{"id":"fb471b1f-952e-40c6-8566-4864dc5428b1","arxiv_id":"2508.21116","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":12,"one_line_summary":"Post-merger blue supergiants with fast-spinning envelopes can explain 1987A-like supernovae, ultra-long gamma-ray bursts, and fast luminous transients.","lead":"This paper models what happens when two massive stars merge before the larger one dies, finding that the survivors can stay blue and spinning until they explode. Such blue supergiant mergers are presented as likely parents of SN 1987A-like supernovae, ultra-long gamma-ray bursts, and some fast bright optical transients.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Merger prescription is the load-bearing pivot: neglecting the secondary's He core and dredge-up could shift both the BSG threshold and the outer-envelope AM that sets disk/jet powers.","rationale":"The reader identified the same weakest assumption, and I agree. The 1987A-like-SN part of the claim is comparatively robust because it depends mainly on the BSG/RSG dichotomy, which is supported by previous work and by the paper's HR tracks; including the secondary's He core likely makes stars bluer, strengthening that channel. The ultra-long-GRB and FBOT parts are less robust because they depend quantitatively on the outer-envelope AM profile. The paper's own Appendix B shows that ⟨jrot⟩ is the most variable output when input physics is changed, and those tests do not vary the secondary's internal structure. The Section 5 admission is therefore the weakest link in the central claim, and a targeted test with a more realistic merger-remnant construction would settle whether the concern actually degrades the predicted landscape. Until that test is performed, a CONDITIONAL verdict remains appropriate, so I recommend no change to the reader's verdict.","tokens_in":31778,"tokens_out":12942,"duration_ms":142824,"concrete_test":"Reconstruct the 15 M⊙ + 8 M⊙ and 25 M⊙ + 16 M⊙ merger products using a helium-core-sinking prescription (e.g., initial entropy/composition profiles from Patton et al. 2025 or a 3D SPH merger simulation), keeping total accreted mass and the Eq. (2) AM fixed; evolve both with identical MESA settings to central C depletion. Compare Teff,fin and the mass with j > jISCO (aBH = 0) to Table 1. If Teff,fin stays above 10^3.9 K and M_j>jISCO changes by <30%, the central claim is robust; if the threshold crosses or M_j>jISCO changes by >2×, the ultra-long GRB/FBOT landscape in Figs. 7–10 shifts and the paper's conclusions need revision.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim—that merger-origin BSGs yield 1987A-like SNe, ultra-long GRBs, and FBOTs—requires that the post-merger models have the correct final Teff (BSG vs RSG) and, especially for the engine-powered transients, the correct specific AM profile. The failed-explosion predictions in §4.2 depend directly on comparing jrot to jISCO (Eq. 9, Fig. 4): Mcirc, Mdot, Ljet, and Lwind all scale with the mass whose j exceeds jISCO. That AM profile is inherited from the §2 merger prescription, which adds mass at the primary's surface with surface entropy/composition and jacc from Eq. (2), with no mass loss and no secondary structure. Section 5 explicitly admits this neglects the secondary's helium-rich core and dredge-up. A q≈0.5–0.8 secondary is itself evolved and has a low-entropy He core that can sink, while dredge-up enriches the envelope in He; both can change the BSG threshold and the composition gradients that control AM transport in the radiative envelope. The Appendix B sensitivity suite varies merger radius, wind efficiency, AM-transport scheme, and resolution, but not the internal entropy/composition structure of the accreted material. Since the disk/jet outputs are highly sensitive to the mass above jISCO, an untested change in that mass can move models across the BSG boundary and change Ljet/Lwind by orders of magnitude. Thus the most load-bearing assumption is not tested by the paper's own sensitivity runs.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper constructs a MESA grid of 32 rotating supergiant models from post-main-sequence binary mergers, spanning four primary masses (10–25 Msun) and accreted masses up to 0.8 M1, at LMC-like metallicity. The key findings are that larger accreted mass favors blue supergiants (BSGs) at core collapse, that BSGs retain rapidly rotating outer envelopes (with lower-mass BSGs rotating faster), and that these rotation profiles lead to a landscape of transients: successful neutrino-driven explosions produce 1987A-like SNe with long-rising light curves, while failed explosions produce fallback-driven disk winds and possibly relativistic jets, proposed as progenitors of ultra-long GRBs and AT2018cow-like fast luminous transients. The paper includes light-curve calculations with SNEC, an accretion-disk model from Fuller & Lu (2022), and a jet-breakout model, and compares the resulting surface rotation rates to observed LMC BSGs.","tokens_in":32328,"tokens_out":5211,"duration_ms":54310,"significance":"If the main claims hold, this is a valuable unification: a single binary-merger channel produces a diversity of core-collapse transients, from ordinary Type II-P SNe to 1987A-like SNe, ultra-long GRBs, and fast optical transients. The paper's strengths include the systematic 32-model grid, the comparison to observed LMC BSG rotation (which provides an external anchor), the explicit sensitivity study in Appendix B showing that final mass, radius, and Teff vary by about 10% under altered physics, and the qualitatively reasonable fit to the observed rate of 1987A-like SNe. The modeling is state-of-the-art in using MESA with modern AM transport, and the authors are candid about the limitations of their 1D merger prescription. However, the central transient predictions inherit several untested assumptions, as detailed below, so the significance will be fully realized only after those assumptions are shown to be benign or are appropriately bracketed.","major_comments":[{"comment":"The merger is modeled as rapid accretion onto the primary at R* = 50 Rsun, with accreted material set to the primary's surface specific entropy and composition, no mass loss from the system, and jacc from Eq. (2). Section 5 explicitly concedes that this neglects the secondary's helium-rich core and dredge-up, which can affect envelope helium abundance and hence the BSG threshold. However, the sensitivity study in Appendix B varies merger radius, wind efficiency, AM transport scheme, and resolution, but not the entropy/composition structure of the accreted material. Since the final AM profile, and therefore the mass with j > jISCO (Table 1), directly controls Mcirc, Mdot, Ljet, and Lwind (Eqs. 14–16, Fig. 7), this untested variation is load-bearing. I request additional models (or a quantitative bounding argument) that vary the accreted material's entropy/composition, or include a He-rich","section":"Section 2, Section 5, Appendix B"},{"comment":"The engine model adopts constant disk and jet parameters: theta_disk = 45 degrees, s = 0.5, eta_jet = 0.01, and Mej scaling from Ivanov & Fernandez (2021). No sensitivity tests or error bars are presented for any of these. The BH accretion rate (Eq. 16) and the wind/jet luminosities (Eqs. 14–15) scale directly with these choices; for instance, the disk mass is proportional to sin(theta_disk) ~ 0.7, and s determines the split between accretion and wind. Moreover, the jet efficiency eta_jet is taken from MAD simulations at accretion rates orders of magnitude higher than the 1e-6–1e-3 Msun/s rates in these models (Section 4.2.2). The identification of these models as viable ultra-long GRB and FBOT progenitors therefore needs either a parameter exploration or a clear statement of how the conclusions should be revised as these parameters vary.","section":"Section 4.2.1–4.2.2"}],"minor_comments":[{"comment":"Typo: 'increaesd' should be 'increased'.","section":"Figure 6 caption"},{"comment":"Typo: 'throughtout' should be 'throughout' in the first paragraph.","section":"Section 1"},{"comment":"Missing space in 'thatthe' in the sentence 'If we assume thatthe efficiencies...'.","section":"Section 4.2.2"},{"comment":"The notation 'Mj>jISCO,Sch' is used in the table, but the text sometimes refers to 'mass with j > jISCO'; please define consistently. Also, the gray region in Figure 4 would benefit from an explicit statement of the BH spin range that bounds it.","section":"Table 1 and Figure 4"}],"recommendation":"major_revision","confidential_remarks":"The paper is well written and the authors are candid about limitations. My main concern is the untested sensitivity of the transient landscape to the internal structure of the accreted material; this is the load-bearing assumption that the current Appendix B does not cover. If the authors can add models or diagnostics addressing this, the paper could become acceptable. No concerns about novelty or citation practices; the use of Fuller et al. 2019 and Fuller & Lu 2022 is appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague, here's my read on Tsuna et al. The genuinely new thing is the systematic grid: 32 MESA models of post-main-sequence merger products with AM transport, run to carbon depletion, with an appendix that checks sensitivity to merger radius, wind efficiency, AM prescription, and resolution. The finding that larger mass gain pushes the star blue, and that the lower-mass BSGs keep faster outer-envelope rotation, is credible and consistent with the LMC rotation data they compare to. The SN light curves from successful explosions really do look 1987A-like, which is a nice, concrete payoff.\n\nThe soft spot is exactly where the reader's report puts it. The transient landscape for failed explosions depends on the outer-envelope specific AM profile, and that profile is inherited from a merger prescription that adds mass with the primary's surface entropy and composition, no secondary structure, no dredge-up. The authors say so themselves in Section 5, and they cite recent work that shows merger prescriptions differ. The Appendix B sensitivity runs vary merger radius, wind, AM transport, and resolution, but not the entropy/composition of the accreted material. That missing variation is load-bearing for Mcirc, Mdot, Ljet, and Lwind. On top of that, the disk/jet modeling uses adopted parameters—theta_disk=45°, s=0.5, eta_jet=0.01—without sensitivity tests. So the BSG-collapsar predictions for ultra-long GRBs and FBOTs are a plausible, well-framed hypothesis, not a result that stands on the same footing as the stellar grid.\n\nGrading the paper: the stellar evolution part is sound, the authors are honest about the simplifications, and the citation pattern is fine—Fuller & Lu is the natural model for the disk, and they cite the prior BSG-merger literature. The main thing I'd want in revision is sensitivity of the transient predictions to the merger prescription and the disk parameters, and the promised data release. That is a revision, not a rejection. A serious referee should spend time on it; I'd send it out. I would cite the stellar grid in my own work. Yes, bring to reading group.","headline":"A credible rotating-merger stellar grid whose transient landscape is a plausible but less-tested extension; worth a serious referee with a focus on the merger prescription and disk parameters.","tokens_in":32729,"tokens_out":2224,"would_cite":true,"duration_ms":25604,"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":"Blue supergiants forged in post-main-sequence binary mergers are viable progenitors of SN 1987A-like supernovae, ultra-long gamma-ray bursts, and a subset of fast luminous transients.","keywords":["blue supergiants","stellar mergers","stellar rotation","core-collapse supernovae","SN 1987A-like supernovae","ultra-long gamma-ray bursts","fast blue optical transients","fallback accretion"],"falsifier":"A three-dimensional hydrodynamic simulation of an early Case B merger at mass ratio q ≈ 0.5–0.8, checking whether the full orbital angular momentum of Eq. (2) is retained, how much of the secondary is actually kept, and whether helium-core material reaches the surface. Large deviations in any of these would shift or erase the mapped transient classes. Observationally, measuring a fast-rotating, helium/nitrogen-enriched envelope in a nearby SN 1987A-like progenitor would confirm the channel, while finding none in a sample of a dozen such events would cap its contribution to the observed rate.","tokens_in":31719,"feed_emoji":"💥","tokens_out":15532,"duration_ms":141999,"temperature":0.7,"pith_summary":"Blue supergiants — hot, compact evolved stars — are hard to make in single-star evolution, but this paper argues they are made naturally when two stars in a binary merge after one leaves the main sequence, and that the deaths of these merged stars can account for several distinct classes of observed transients. The authors evolve 32 merger products in a one-dimensional stellar evolution code and find that mergers with mass ratio roughly 0.5–0.6 or higher die as blue supergiants whose outer envelopes are still rotating fast — faster for lower-mass products, whose weaker winds shed less angular momentum. If the neutrino-driven explosion succeeds, these stars die as SN 1987A-like supernovae with long-rising light curves, at roughly the observed rate; if it fails, the envelope falls back onto a newborn black hole and powers accretion-driven winds and jets that could appear as ultra-long gamma-ray bursts and as AT2018cow-like fast luminous transients. The central claim is that this single merger channel, at Large Magellanic Cloud-like metallicity, is a viable origin for all three classes.","feed_headline":"One channel yields SN 1987A twins, ultra-long GRBs, fast transients","feed_subtitle":"A single stellar-death route ties together three transient classes without invoking near-zero metallicities.","key_machinery":"Three pieces carry the argument. (1) A one-dimensional merger prescription: when the expanded primary reaches 50 solar radii, the secondary's mass is added at 10^-2 solar masses per year carrying the full specific angular momentum of the binary orbit, j_acc = M1 sqrt(G R_*/(M1+M2)) (Eq. 2), with added gas set to the primary's surface entropy and composition. (2) An angular-momentum transport model based on the Tayler instability — magnetic field amplification that drives nearly rigid rotation in radiative zones — which fixes the rotation profile that survives to collapse. (3) A death criterion: comparing the envelope's specific angular momentum with j_ISCO (Eq. 9), the threshold for material","core_discovery":"On the paper's own terms, the discovery is a connected set of fates. Post-main-sequence binaries that merge with mass ratio roughly M2/M1 ≳ 0.5–0.6 die as blue supergiants, not red supergiants, and retain much of the merger's orbital angular momentum in their radiative envelopes until core collapse; lower-mass products rotate faster because weaker winds shed less angular momentum. A successful neutrino-driven explosion produces a long-rising light curve of SN 1987A morphology, at an estimated rate (≈2–4% of core collapses) matching the observed fraction of such supernovae. A failed explosion makes the envelope fall back over 10^3–10^5 seconds, circularizing 0.1–several solar masses into an a","pith_inferences":["Because the one-dimensional prescription neglects the secondary's helium-rich core, a three-dimensional hydrodynamical simulation of an early Case B merger at q ≈ 0.5–0.8 is the cleanest test of whether the true blue-supergiant threshold and the retained angular momentum move; any shift there rescales all the quoted rates.","If late-stage LBV-like mass loss is as strong as radio observations of fast blue optical transients imply, it could strip the rotating envelope and quench the very accretion that powers them — a self-limiting tension that wind prescriptions for the final centuries could settle.","The predicted anti-correlation between black-hole spin and final black-hole mass offers a pathway to the massive, spinning black holes seen in gravitational-wave events, which hierarchical mergers struggle to produce.","The predicted cocoon flash (near-UV, ~10^42–10^43 erg/s, peaking days after jet breakout, detectable to z ≈ 0.2) is a discriminator: prompt ultraviolet follow-up of a nearby fast transient could tell a blue-supergiant collapsar from a magnetar-powered engine."],"forward_implications":["A single route — post-main-sequence binary mergers at sub-solar metallicity — can populate the observed landscape from SN 1987A-like supernovae to ultra-long gamma-ray bursts to fast luminous transients, tying together classes usually modeled separately.","Fast-rotating blue supergiants can be produced at Large Magellanic Cloud-like metallicity, so ultra-long gamma-ray burst engines do not require the near-zero metallicities previously invoked.","Failed collapse of the lower-mass merger products sustains accretion at 10^-6–10^-3 solar masses per second for hours, and the resulting jets break out of the stellar envelope before they shut off, making these stars workable collapsar engines.","The channel's estimated rate (≈2–4% of core collapses) agrees with the observed fraction of 1987A-like supernovae, supporting the merger origin for that class.","High-mass (25 solar mass) merger products that fail to explode yield week-long, ~10^44 erg/s wind-driven transients with hydrogen, helium, fast asymmetric ejecta, and little nickel — the hallmarks of AT2018cow-like fast blue optical transients."],"supporting_citations":[{"why":"Pioneered the one-dimensional rapid-accretion construction of post-main-sequence merger products that this paper follows, and supplied the rate-estimation approach for 1987A-like supernovae.","marker":"Justham et al. 2014"},{"why":"Comparison grid for merger-produced blue supergiants; its picture of convective hydrogen-shell burning is invoked to explain why large accreted mass keeps stars blue, and it motivated the added-mass composition assumption.","marker":"Schneider et al. 2024"},{"why":"Supplies observed projected rotation velocities of LMC blue supergiants used to validate the models' envelope rotation, and serves as a comparison model grid that includes merger dredge-up which this paper lacks.","marker":"Menon et al. 2024"},{"why":"Supplies the angular-momentum transport prescription (magnetic Tayler instability) that fixes the nearly rigid rotation of the radiative envelope.","marker":"Fuller et al. 2019"},{"why":"Provides the one-zone fallback-disk model and the angular-momentum transport parameters used to compute every failed-explosion transient prediction.","marker":"Fuller & Lu 2022"},{"why":"Jet propagation model used to compute whether the relativistic jet breaks out of the blue-supergiant envelope before the accretion shuts off.","marker":"Bromberg et al. 2011"},{"why":"The radiation-hydrodynamic light-curve code used to compute the SN 1987A-like light curves from the blue-supergiant explosion models.","marker":"Morozova et al. 2015"},{"why":"Prior proposal of rotating blue supergiants at extremely low metallicity as ultra-long gamma-ray burst progenitors, which this paper extends to merger products at LMC-like metallicity.","marker":"Kashiyama et al. 2013"},{"why":"The stellar evolution release whose mass-addition scheme sets added gas to the surface entropy, the basis of the one-dimensional accretion prescription.","marker":"Paxton et al. 2015"}],"fun_headline_variants":["Merger-born blue supergiants tie SN 1987A to GRBs and fast transients","One merger path may explain SN 1987A, ultra-long GRBs, and fast transients","Merger-made blue supergiants may yield SN 1987A, long GRBs, and fast transients","Post-merger blue supergiants unify SN 1987A, ultra-long GRBs, and fast transients","Mergers spawn rotating blue supergiants behind SN 1987A, GRBs, fast transients"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The whole landscape rests on treating a post-main-sequence binary merger as one-dimensional rapid accretion of the secondary's mass onto the primary, with no mass lost, the added gas given the primary's surface composition, and the full orbital angular momentum of Eq. (2) retained — a picture that ignores the secondary's helium-rich core and merger dredge-up, which would change the envelope composition, the blue-supergiant threshold, and the rotation profile powering every di","fun_headline_variants_meta":{"raw":{"variants":["Merger-born blue supergiants tie SN 1987A to GRBs and fast transients","One merger path may explain SN 1987A, ultra-long GRBs, and fast transients","Merger-made blue supergiants may yield SN 1987A, long GRBs, and fast transients","Post-merger blue supergiants unify SN 1987A, ultra-long GRBs, and fast transients","Mergers spawn rotating blue supergiants behind SN 1987A, GRBs, fast transients"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001355,"raw_usage":{"total_tokens":5385,"prompt_tokens":841,"completion_tokens":4544,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":585,"completion_tokens_details":{"reasoning_tokens":4409}},"tokens_in":585,"tokens_out":4544,"duration_ms":27603,"temperature":1.0,"reasoning_tokens":4409,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T14:34:04.075611+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A three-dimensional hydrodynamic simulation of an early Case B merger at mass ratio q ≈ 0.5–0.8, checking whether the full orbital angular momentum of Eq. (2) is retained, how much of the secondary is actually kept, and whether helium-core material reaches the surface. Large deviations in any of these would shift or erase the mapped transient classes. Observationally, measuring a fast-rotating, helium/nitrogen-enriched envelope in a nearby SN 1987A-like progenitor would confirm the channel, while finding none in a sample of a dozen such events would cap its contribution to the observed rate.","supporting_citations":[{"cited_title":"2013, , 770, 8, 10.1088/0004-637X/770/1/8","cited_arxiv_id":null,"evidence_quote":"Prior proposal of rotating blue supergiants at extremely low metallicity as ultra-long gamma-ray burst progenitors, which this paper extends to merger products at LMC-like metallicity."}],"review_version":1}