evolution over ~10^5 yr. The central claims are that in the later quiescent phase (t ≳ 50 kyr) accretion proceeds predominantly through the disc surface layers, that the full disc mass reservoir is replenished on ~10 kyr timescales, and that massive streamers episodically truncate discs by ~65% in radius and ~40% in mass, making the outer regions of young discs inhospitable to planet formation.","tokens_in":33723,"tokens_out":4552,"duration_ms":44058,"significance":"If correct, the paper would substantially advance the picture of Class 0 disc evolution: rather than isolated, viscously spreading discs, these systems are open, continuously replenished reservoirs whose outer regions are too dynamic for early planet formation. The work benefits from a realistic cloud-scale environment, a sample of nine systems, and unusually detailed appendices, including a resolution study and a transparent description of the sink model and disc-fitting procedures. The qualitative scenario is internally consistent and the simulation campaign is a technical achievement. However, the most general claims — surface-layer accretion and 10 kyr replenishment — are demonstrated in detail for only one, deliberately quiet system, and the paper's own resolution study warns that quantitative accretion rates have not fully converged. The significance of the universal conclusion is therefore not yet established at the level claimed.","major_comments":[{"comment":"The central universal claim is supported by only one system. The replenishment-time analysis (Eqs. 12–13) and Fig. 10 are computed exclusively for system 180, which was explicitly selected because it did not experience a major disruptive streamer event and has low magnetic field, low mass budget, and low rotational energy (Sec. 4). Section 8 nevertheless concludes that 'accretion flows predominantly along the disc surface' are 'common to all systems', and the abstract generalizes to 'young discs'. Section 5 shows large diversity — e.g., systems 82 and 122 do not approach a quasi-steady state (Sec. 5.5) — but no equivalent spherical-shell or cylindrical mass-flux decomposition is presented for the other eight systems. The manuscript should either provide the surface-layer/replenishment analysis for the full sample or explicitly restrict the claim to the quiescent, ideal-MHD regime exempli","section":"Sec. 4.5, Sec. 8, Abstract"},{"comment":"The quantitative accretion and replenishment rates are resolution-dependent. Figure A.1 shows that stellar accretion rates decrease with increasing resolution for systems 13 and 225, and the text estimates that at 0.8 au resolution the stellar mass is overestimated by up to 30%. The headline values Mdot ~ 10^-5 M_sun/yr and tau_rep ~ 10 kyr are derived from system 180 at the fiducial 0.8 au resolution, but no convergence test is shown for that system or for the cylindrical mass-flux measurement underlying Eq. (13). If the true accretion rate is lower, the replenishment time is correspondingly longer. The authors should quantify this uncertainty for the specific quantities used in the central claim, or explicitly state that tau_rep is a resolution-dependent upper/lower bound.","section":"Appendix A.4, Sec. 4.5"},{"comment":"The applicability of the surface-layer accretion and replenishment picture to real discs rests on the ideal-MHD assumption, which the authors themselves call 'a crude approximation during the initial stages of disc growth on small scales ≲10 au'. The 10 kyr replenishment calculation includes radii r < 20 au (Fig. 10), and the surface-layer/toroidal-field-reversal pattern is an ideal-MHD result. Non-ideal effects such as ambipolar diffusion are known to reduce magnetic braking and alter the vertical current distribution, so they could change both the accretion geometry and the replenishment timescale. Since the planet-formation implications in Sec. 7 depend on high replenishment rates and strong turbulence, the manuscript should either provide a quantitative estimate of the expected non-ideal correction or clearly limit the conclusions to the well-ionised, ideal-MHD regime.","section":"Sec. 6.1, Sec. 7"}],"minor_comments":[{"comment":"The abstract states streamers reduce disc size 'by half', while Sec. 5.5 and the conclusion state 'an average reduction by approximately 65 %'. These should be reconciled.","section":"Abstract vs. Sec. 5.5 / Sec. 8"},{"comment":"The term 'destroyer-class streamer' is introduced informally. Since it is used as a classification criterion (M_str > M_disc), it would be clearer to state this condition explicitly at first use and give the threshold for the class.","section":"Sec. 5.4"},{"comment":"The text says 'accretion rates decrease with increasing levels of refinement' but does not state the magnitude of the change for the two test systems. Adding the percentage decrease or a range in the caption of Fig. A.1 would help readers gauge the convergence uncertainty.","section":"Appendix A.4"},{"comment":"The sentence 'The dashed grey line in panel b of Figure 8 shows the thermal scale height for a nearly isothermal disc' is followed by reference to a 'black dash-dotted line' without a clear colormap/line-style key in the text. Consider making the line styles easier to distinguish in the printed figure.","section":"Sec. 4.4.2 / Fig. 8"},{"comment":"The reference to 'recent observations hint that effective ionisation rates are high near young stellar objects' is supported by several citations, but the sentence does not specify which of the cited observations corresponds to B335 versus L1157; moving the parenthetical objects to the relevant citations would improve clarity.","section":"Sec. 6.1"}],"recommendation":"major_revision","confidential_remarks":"The core simulation campaign and the detailed appendices are strong. My recommendation is driven by the gap between the single-system analysis of the surface-layer/replenishment claim and the universal phrasing in the abstract and conclusions. This is fixable: either the analysis is extended to the full sample, or the claims are substantially tempered. The ideal-MHD caveat is acknowledged by the authors, but because it interacts with the central quantitative claim, it should be treated as a load-bearing limitation rather than a routine caveat. I see no concern about novelty or citation practices."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The useful new content here is the setup, not the individual physics. Nine Class 0 systems followed in a cloud-fed zoom-in at 0.8 au, with a streamer census, a 'destroyer-class' truncation event, and a replenishment timescale calculation. That combination is new and worth engaging with. The surface-layer accretion and toroidal field reversal were already in isolated ideal-MHD disc simulations, but showing them in an embedded, cloud-fed context is a fair extension, and the authors are transparent that they recover a known mechanism rather than inventing a new one.\n\nThe paper also earns credit for its own caveats: Appendix A.4 shows accretion rates are resolution-dependent and that stellar masses may be overestimated by up to 30%; Section 6.1 admits ideal MHD is crude at small scales; and the sink parameters are discussed openly. Those are real limitations, but they are stated rather than hidden.\n\nThe soft spot the stress-test flags is real. The detailed mass-flux and replenishment analysis in Sections 4.2–4.5 is presented for system 180 only. That system was explicitly chosen because it is quiet, weakly magnetized, and had no major streamer. The conclusion then says that rapid replenishment and surface-layer accretion are 'common to all systems', and the abstract generalizes to 'young discs'. I do not see equivalent angular-resolved mass-flux or replenishment analysis for the other eight systems in the paper. Systems 82 and 122, by the authors' own account, do not approach quasi-steady state. The universal claim may be true, but this paper has not shown it. The authors should either provide the evidence for the full sample or soften the claim to a demonstration in one representative case plus suggestive agreement in the accretion rates of the others.\n\nThere is also a direct internal inconsistency: Section 5.5 and the conclusion say streamers reduce disc size by an average of 65%, while the abstract says 'by half'. That is the kind of number that should agree.\n\nOverall the paper is a solid computational study with genuine new data and a plausible qualitative scenario, but its strongest conclusion is undersupported as written. I would send it to peer review without hesitation — the referee can push for the sample-wide evidence and the reconciled numbers. This is useful reading for anyone working on Class 0 disc formation, streamers, or the environment dependence of early disc evolution. I would not cite the universal replenishment claim in its current form, but I would cite the setup and the streamer census.\n\nRecommendation: serious referee, likely with requested revisions rather than immediate acceptance.","headline":"Worth reading and refereeing: a genuinely new cloud-fed zoom-in sample of nine discs with a streamer census and replenishment calculation, but the headline claim that the 10 kyr replenishment and surface-layer mode are common to all systems is only demonstrated for one quiet system, and the truncation numbers disagree with themselves.","tokens_in":34322,"tokens_out":1648,"would_cite":true,"duration_ms":15940,"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":"Young protoplanetary discs are open, fast-replenished systems: the full gas reservoir is exchanged on ~10 kyr timescales, and massive streamers repeatedly truncate the discs.","keywords":["protoplanetary discs","star formation","ideal magnetohydrodynamics","accretion","streamers","disc replenishment","Class 0 protostars","planet formation"],"falsifier":"Run the same nine cloud-fed cores with non-ideal MHD (ambipolar diffusion and Ohmic resistivity) and radiative transfer at comparable resolution: if the surface-layer accretion channel and toroidal-field reversal disappear, or if the outer-disc replenishment time rises above ~100 kyr, the central claim fails. Observationally, mapping ionisation tracers and infall kinematics in the outer regions of Class 0 discs could test whether the assumed well-ionised, strongly coupled regime actually holds.","tokens_in":33308,"feed_emoji":"🪐","tokens_out":5804,"duration_ms":49725,"temperature":0.7,"pith_summary":"The paper tries to establish that the earliest protoplanetary discs—still embedded in their natal cloud—are open systems, not closed reservoirs. From zoom-in ideal-magnetohydrodynamic simulations of nine forming star–disc systems resolved to 0.8 au, it argues that after roughly 50 kyr accretion onto the star proceeds through the midplane and a high-density surface layer, tied to a toroidal magnetic field with reversals across the disc, and that the entire disc mass is replenished on ~10 kyr timescales. It further identifies massive, ~0.2 solar-mass streamers that deliver gas at rates comparable to the background accretion flow and, on impact, shrink discs by about 65% in radius and 40% in mass. If true, these results imply that the outer parts of very young discs are too rapidly refreshed and too turbulent to build planets, and that quiescent planet-forming conditions must wait until later evolution.","feed_headline":"Young discs refill gas every 10,000 years","feed_subtitle":"Surface-layer accretion and streamer impacts keep very young outer discs hostile to planet formation.","key_machinery":"The central mechanism is layered 'surface-layer accretion' driven by a wound-up toroidal magnetic field. The field, with reversals across both disc surfaces and a current sheet in the midplane, transports angular momentum vertically and funnels gas inwards along the disc surfaces, while the midplane remains turbulent. The paper quantifies this with a replenishment timescale tau_rep(r) = M_disc(r) / Mdot_disc(r), computed through cylindrical shells, and identifies 'destroyer-class' streamers—overdense infalling filaments that deposit mass comparable to the background accretion flow and truncate the disc. The numerical machinery is zoom-in adaptive-mesh ideal-MHD simulation of cloud-fed collap","core_discovery":"On its own terms, the paper claims that Class 0 discs pass through two stages. During the first ~50 kyr, the disc forms and grows through filamentary infall while polar outflows carve cavities. In the later quiescent stage, a layered accretion mode sets in: the midplane is turbulent while the disc surface layers carry the bulk of the inwards mass flux at ~1e-5 solar masses per year, and the toroidal magnetic field develops sign reversals across the surfaces and a current sheet in the midplane. The defining quantitative result is a replenishment timescale tau_rep = M_disc / Mdot_disc of about 10 kyr in the outer disc and 0.1 kyr in the inner disc, meaning the full disc mass reservoir is excha","pith_inferences":["If the 10 kyr replenishment is representative, the onset of planet formation is time-sequenced: the inner few au may quench first while the outer disc remains hostile; searches for planet formation tracers should be weighted toward older, less infall-dominated systems.","The streamer truncation and regrowth cycle predicts observable variability in disc outer radii and accretion rates at ~10 kyr intervals; high-cadence monitoring of Class 0 or outbursting young stellar objects could test this directly.","Because the simulations are ideal MHD, they likely represent a maximally magnetically coupled regime; a non-ideal MHD rerun could weaken the surface-layer channel and change the replenishment timescale, so the 10 kyr number is best read as contingent on that assumption.","With roughly five massive streamers among nine systems over 100 kyr, most young discs should experience at least one destroyer-class event, implying that snapshot measurements of Class 0 disc sizes may often catch discs in a post-truncation regrowth phase."],"forward_implications":["Outer regions of very young discs are not planet-forming sites: high turbulence, rapid gas refreshment, and streamer disruption hinder dust growth and planetesimal formation.","Discs can remain gravitationally stable (disc-to-star mass ratio below 10%) even while accreting at ~1e-5 solar masses per year; episodic bursts correlate with transient regions of marginal Toomre-Q instability.","Destroyer-class streamers naturally produce periods of disc truncation (size down ~65%, mass down ~40%) followed by regrowth on ~10 kyr timescales, offering an explanation for compact discs seen around outbursting young stars.","Rapid replenishment erases spatially separated chemical or isotopic reservoirs in the early disc, so any such reservoirs must be established later, when infall has declined.","Surface-layer accretion with toroidal-field reversal, previously seen in ideal-MHD disc models, operates in realistic embedded discs and connects stellar accretion to disc surface layers."],"fun_headline_variants":["Outer young discs replenish every 10k years, delaying planet birth","Disc gas turnover every 10,000 years keeps outer regions planet-free","Streamers and layered accretion refill young discs in 10k years","Rapid disc replenishment every 10 millennia blocks outer planet formation","Outer discs exchange mass every 10k years, hindering planet building"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The load-bearing premise is that ideal MHD with a barotropic equation of state, at 0.8 au resolution, faithfully captures how gas accretes in the outer disc; if non-ideal magnetic effects or unresolved heating change the accretion layers, the 10 kyr replenishment picture could collapse.","fun_headline_variants_meta":{"raw":{"variants":["Outer young discs replenish every 10k years, delaying planet birth","Disc gas turnover every 10,000 years keeps outer regions planet-free","Streamers and layered accretion refill young discs in 10k years","Rapid disc replenishment every 10 millennia blocks outer planet formation","Outer discs exchange mass every 10k years, hindering planet building"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000362,"raw_usage":{"total_tokens":1873,"prompt_tokens":906,"completion_tokens":967,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":650,"completion_tokens_details":{"reasoning_tokens":870}},"tokens_in":650,"tokens_out":967,"duration_ms":7302,"temperature":1.0,"reasoning_tokens":870,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-04T11:30:01.640431+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same nine cloud-fed cores with non-ideal MHD (ambipolar diffusion and Ohmic resistivity) and radiative transfer at comparable resolution: if the surface-layer accretion channel and toroidal-field reversal disappear, or if the outer-disc replenishment time rises above ~100 kyr, the central claim fails. Observationally, mapping ionisation tracers and infall kinematics in the outer regions of Class 0 discs could test whether the assumed well-ionised, strongly coupled regime actually holds.","supporting_citations":[],"review_version":1}