{"id":"c72e03d5-f938-4dd1-9405-52110e48516d","arxiv_id":"2411.17114","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"Massive, low-turbulence, small-grain-depleted disks around one to five solar-mass stars can retain gas for 10-100 million years, with the longest lifetimes near two solar masses, supporting a primordial origin for gas-rich debris disks and predicting ongoing accretion.","lead":"This paper simulates whether old gas disks around young stars can be leftovers from the original planet-forming disk. The model finds that massive, calm, small-grain-depleted disks can retain gas for over ten million years, with the longest survival around two-solar-mass stars, and predicts these old disks should still be accreting gas onto their stars.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"FUV photoevaporation is switched off by fiat; the central claim depends on small-grain depletion occurring early enough and persistently, which is not modeled self-consistently.","rationale":"The reader's weakest_assumption is exactly the same load-bearing concern: small-grain depletion must be early and persistent enough to suppress FUV photoevaporation, and this is not modeled self-consistently in Section 4.3. I agree with that identification. The paper is internally coherent and provides a useful 1D extension of the earlier 0D model, with new spatial predictions and a falsifiable accretion signature. However, the central claim is conditional on an unverified prerequisite. The FUV issue is more load-bearing than the other listed uncertainties: the X-ray suppression factor of 0.1 is an externally motivated update from Sellek et al. (2024), and accretion-generated EUV is already tested in Appendix A with only moderate lifetime reduction. In contrast, if FUV photoevaporation is not suppressed, the lifetimes drop well below 10 Myr, which would remove the proposed primordial-origin parameter space. The paper itself flags this limitation clearly, so the concern is not a straw man. The proposed test is a targeted numerical experiment that directly determines whether the small-grain-depletion premise is sufficient in the critical 2-3 Msun mass range. Since the reader already assigned CONDITIONAL with moderate confidence, my stress-test does not change the verdict; it reinforces it.","tokens_in":28747,"tokens_out":2276,"duration_ms":24142,"concrete_test":"Extend the 1D model to include FUV photoevaporation with a parameterized small-grain/PAH abundance f_grain, using the FUV luminosity evolution for 2 Msun from Kunitomo et al. (2021). Run FID-2 with f_grain = 1, 0.1, 0.01, and 0.001 times ISM, and with depletion imposed at ages 0, 0.5, 1, and 3 Myr. If no combination with f_grain <= 0.01 and depletion by 1 Myr yields a disk lifetime >10 Myr and CO mass near 0.06-0.6 Mearth, then the primordial scenario fails for the claimed parameter space. Preferably, couple a dust evolution model to compute the depletion self-consistently and repeat the FID-2 run to verify that small grains are actually removed before the FUV luminosity rise.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central claim is that gas in initially massive, low-alpha, small-grain-depleted disks can survive beyond 10 Myr. The load-bearing premise is the depletion of very small grains and PAHs to roughly 0.1-1% of ISM abundance (Section 4.3), which suppresses FUV-driven photoevaporation. This premise is not simulated: Section 2.3 simply omits FUV photoevaporation, with the justification that the disks of interest are small-grain-depleted. The ablation is that FUV photoevaporation becomes significant at about 1-3 Myr for 2-3 Msun stars as FUV luminosity rises with stellar evolution (Kunitomo et al. 2021; Ronco et al. 2024). For the primordial-origin scenario to hold, depletion must occur before this rise and persist for tens of Myr. The paper itself acknowledges in Section 4.3 that the threshold abundance is uncertain and that no self-consistent calculation including grain growth and FUV photoevaporation is performed. Observational evidence for small-grain depletion is suggestive but does not establish that the depletion is early enough, strong enough, or global enough to suppress FUV photoevaporation across the whole disk. If the depletion is delayed or partial, FUV photoevaporation would shorten lifetimes and the 'survival beyond 10 Myr' result would not be realized, removing the primordial-origin interpretation for the modeled parameter space. This is not an internal inconsistency, but it is the least secure external condition on which the central claim rests.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper explores the primordial-origin scenario for gas in debris disks by simulating 1D secular evolution of protoplanetary disks that are assumed to be depleted in very small grains and PAHs. The model includes viscous accretion, MHD disk winds, EUV/X-ray photoevaporation with time-dependent stellar evolution, and wind shielding. The authors find that initially massive disks (M_disk ~ 0.1 M_*) with weak turbulence (alpha << 1e-2) can survive beyond 10 Myr, with the longest lifetimes around 2 M_sun, that gas persists at roughly 10-1000 au, and that accretion continues as long as the disk survives. They compare estimated CO masses to gas-rich debris disks and argue that searching for accretion signatures can distinguish primordial from secondary gas origins. The paper is an extension of the one-zone model of Nakatani et al. (2023) to a spatially resolved model and is framed as a plausibility study for a specific, long-lived disk population.","tokens_in":29058,"tokens_out":5111,"duration_ms":48348,"significance":"If the results hold, the paper provides a concrete pathway for the primordial-origin scenario, explaining the relatively high incidence of gas-rich debris disks around early A stars and offering a unified explanation for long-lived accreting disks including Peter Pan disks. The work is valuable for its systematic parameter survey, its inclusion of MHD wind shielding, its spatial predictions that can be compared with ALMA observations, and its explicit, falsifiable prediction that accretion should persist in old gas disks. The simulations are internally consistent and the parameter variations are transparent; the conditional claim 'if small grains are depleted early and persistently and alpha is small, disks can survive beyond 10 Myr' is defensible. The main weakness is that the central premise, early and persistent small-grain depletion, is not modeled self-consistently, so the paper's broader conclusions about observational plausibility rest on an external, unverified condition.","major_comments":[{"comment":"The central claim that disks survive beyond 10 Myr depends on neglecting FUV-driven photoevaporation, justified by the assumption that very small grains and PAHs are depleted. This depletion is not simulated: the paper states in Section 4.3 that the threshold abundance of 0.1-1% of ISM is uncertain and that no self-consistent calculation including grain growth and FUV photoevaporation is performed. The same section notes that FUV photoevaporation becomes significant at 1-3 Myr for 2-3 M_sun stars as FUV luminosity rises with stellar evolution. For the longevity result to apply, depletion must occur before this rise and persist across the disk. The observational evidence cited is suggestive but does not establish the required timing or spatial extent. This is the load-bearing external condition of the paper, and it needs to be addressed directly, for example by coupling a grain evolution model or by computing the epoch of depletion relative to the FUV rise, or by explicitly restricting the conclusions to disks where such early depletion can be demonstrated.","section":"Sections 2.3 and 4.3"},{"comment":"The main-text claim of a pronounced lifetime peak at 2 M_sun is substantially weakened by the authors' own EUVACC models, where 4% of accretion energy converted to EUV radiation flattens the stellar-mass dependence and reduces lifetimes to about 10 Myr for all stellar masses. Since the model predicts ongoing accretion, accretion-generated EUV is not a negligible perturbation; it removes the distinctive A-star peak that the paper emphasizes in the abstract and introduction. The appendix is clearly written, but the main conclusions and the abstract should be qualified to reflect that a plausible and internally motivated process erases the central longevity trend. At minimum, the abstract should state that the >10 Myr lifetimes and the 2 M_sun peak apply only when accretion-generated EUV is small.","section":"Appendix A and Figure 14"},{"comment":"The estimated CO masses of 0.06-0.6 M_Earth are quoted as comparable to the most massive gas-rich debris disks, but this estimate assumes interstellar carbon abundance and neglects photodissociation and carbon chemistry. The authors do describe this as an upper limit in the text, but the abstract and Section 5 present the alignment without this caveat. The comparison is therefore weaker than the summary suggests. I recommend rewording the conclusions to state that the predicted CO masses are order-of-magnitude upper limits that are consistent with the most massive observed disks only if CO survives efficiently, and that thermochemical modeling is needed for a quantitative comparison.","section":"Section 4.2 and Figure 13"}],"minor_comments":[{"comment":"The text uses 'MIRact-2' once in the discussion of Figure 9; this should be 'MRIact-2' for consistency with Table 1 and the rest of the paper.","section":"Section 3.3"},{"comment":"The sentence 'The remaining mass in 10-100 au disperses finally, which is no later than 2 Myr' appears inconsistent with Figure 3 and with the quoted lifetimes of roughly 15-20 Myr for FID-1 and FID-5; this is likely a typo for '20 Myr' and should be corrected.","section":"Section 3.1.1"},{"comment":"In the paragraph following Equation (6), the second occurrence of 'On the left-hand side, the first and second terms' should read 'On the right-hand side' to match the physical meaning.","section":"Section 2.2, Equation (6)"},{"comment":"The entry 'Rcut1-N' lists rcut as '30 × N', which is ambiguous because N denotes the stellar mass suffix; the caption or table should explicitly state that rcut = 30 au × (M_*/M_sun).","section":"Table 1"},{"comment":"The phrase 'unless such a survey has not already been undertaken' is confusing; it should be rephrased to state whether such a survey has already been conducted or to recommend one without the double negative.","section":"Section 4.1"}],"recommendation":"major_revision","confidential_remarks":"The paper is honest about its limitations and the simulations are well executed, but the strongest conclusions depend on an unmodeled premise (early, persistent small-grain depletion) and on ignoring accretion-generated EUV, which the authors' own appendix shows can remove the central mass dependence. I would support publication after the authors either add a self-consistent treatment or substantially soften the abstract and conclusions to reflect these conditionalities. The manuscript fits the journal's scope and the accretion-signature prediction is a valuable, testable contribution."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear X,\n\nThe headline for you: this is a competent 1D extension of Nakatani et al.'s 0D model, and its main new contribution is a falsifiable prediction—accretion persists as long as the disk survives, and gas lingers at 10–1000 au for 2 M_sun stars. The longevity peak at 2 M_sun is not new; the authors say so themselves. What is new is the spatial structure and the accretion-persistence diagnostic.\n\nThe paper does several things well. The parameter study is systematic: disk mass, alpha, MHD wind model, shielding, and accretion-generated EUV are all varied, and the qualitative result (lifetimes above 10 Myr for M_disk ~ 0.1 M_* and alpha << 1e-2, peaking near 2 M_sun) is robust across those variations. Adopting the 0.1 suppression of Owen et al. X-ray rates follows Sellek et al. and is reasonable. The MHD-wind shielding test is a genuine addition. And Section 4.3 is unusually candid about what the model does not include.\n\nThe soft spot is exactly what the stress-test flags: FUV photoevaporation is turned off by assumption, not by model. Section 2.3 omits it, and Section 4.3 admits the depletion threshold (0.1–1% of ISM) is uncertain and that no self-consistent grain-growth plus FUV calculation is done. So the central claim is conditional on an unmodeled premise. That said, the paper does not overclaim: it says \"plausibility\" and \"could.\" The internal dynamics do support the conditional statement—if such disks exist, they can live long and keep accreting. An uncertain threshold is a real limitation, but it is not a flaw in the calculations themselves. The 0.1 X-ray suppression is a large knob, though they cite Sellek et al. and argue the conclusions would not change with the full fits. No code or data is shipped, which is annoying but not unusual for this field.\n\nVerdict: this deserves a serious referee. It sharpens a live debate and hands observers a direct test (accretion signatures in gas-rich debris disks). The reader's conditional take and the stress-test note are both fair, but the paper already self-identifies the depletion premise, so I would not call it fatal—just the price of admission. I might not bring it to reading group unless someone works on disk evolution or debris gas, but I would cite it for the persistent-accretion prediction, with a caveat on the depletion assumption.\n\nRecommendation: send it to peer review, and let the referee weigh the depletion assumption carefully.","headline":"A careful 1D extension of the authors' 0D small-grain-depleted disk model: the conditional longevity claim is internally supported, but the whole result leans on an unmodeled premise—early and persistent small-grain depletion that switches off FUV photoevaporation.","tokens_in":29679,"tokens_out":2678,"would_cite":true,"duration_ms":23363,"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":"The paper argues that gas-rich debris disks can be primordial remnants: initially massive, weakly turbulent, small-grain-depleted protoplanetary disks survive beyond 10 Myr, peak in lifetime near 2 solar masses, and keep accreting as long…","keywords":["protoplanetary disks","debris disks","photoevaporation","disk evolution","accretion signatures","primordial gas origin","small-grain depletion","A-type stars"],"falsifier":"Run the same disk evolution with a self-consistent model of carbonaceous grain growth, radial drift, and far-ultraviolet photoelectric heating: if realistic starting grain populations do not fall below roughly 0.1--1 percent of interstellar abundance before the far-ultraviolet luminosity rises at 1--3 Myr, the long-lived population becomes too rare to explain the observed gas-rich debris disks.","tokens_in":28514,"feed_emoji":"🪐","tokens_out":13244,"duration_ms":109495,"temperature":0.7,"pith_summary":"The paper asks whether the gas seen in old debris disks can be leftover from the original protoplanetary disk rather than freshly released by colliding planetesimals. Using one-dimensional disk evolution simulations that include stellar evolution and time-varying photoevaporation, it argues that gas can survive beyond 10 Myr, and near 2 solar masses even past 100 Myr, if the disk started massive (about 10% of the stellar mass), is only weakly turbulent ($\\alpha \\ll 10^{-2}$), and has lost its very small grains and carbon-rich molecules (PAHs) early. If that is right, the primordial-origin scenario is alive for a defined subset of gas-rich disks around early A stars, and ongoing accretion should still be detectable in those systems. This matters because it converts a long-standing tension over debris-disk gas into falsifiable predictions about accretion and grain depletion.","feed_headline":"Protoplanetary gas can survive 100 Myr around 2-solar-mass stars","feed_subtitle":"Simulations tie old gas-rich debris disks to leftover protoplanetary gas, with a testable accretion signature.","key_machinery":"The machinery is a one-dimensional secular evolution equation for the gas surface density with separate terms for turbulent accretion, magnetohydrodynamic disk winds, and photoevaporation, using the standard $\\alpha$-parameter prescription in which turbulent stress is set by a dimensionless number $\\alpha$. The load-bearing element is stellar evolution: for a $2\\,M_\\odot$ star the disappearance of the surface convective layer at about 4 Myr sharply cuts the X-ray and magnetic EUV radiation that powers photoevaporation, while for 1 and 5 solar-mass stars the radiation stays strong at different epochs. With weak turbulence ($\\alpha \\sim 8 \\times 10^{-5}$), accretion is slow enough that this radiation drop lets the disk survive far beyond the usual few-million-year dispersal timescale; the assumed early depletion of small grains and PAHs is what removes far-ultraviolet photoevaporation from the competition.","core_discovery":"The central claim is that a protoplanetary disk with a depleted population of very small grains and carbon-rich molecules (PAHs) can evolve into a gas-rich debris disk instead of dispersing within 10 Myr. In the model, weak turbulent stress ($\\alpha \\sim 8 \\times 10^{-5}$) and a massive initial disk ($M_{\\mathrm{disk}}\\sim 0.1\\,M_*$) let gas persist; for a $2\\,M_\\odot$ host, the surface convective zone disappears around 4 Myr, collapsing the stellar X-ray and magnetic EUV output and therefore the photoevaporation rate, so the disk retains gas at 10--1000 au for more than 100 Myr. The same model predicts that accretion continues as long as the disk survives, with rates around $10^{-11}$--$10^{-10}\\,M_\\odot\\,\\mathrm{yr}^{-1}$ for $2\\,M_\\odot$ hosts after 10 Myr, and it estimates CO masses comparable to the most massive gas-rich debris disks around early A stars.","pith_inferences":["The paper does not model small-grain depletion; an explicit consequence of its logic is that the timing and completeness of that depletion, rather than initial disk mass alone, selects which disks become long-lived. Coupling grain growth and drift to the same evolution calculation would turn the scenario into a predicted population fraction.","The 2--3 solar-mass peak implies a physical filter: only intermediate-mass stars lose their surface convective zone early enough to cut X-ray and EUV photoevaporation before the disk is gone, so the observed A-star bias in gas-rich debris disks may be partly a longevity selection effect rather than only a detection bias.","The appendix's result that accretion-generated EUV can shorten lifetimes toward 10 Myr suggests the longest survivors may need accretion-inhibiting processes such as inner planets or magnetization; future accretion observations could therefore constrain unseen planetary architecture, not just gas origin.","For low-CO debris disks, the secondary-origin scenario may still win; the model's parameter requirements make primordial gas a minority outcome, so a mixed population is the most plausible observational reality."],"forward_implications":["Gas can survive beyond 10 Myr for all stellar masses considered, provided the disk starts massive ($M_{\\mathrm{disk}}\\sim 0.1\\,M_*$) and weakly turbulent ($\\alpha \\ll 10^{-2}$), with the longest lifetimes exceeding 100 Myr at $2\\,M_\\odot$.","The long-lived gas sits at roughly 10--1000 au, matching the ring-like radial extents of gas observed in gas-rich debris disks.","Accretion persists as long as the disk survives, so searching for accretion signatures is a direct way to distinguish primordial from secondary gas origins.","The estimated CO masses in the long-lived models overlap the observed range for the most massive gas-rich debris disks around early A stars.","The same evolution can explain old accreting disks around low-mass stars, including Peter Pan disks."],"supporting_citations":[{"why":"Supplies the one-zone model and the small-grain/PAH depletion premise whose extension to 1D is this paper's aim.","marker":"Nakatani et al. (2023)"},{"why":"Provides the 1D secular evolution equation and the alpha prescriptions for turbulent accretion and MHD disk winds used throughout.","marker":"Suzuki et al. (2016)"},{"why":"Provides time-dependent stellar X-ray and EUV emissivity from stellar evolution, creating the 4 Myr radiation drop for 2 solar-mass stars.","marker":"Kunitomo et al. (2021)"},{"why":"Gives the EUV photoevaporation base density profile that sets the outer-disk mass-loss rate.","marker":"Tanaka et al. (2013)"},{"why":"Gives the X-ray photoevaporation rate profiles, scaled by a factor 0.1 following updated cooling results.","marker":"Owen et al. (2012)"},{"why":"Supplies the MHD disk wind shielding treatment for EUV and X-ray radiation and the column-density criterion.","marker":"Weder et al. (2023)"},{"why":"Justifies the suppression factor applied to the Owen et al. X-ray photoevaporation rates.","marker":"Sellek et al. (2024)"},{"why":"Provides comparison long-lived disk models and the revenant-disk behavior that the MRI-active runs reproduce.","marker":"Ronco et al. (2024)"}],"fun_headline_variants":["Protoplanetary gas outlasts 100 Myr around 2-solar-mass stars","Old debris disks may be leftover protoplanetary gas disks","Massive disks with weak turbulence retain gas for 100 Myr","Accretion in old disks hints at primordial gas origin","Gas-rich debris disks trace back to long-lived protoplanetary disks"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the very smallest dust grains and carbon-rich molecules are removed early and stay gone, so far-ultraviolet starlight cannot heat the disk and drive photoevaporation; that removal is not modeled self-consistently.","fun_headline_variants_meta":{"raw":{"variants":["Protoplanetary gas outlasts 100 Myr around 2-solar-mass stars","Old debris disks may be leftover protoplanetary gas disks","Massive disks with weak turbulence retain gas for 100 Myr","Accretion in old disks hints at primordial gas origin","Gas-rich debris disks trace back to long-lived protoplanetary disks"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000406,"raw_usage":{"total_tokens":2192,"prompt_tokens":1106,"completion_tokens":1086,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":722,"completion_tokens_details":{"reasoning_tokens":993}},"tokens_in":722,"tokens_out":1086,"duration_ms":10197,"temperature":1.0,"reasoning_tokens":993,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T12:30:18.063302+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same disk evolution with a self-consistent model of carbonaceous grain growth, radial drift, and far-ultraviolet photoelectric heating: if realistic starting grain populations do not fall below roughly 0.1--1 percent of interstellar abundance before the far-ultraviolet luminosity rises at 1--3 Myr, the long-lived population becomes too rare to explain the observed gas-rich debris disks.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the EUV photoevaporation base density profile that sets the outer-disk mass-loss rate."},{"cited_title":"P., Schreiber, M","cited_arxiv_id":null,"evidence_quote":"Provides comparison long-lived disk models and the revenant-disk behavior that the MRI-active runs reproduce."}],"review_version":1}