{"id":"e0e39921-89bd-46ba-96f8-d3c54c932bf1","arxiv_id":"2505.14780","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Analysis of 16 Be+sdOB binaries shows that most transferred mass is retained by the accretor, with half of the systems requiring efficiencies above 50%.","lead":"A new analysis of 16 binary systems with a stripped hot subdwarf and a rapidly spinning Be star reconstructs how much mass each Be star gained during an earlier phase of mass transfer. The authors find that accretion was mostly efficient, which contradicts widely used assumptions in binary evolution models.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 'half of systems require >50% accretion' claim depends on the +20% residual-envelope relation (Eq. B2) rather than the strict lower limit (Eq. B1); switching between them changes the count from 7 to 4 of 14 systems.","rationale":"The reader correctly identified the initial-mass--stripped-mass relation as the weakest assumption. My stress test sharpens the direction of the concern: for fixed observed MsdOB, incomplete stripping actually makes the true donor initial mass smaller and the true beta larger than the strict lower-bound estimate, so the strict analysis is conservative. The vulnerability is specifically the step from the strict lower limit (Eq. B1) to the best-guess relation (Eq. B2): the 50% headline is produced only after applying the +20% residual-envelope correction. I verified this quantitatively with Table 1: using Eq. B1 gives 4 of 14 systems with beta_min > 0.5, whereas Eq. B2 gives 7 of 14. Thus the 'half of systems' claim is a model-dependent best guess, not a robust lower-bound result. The paper is transparent about this in Section 4, but the abstract and conclusion state the 50% figure without the necessary qualifier. I do not see a fatal flaw: the disagreement with rotationally limited accretion is strong even with the strict lower bounds, since those bounds already exceed the few-percent efficiencies predicted by that scheme, and the MESA input files are deposited. The 10^-10 probability for the thermally limited tension is another under-derived number, but it supports a secondary claim rather than the central efficient-transfer result, so I do not make it the primary attack. The appropriate verdict remains CONDITIONAL: the core result is credible, but the headline quantitative statement should be rephrased as conditional on the adopted stripped-mass relation, and the thermally limited probability should be derived or removed.","tokens_in":25618,"tokens_out":11133,"duration_ms":73648,"concrete_test":"Recompute Table 1's beta_min and best-guess ranges with the same observed masses but a family of stripped-mass relations: (a) Eq. B1 with no residual envelope, (b) +10% residual envelope, (c) Eq. B2 with +20%, (d) +30%, and (e) an alternative overshooting calibration such as Choi+16/M67 for masses below about 4 solar masses. Count systems with best-guess beta_min > 0.5 after excluding HR 2142 and HR 6819. If the count drops from 7 to 4 or 5 under (a), (b), or (e), then the 'half of systems' claim must be explicitly qualified as conditional on the adopted +20% residual-envelope relation.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing step is the mapping from the observed sdOB mass to the donor's initial mass via the MESA initial-mass--stripped-mass relations in Appendix B (Eqs. B1 and B2, used through Eqs. B3 and B4). For a fixed observed MsdOB, the inferred donor initial mass M_donor,initial = f(MsdOB), and beta_min = max(0, (MBe - M_donor,initial)/(M_donor,initial - MsdOB)) in the q->1 limit. Relation (1) is the helium-core mass from single-star models, used as a 'strict lower limit' on the stripped mass. This is conservative with respect to incomplete stripping: if true remnants retain 10-25% of the envelope, as the binary grids in Figure 5 indicate, then for the same observed MsdOB the true initial donor mass is smaller and beta_min is larger. Switching from relation (1) to relation (2), which adds 20% residual envelope, raises the number of systems with beta_min > 0.5 (excluding the two Case A candidates, HR 2142 and HR 6819) from 4 to 7, i.e. from 'more than a quarter' to 'half'. The headline 'half of systems' therefore rests on the +20% best-guess assumption, not on the strict lower bound. The paper itself flags the overshooting dependence in Section 4, but the abstract and conclusion present the 50% figure without this qualifier. A smaller residual envelope or a lower overshooting calibration (e.g. Choi+16/M67 at low masses) would move several systems below the 0.5 threshold. This does not invalidate the broader conclusion that typical efficiencies are far above the few-percent rotationally limited prediction, but it does weaken the specific 'half > 50%' statement as written.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper compiles a sample of 16 Be+sdOB binaries with mass determinations of both components, and uses the relation between the present-day stripped-star mass and the initial donor mass (from MESA models) to derive lower limits on the mass transfer efficiency beta. The central result is that mass transfer in these systems must often have been highly efficient: more than a quarter of the 14 systems relevant to early Case B require beta_min > 0.5 under a strict lower-limit stripped-mass relation, and about half do so under a best-guess relation that includes a residual envelope. The authors argue that these constraints are inconsistent with rotationally limited accretion and in tension with a thermally limited accretion scheme as implemented in rapid population-synthesis codes.","tokens_in":25986,"tokens_out":6719,"duration_ms":57013,"significance":"If the result holds, it is a significant observational constraint on mass transfer efficiency in intermediate-mass (2-9 Msun) binaries, directly challenging widely used prescriptions in binary evolution and population synthesis. The paper's method is clean: beta_min follows from observed masses, an independent MESA initial-mass--stripped-mass relation, and the evolutionary constraint that the donor was initially more massive than the accretor; it does not fit beta to the data. The authors are transparent about the sample's heterogeneous quality, provide analytical fitting formulas (Eqs. B3 and B4), and make their MESA inlists and history files publicly available on Zenodo. The main weakness is that the headline quantitative claim ('half of the systems') is sensitive to the assumed stripped-mass relation and to the adopted overshooting prescription, and this sensitivity is not fully quantified in the abstract and conclusions.","major_comments":[{"comment":"The abstract states 'half of the systems require mass transfer efficiencies above 50%' without qualification, but this number comes from the best-guess relation (2), which adds 20% residual envelope mass. Under the strict lower-limit relation (1), the count is 4 of 14 (Section 3, first bullet), not 7 of 14. The conclusion does say 'using our best-guess estimates,' but the abstract does not. Since the central headline depends on this assumption, the abstract should either state the strict-lower-limit count or explicitly attribute the 50% figure to the best-guess relation.","section":"Abstract and Section 3, Eqs. (B1)-(B2)"},{"comment":"The claim that relation (1) is a 'strict lower limit' to the stripped mass is conditional on the adopted overshooting prescription. The paper's single-star grid uses a mass-dependent overshooting calibrated by Castro et al. (2014) and Brott et al. (2011), while the Temmink et al. (2023) grid uses the Choi et al. (2016) M67 calibration, which gives smaller cores at low masses. If a lower-overshooting relation is appropriate, the inferred initial donor mass for a given observed MsdOB would be larger, and beta_min would be smaller (Section 2, Eqs. B3-B4). The paper mentions this dependence in Section 4 but does not quantify how many systems would fall below the beta_min = 0.5 threshold. Please provide a sensitivity test using a lower-overshooting initial-mass--stripped-mass relation, or otherwise justify why the adopted prescription yields a conservative (lower) bound on beta.","section":"Appendix B, Section 4, and Fig. 5"},{"comment":"The analysis excludes HR 2142 and HR 6819 as likely Case A systems, but for HR 2142 the main result adopts the orbital mass of 17.6 Msun for the Be star, while a separate spectral fit (BeAtlas, Rubio et al. 2023) yields about 9.6 Msun and a companion mass of about 0.7 Msun, which would give beta_min = 0.85 and a Case-B-consistent initial period (Section 4). The paper acknowledges this but does not explicitly state how the sample-level conclusions change if HR 2142 is included under the alternative mass. Since the sample is small and the 'half of systems' statement is a count, this systematic uncertainty in one system should be made explicit in the results, not only in the discussion.","section":"Section 3 and Table 1, HR 2142 and HR 6819"}],"minor_comments":[{"comment":"The robustness bullet 'Even when pessimistically considering the 1-sigma uncertainties, we still find at least 4 systems with evidence for modest accretion (beta_min > 0.15)' uses a threshold of 0.15, which is much lower than the headline threshold of 0.5; reporting the 1-sigma range for the number of systems with beta_min > 0.5 would be more directly relevant to the central claim.","section":"Section 3, bullet list"},{"comment":"The probability 'of the order of 10^-10' that ten systems lie in the narrow consistent region of the thermally limited model is stated without derivation; please specify how it was computed (e.g., from the fractional area of the initial parameter space that is consistent) and note that it is an order-of-magnitude estimate.","section":"Section 3, fourth paragraph"},{"comment":"The top axis of Fig. 2 is labeled 'Initial mass of the stripped star'; this is the inferred initial mass of the donor, and the label should be consistent with the text (e.g., 'Initial donor mass') to avoid confusion.","section":"Fig. 2 caption and Appendix B"},{"comment":"The term 'CASE A' is capitalized inconsistently; use 'Case A' to match standard nomenclature.","section":"Throughout, e.g., Fig. 4 and Appendix C"},{"comment":"For the Kendall rank correlation results, reporting the tau values and p-values would help the reader judge the significance; the text only says 'strong' or 'moderate but still significant.'","section":"Appendix C, Section C.1"}],"recommendation":"major_revision","confidential_remarks":"The paper is well within the scope of the journal and makes a valuable contribution. The main reservation is the disconnect between the strong, unqualified abstract claim and the assumption-dependence of the underlying count; a sensitivity analysis and a revised abstract would resolve this. I see no grounds for rejection, as the central derivation is sound and the assumptions are clearly stated."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is the first paper to turn the Pols (2007) single-system argument into a sample-level statement about stable Case B mass transfer in intermediate-mass binaries. That is worth having. The central logic is clean: observed sdOB mass plus a MESA initial-mass–stripped-mass relation gives the donor's initial mass; requiring the donor to start more massive than the accretor gives beta_min. The paper is transparent about the two relations and deposits the MESA files on Zenodo. Good reproducibility.\n\nThe result that rotationally limited accretion (few percent) is inconsistent with the lower bounds is robust. Even with the strict lower-limit relation, 4 of 14 systems need beta > 0.5 and 8 of 14 need beta > 0.3; that is far above the few-percent prediction. The analytical formulas in Appendix B are useful, and the comparison with the thermally limited model is suggestive.\n\nSoft spots, in proportion. The 'half of systems >50%' claim in the abstract and conclusion is the best-guess estimate, not the strict lower bound. Switching from Eq B1 to Eq B2 changes the count from 4 to 7 of 14. The body does flag this, and the conclusion says 'using our best-guess estimates,' but the abstract does not. That wording needs fixing. The 10^-10 probability for the thermally limited fine-tuning is stated without derivation or error budget; it is not load-bearing for the main conclusion, but it is a quantitative claim that should either be derived or softened. The sample is heterogeneous by tier, and the paper acknowledges the selection biases. The Case A handling of HR 2142 and HR 6819 is honest and does not hide the awkwardness.\n\nOverall, the central argument holds. This paper is for anyone working on mass transfer in intermediate-mass binaries, Be+sdOB populations, or population synthesis. It deserves a serious referee. My recommendation: send it to peer review, with a request that the authors qualify the headline number in the abstract and either derive or moderate the 10^-10 claim.","headline":"First multi-system lower bounds on mass transfer efficiency in Be+sdOB binaries; the rotationally-limited result is robust, but the 'half >50%' headline rests on the best-guess stripped-mass relation and needs a qualifier.","tokens_in":26563,"tokens_out":2447,"would_cite":true,"duration_ms":20761,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Mass transfer in Be+sdOB binaries is mostly conservative: half of the systems require at least 50 percent accretion efficiency, contradicting standard binary models.","keywords":["Binary stars","Stellar accretion","Roche lobe overflow","Be stars","Subdwarf stars","Stellar evolutionary models","Mass transfer efficiency"],"falsifier":"A volume-limited survey of Be+sdOB binaries that fills the currently empty lower-right region of the accretor-mass versus stripped-mass diagram with systems whose inferred efficiencies fall below 0.3 would contradict the claim that mass transfer is predominantly conservative; the present sample's emptiness in that region is suggestive, but a selection-corrected census could turn it into a decisive test.","tokens_in":25428,"feed_emoji":"💫","tokens_out":9416,"duration_ms":82495,"temperature":0.7,"pith_summary":"This paper uses a new sample of 16 Be+sdOB binaries—pairs where a rapidly spinning Be star orbits a stripped hot subdwarf—to measure how much mass the Be star actually kept when its companion transferred mass to it. The authors argue that mass transfer in these systems was predominantly efficient: about half of the sample requires that at least 50 percent of the transferred mass was retained by the accretor, and several systems require considerably more. If true, this contradicts the widely used rotationally limited accretion prescription in detailed binary evolution codes, which predicts that only a few percent can be accreted once the gainer spins up, and it also strains thermally limited prescriptions used in population synthesis. The result matters because the mass transfer efficiency sets the final masses and orbits of almost all interacting binary products, from white dwarfs and blue stragglers to X-ray binaries and gravitational-wave sources.","feed_headline":"Mass transfer in binaries is mostly conservative, 16 systems show","feed_subtitle":"Half of them require at least 50 percent retention, contradicting standard binary evolution models.","key_machinery":"The load-bearing object is the initial-mass–stripped-mass relation: a mapping from a star's zero-age main-sequence mass to the helium-core mass it has when hydrogen-shell burning begins, computed from a grid of single-star models and used as a lower bound on the final stripped-star mass, with a plus-20-percent variant as a best guess. Combined with the evolutionary constraint that the donor must have started out more massive than the accretor, it yields analytical expressions for the minimum mass transfer efficiency $\\beta_{\\min}$ as a function of the two observed masses. This relation converts a static snapshot—present-day masses of the Be star and the subdwarf—into a statement about the integrated history of mass transfer.","core_discovery":"The central claim is that stable Case B mass transfer in binaries with initial donor masses of 2 to 9 solar masses is largely conservative, with the accretor keeping more than half of the donated mass in many systems. The argument works by treating each present-day hot subdwarf as the helium core of the original donor: a calibrated initial-mass–stripped-mass relation gives the donor's initial mass, and the requirement that the donor was initially the more massive star gives a lower bound on how much mass the Be star must have gained. Applying this to the 16 systems, the authors find that 4 of the 14 non-Case-A systems require efficiencies above 50 percent even under strict lower-limit assumptions, and that the sample as a whole is incompatible with rotationally limited accretion, which allows only a few percent. They further argue that a thermally limited model is consistent only if nearly all systems originate from a narrow 10 percent sliver of initial parameter space, which they deem unlikely.","pith_inferences":["A natural test is to measure projected rotation velocities of the Be accretors: efficient disk accretion that still removes angular momentum should leave many accretors rotating below critical speed, which can be checked against the observed distribution.","Extending the same analysis to Be X-ray binaries, whose progenitors pass through the same channel above 10 solar masses, would test whether conservative mass transfer persists at higher masses.","The empty lower-right corner of the mass diagram could be a selection effect or a real absence; a volume-limited, bias-corrected census of Be+sdOB binaries would distinguish these and would double as a direct falsification test of the paper's main claim.","The closed-form $\\beta_{\\min}$ formulas could be applied as a fast observational prior in future binary population synthesis, so that each newly measured Be+sdOB system immediately updates the inferred accretion efficiency distribution."],"forward_implications":["Rotationally limited accretion as implemented in detailed binary evolution codes cannot reproduce the sample; the inferred lower bounds require far more accretion than those models allow.","Rapid population synthesis codes using thermally limited accretion face tension: the observed systems would have to be drawn from a narrow band covering about 10 percent of the stable mass transfer parameter space.","A constant mass transfer efficiency of 60 to 80 percent can explain the whole sample, suggesting that simple fixed-efficiency prescriptions may be closer to reality than spin-up-based limits.","The empty lower-right region of the mass diagram may indicate that systems with very low mass transfer efficiency are rare or do not survive stable mass transfer, though the sample selection is not homogeneous enough to be conclusive.","If the same efficiency applies to higher-mass binaries, predictions for supernova types, X-ray binaries, and gravitational-wave merger rates would shift toward heavier and more widely separated remnants."],"supporting_citations":[{"why":"Supplies the method of converting present-day stripped-star masses into lower bounds on mass transfer efficiency, originally applied to phi Persei.","marker":"O. R. Pols (2007)"},{"why":"Provides masses and orbital solutions for several Tier 3 systems and the expanded sample that makes this analysis possible.","marker":"L. Wang et al. (2023)"},{"why":"Supplies the bulk of the sample: interferometric and spectroscopic orbital solutions for seven Tier 2 systems including HR 2142.","marker":"R. Klement et al. (2024)"},{"why":"Provides the first dynamical masses for HR 6819, completing the sample with a Tier 1 system.","marker":"R. Klement et al. (2025)"},{"why":"The stellar evolution code used to compute the initial mass–stripped mass relations underlying the beta inferences.","marker":"B. Paxton et al. (2015)"},{"why":"Independent binary grid whose stripped masses justify treating the adopted relation as a lower bound with a separate best-guess variant.","marker":"K. D. Temmink et al. (2023)"},{"why":"Provides the thermally limited accretion predictions and initial-parameter map used for the tension test.","marker":"F. R. N. Schneider et al. (2015)"},{"why":"Origin of the rotationally limited accretion expectation that only a small fraction of mass can be accreted, the main model the paper contradicts.","marker":"W. Packet (1981)"}],"fun_headline_variants":["Binary mass transfer more efficient than models predict","16 stripped-star binaries show accretor keeps over half","Conservative mass transfer common in Be+sdOB systems","Rapid rotators spoil non-conservative binary evolution models","Stable mass transfer: 50% retention seen in half of sample"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The whole calculation assumes that the present-day mass of the stripped subdwarf maps uniquely to the donor's initial mass through a stellar-evolution relation; if the true stripped mass is lower than the adopted helium-core mass, the inferred donor mass, donated mass, and minimum efficiency all shrink.","fun_headline_variants_meta":{"raw":{"variants":["Binary mass transfer more efficient than models predict","16 stripped-star binaries show accretor keeps over half","Conservative mass transfer common in Be+sdOB systems","Rapid rotators spoil non-conservative binary evolution models","Stable mass transfer: 50% retention seen in half of sample"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000213,"raw_usage":{"total_tokens":1463,"prompt_tokens":1029,"completion_tokens":434,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":645,"completion_tokens_details":{"reasoning_tokens":354}},"tokens_in":645,"tokens_out":434,"duration_ms":5125,"temperature":1.0,"reasoning_tokens":354,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T15:29:55.380354+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A volume-limited survey of Be+sdOB binaries that fills the currently empty lower-right region of the accretor-mass versus stripped-mass diagram with systems whose inferred efficiencies fall below 0.3 would contradict the claim that mass transfer is predominantly conservative; the present sample's emptiness in that region is suggestive, but a selection-corrected census could turn it into a decisive test.","supporting_citations":[],"review_version":1}