{"id":"24011776-51af-429a-9fd5-19e015237518","arxiv_id":"2411.15346","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Stars stripped by partial tidal disruptions re-inflate into giants with normal radii, about twice the luminosity of same-mass giants, and can be repeatedly stripped down to 0.6 to 0.7 solar masses.","lead":"This paper simulates what happens when a supermassive black hole tears part of a giant star away, and finds the surviving core quickly puffs back up into a giant again. The result points to a new population of lightweight giant stars near the center of the Milky Way that could be identified as remnants of past tidal disruptions.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central remnant-structure prediction rests on the untested assumption that a spherical quasi-static wind reproduces an impulsive aspherical PTDE; chemical homogeneity does not justify this, and a 3D simulation is required.","rationale":"The paper's strongest claim is that stripped giant remnants re-inflate to a giant configuration with a similar radius to the progenitor, a luminosity about twice that of a same-mass normal giant, and a comparable lifetime, and that repeated PTDEs converge to low-mass (~0.6-0.7 Msun) giants observable in the Galactic center. All of these results are obtained by evolving, in MESA, a star from which a fraction of the envelope has been removed by a spherical, quasi-static wind. The only justification for equating this with a real PTDE is the statement that the giant envelope is hydrogen, so hydrodynamic effects can be ignored. This justification is logically insufficient: hydrodynamic effects are not limited to compositional mixing. Shock heating, adiabatic cooling of the expanding surviving envelope, and non-spherical readjustment can all change the thermal and entropy structure of the remnant, and they are absent from the 1D quasi-static model. The paper itself flags this in Section 3: 'Indeed, this stage should be done using a full 3D hydro simulations.' The fact that the final thermally relaxed structure might be unique for a given total mass and core mass does not rescue the prediction, because the 3D dynamics also determine the remnant mass and core mass; Eq. 2 is an analytic estimate calibrated for main-sequence stars, not for giants with a core-envelope density contrast of ~1e6. The paper's own resolution tests (Fig. 13) and reproducible inlists are valuable, but they do not test this equivalence. The reader's weakest assumption identifies exactly this point, and we agree. Therefore the reader's CONDITIONAL verdict is appropriate; our stress test does not move the verdict, but it sharpens the required validation: a 3D PTDE simulation of a giant, mapped back into a 1D evolutionary code, is the decisive check.","tokens_in":62,"tokens_out":15647,"duration_ms":267507,"concrete_test":"Run a 3D hydrodynamics simulation (SPH or moving-mesh) of a 2 Msun, 10 Rsun horizontal-branch giant (Z=0.1 Zsun) on a parabolic orbit around a 1e6 Msun BH with pericenter rp=130 rg, matching the paper's repeated-PTDE fiducial case. After the star recedes beyond the tidal sphere, identify the self-bound remnant, record its mass, and map its spherically averaged density and entropy profiles into MESA (e.g., via relax_initial_structure). Evolve to thermal equilibrium and compare the resulting radius, luminosity, and core mass with the paper's relaxed MESA remnant stripped by ΔM=0.5 Msun at f_He=0.5. If the radius or luminosity differ by >10%, or if the remnant mass differs from the Eq. 2 prediction by >0.1 Msun, the central quantitative predictions (and the repeated-PTDE mass sequence) are not robust and the paper's conditional claims should be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 3 replaces the true impulsive, aspherical partial tidal disruption with a spherical, quasi-static wind (relax_mass_to_remove_H_env) and justifies this by stating that because giant envelopes consist of hydrogen, hydrodynamic effects can be ignored. This is a non sequitur: the absence of a chemical gradient only excludes compositional mixing; it does not exclude shock heating, adiabatic expansion, aspherical oscillations, or additional mass loss during the sudden removal of the envelope. The paper itself acknowledges that the pericenter passage time is much shorter than the thermal timescale (tp << tKH), so the MESA relaxation path—which maintains thermal equilibrium throughout the stripping—is not the physical path. The load-bearing predictions (re-inflation to the progenitor radius, luminosity boost by ~2 relative to a same-mass giant, and the repeated-PTDE convergence to 0.6-0.7 Msun) depend on the relaxed remnant having the same mass and core mass as the spherical-wind model. If the real remnant mass differs (e.g., because Eq. 2 is uncalibrated for giant core-envelope profiles) or if the surviving envelope's entropy is altered by the dynamical encounter, these predictions shift. The paper provides no 1D test of stripping-timescale dependence and no 3D test of the equivalence; the only support is the plausibility argument in Section 3.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the long-term evolution of remnants left by partial tidal disruption events (PTDEs) of horizontal-branch giant stars by supermassive black holes. The authors use MESA to evolve 1, 1.5, 2, and 3 Msun stars at 0.1 Zsun, strip a fraction of the envelope with an artificial spherical wind, and follow the remnant to white-dwarf formation. They find that the stripped stars re-expand to a radius comparable to the progenitor's, have luminosities about a factor of two larger than ordinary giants of the same total mass, have lifetimes that differ from their progenitors by only a few million years, and, if their orbits survive, undergo successive PTDEs that converge to a 0.6-0.7 Msun giant. They estimate that a few dozen to a few hundred such remnants may currently reside in the Galactic center.","tokens_in":109,"tokens_out":7522,"duration_ms":190274,"significance":"If the mapping from an impulsive, aspherical PTDE to a thermally relaxed spherical wind is correct, the paper provides concrete and falsifiable predictions: low-mass (<0.9 Msun) giants in the Galactic center, identifiable by asteroseismology, with a characteristic luminosity excess and a repeated-PTDE convergence mass. The paper also sharpens the argument that giant TDEs around black holes above about 3e5 Msun are partial and that the usual tidal-radius estimates need revision. The MESA setup is well documented, with provided inlists, a detailed appendix, and resolution tests, which are notable strengths. However, the central results rest on an untested equivalence between a quasi-static, thermally relaxed mass-loss process and the actual impulsive, aspherical tidal encounter; the paper acknowledges this limitation, but the main predictions depend on it.","major_comments":[{"comment":"The central assumption of the paper—that removing envelope mass with the MESA routine relax_mass_to_remove_H_env over a thermal timescale reproduces the remnant of an impulsive partial TDE—is load-bearing but untested. The text itself notes that t_p << t_KH, so the real remnant is not in thermal equilibrium along the relaxation path. The argument that the envelope is hydrogen only rules out compositional mixing; it does not rule out shock heating, adiabatic expansion, aspherical oscillations, or prompt additional mass loss during the dynamical encounter. Since the re-inflation radius, the factor-of-two luminosity excess, and the 0.6-0.7 Msun repeated-PTDE convergence mass all follow from the post-disruption mass and entropy profile, the paper needs either a 3D hydrodynamic test of this equivalence or a 1D sensitivity study that varies the stripping duration and the entropy of the retained envelope. As it stands, the results describe thermally relaxed wind-stripped giants, not necessarily PTDE remnants.","section":"Section 3 (Methods)"},{"comment":"The stripped mass is estimated with M(r) ≈ (r/r_p)^3 M_BH from Ryu et al. (2020b), which the paper says was calibrated for main-sequence stars. The order-unity correction factor is dismissed for giants without a quantitative test. Because Eq. (2) sets the mass lost in every event and therefore drives the successive-PTDE sequence, an uncalibrated relation for steep giant density profiles could systematically change the claimed convergence mass and the remnant population estimate. Please validate Eq. (2) against giant-specific hydrodynamic PTDE simulations, or provide a sensitivity analysis over the correction factor and show that the conclusions are robust.","section":"Section 2, Eq. (2), and Figs. 2, 11, 12"},{"comment":"The return-time estimate in the text is 2π R_h^{3/2}/(G M_BH) ≈ 2.5×10^5 yr, but Fig. 12 states that the time between successive disruptions is 2×10^6 yr. The discrepancy is not explained. Since the number of repeated PTDEs and the resulting final remnant mass depend on the cadence of disruptions relative to the thermal relaxation and radius-recovery timescales, the adopted timing should be justified and the inconsistency reconciled.","section":"Section 4.1 and Fig. 12"}],"minor_comments":[{"comment":"Typos appear in several places: 'Schwarzchild' in the captions of Figs. 2 and 11 and in Section 2, 'comparision' in Section 2, 'dicussion' in Section 3, and 'preZAMS' in the caption of Fig. 10 should be 'pre-ZAMS'.","section":"Throughout"},{"comment":"The resolution test is shown for a 1 Msun, 1 Zsun star, while the main simulations use 0.1 Zsun; please state whether the chosen mesh_delta_coeff and time_delta_coeff were also tested at 0.1 Zsun and whether the conclusions are unchanged.","section":"Fig. 13 and Appendix"},{"comment":"The phrase 'unless ≲ 0.15 Msun of the envelope mass is retained' appears to state the opposite of the intended condition; rephrase to indicate that a giant structure is recovered when at least about 0.15 Msun of envelope mass remains.","section":"Section 4, discussion of Fig. 5"},{"comment":"The rate estimate should spell out that 'a TDE per 10^4-10^5 yr' is the assumed total Galactic TDE rate and that the approximately 10% giant fraction is then applied; otherwise the resulting 'few dozen to a few hundred' is not transparent.","section":"Section 5, rate estimate"},{"comment":"The Zenodo DOI for the inlists is given in the Appendix; it would be helpful to also cite it in Section 3 where relax_mass_to_remove_H_env is introduced.","section":"Section 3 and Appendix"}],"recommendation":"major_revision","confidential_remarks":"The paper is transparent about its main approximation, and the MESA work is reproducible. My major concern is not a numerical error but an untested physical-equivalence claim that carries the main results. If the authors can supply a 3D hydrodynamic test for at least one representative case, or a 1D sensitivity study over stripping duration and retained-envelope entropy, the paper would be a solid contribution to the TDE literature. I would not recommend rejection, because the stellar-evolution results are well documented and the predictions are falsifiable, but I would not accept the paper in its current form."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read this if you care about what survives a tidal disruption. Navarro Navarro and Piran run a MESA grid of horizontal-branch giants, strip off envelope mass, and follow the remnants to white dwarfs. The main results: stripped stars re-inflate to a giant radius comparable to the progenitor, they are roughly twice as luminous as an ordinary giant of the same total mass, their lifetimes barely change, and a sequence of repeated partial TDEs converges to a ~0.6–0.7 Msun giant. That last point is a genuinely new prediction, and it is testable in the Galactic center.\n\nThe paper is honest and well documented. The MESA setup is standard, they show resolution tests, and the inlists are on Zenodo. The results are reproducible by construction.\n\nThe soft spot is the stripping prescription. They use MESA's relax_mass_to_remove_H_env, a spherical, quasi-static wind that keeps thermal equilibrium. A real partial TDE is impulsive and aspherical. The paper argues hydrodynamics can be ignored because the envelope is hydrogen, but that only rules out chemical mixing; it does not rule out shock heating, adiabatic expansion, or a different entropy profile in the surviving envelope. They note tp << tKH, so the relaxed path is not the physical path. If the real remnant has a slightly different mass or entropy, the re-inflation radius, the luminosity boost, and the repeated-PTDE endpoint all shift. This is a genuine uncertainty. It is not fatal; the qualitative behavior is likely robust, but the quantitative predictions should be labeled as conditional until tested against a 3D hydrodynamic simulation or at least a 1D impulsive-strip test.\n\nMinor issue: Eq. 2 for the stripped mass comes from MS star simulations, and the paper uses it for giants without calibrating. Could be off by order unity. The rate estimate is order-of-magnitude, fine.\n\nVerdict: worth a serious referee. The paper is a solid first look at an understudied population, reproducible, and self-aware about its main assumption. The referee should ask for a robustness test of the stripping prescription and a clear statement that the predictions are model-dependent on that choice. I would not desk reject this.","headline":"Solid MESA grid with a genuinely new prediction, but the spherical-wind stripping leaves the quantitative results conditional until tested against 3D hydrodynamics.","tokens_in":12648,"tokens_out":2762,"would_cite":true,"duration_ms":25660,"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":"Partial tidal disruptions of giant stars by supermassive black holes leave the surviving core to re-inflate into a new giant of nearly the same radius and roughly twice the luminosity of a normal giant of the same mass, and this sequence…","keywords":["partial tidal disruption events","giant stars","supermassive black holes","MESA stellar evolution code","stellar remnants","white dwarfs","Galactic center","asteroseismology"],"falsifier":"One could run a full 3D hydrodynamical simulation of a 1 solar-mass horizontal-branch giant on a parabolic orbit around a $10^{6}$ solar-mass black hole with pericenter about 130 gravitational radii, and follow the resulting remnant for roughly $10^{5}$ years; if the envelope does not re-expand to approximately the progenitor radius, or if the core's mass or burning structure is altered significantly, the spherical-wind approximation fails.","tokens_in":11617,"feed_emoji":"🌟","tokens_out":10139,"duration_ms":87457,"temperature":0.7,"pith_summary":"Most tidal disruptions of giant stars by supermassive black holes are partial: the dense core survives while part of the envelope is torn away. Using the MESA stellar evolution code, this paper shows that the stripped remnant re-inflates within a thermal timescale to a giant of nearly the same radius as its progenitor, with roughly twice the luminosity of a normal giant of the same mass, and a lifetime barely changed before white-dwarf collapse. The authors argue that low-mass giants in the Galactic center (below about 0.9 solar masses) are therefore likely remnants of such events, and that repeated passages can strip the star successively down to about 0.6–0.7 solar masses. They expect a few dozen to a few hundred such remnants in the Milky Way nucleus.","feed_headline":"Black hole-stripped giants re-inflate and shine twice as bright","feed_subtitle":"Partial tidal disruptions could leave dozens to hundreds of light giant remnants near the Milky Way's center.","key_machinery":"The mechanism that carries the argument is the giant's extreme core–envelope structure: a helium-burning core of about 0.5–0.6 solar masses, roughly $10^{6}$ times denser than the envelope, survives the tidal encounter while the tenuous hydrogen envelope is partially peeled off. The paper models the peeling in MESA as a spherical, thermally relaxed wind (relax_mass_to_remove_H_env), then follows the remnant's evolution to the white-dwarf phase. The approximate bound-mass relation $M(r) \\approx \\frac{4\\pi}{3}\\int_0^r \\rho(r')r'^2\\,dr' \\approx (r/r_p)^3 M_{\\rm BH}$ (Ryu et al. 2020b) links pericenter to stripped mass and drives the successive-PTDE scenario, in which each re-inflated giant returns to the same pericenter and loses progressively less mass.","core_discovery":"The paper's central discovery is that a giant partially stripped by a supermassive black hole does not end its life as a bare core: after a short relaxation of order the thermal timescale, the remnant expands back to a giant with a radius comparable to the progenitor's. The remnant has a slightly more massive and denser core relative to its total mass, a more tenuous envelope, a luminosity about twice that of an ordinary giant of the same total mass, and a lifetime until white-dwarf collapse that differs from the progenitor's by only a few million years. Because stars below roughly 0.9 solar masses cannot reach the giant stage within the age of the Universe, any such light giant near the Galactic center would be identifiable as a partial-tidal-disruption survivor. If the remnant's orbit is unchanged, the sequence is repeated: each encounter removes less mass, and the star converges to a light giant of about 0.6–0.7 solar masses before it eventually becomes a white dwarf.","pith_inferences":["If the spherical-wind approximation is wrong—for instance, if the impulsive encounter heats or mixes the residual envelope—then the factor-of-two luminosity boost and the repeated-PTDE sequence would be weaker; this is testable with a 3D simulation of the encounter itself.","Because the orbital return time (~2.5×10^5 yr) is comparable to the thermal relaxation time, some remnants may be re-disrupted before fully re-inflating, which would change the mass stripped in the next encounter.","The convergence to ~0.6–0.7 solar masses implies a floor set by the core mass; once the envelope is too thin to re-inflate, further encounters would expose the core and quickly produce a helium white dwarf, a distinct endpoint to search for.","The predicted remnant population could be confused with field stripped giants produced by binary interactions, so spatial concentration near the Galactic center is the key observational discriminator."],"forward_implications":["The Galactic center should contain dozens to hundreds of low-mass (<~0.9 solar-mass) giants that are too light to have evolved into giants within the age of the Universe, and asteroseismology could identify them.","A single partial TDE is not the end: successive encounters at the same pericenter strip less and less mass, driving the remnant toward about 0.6–0.7 solar masses.","Stripped giants shine about twice as brightly as ordinary giants of the same mass, which may distinguish them in HR diagrams, though not unambiguously.","The stripping barely changes the remnant's lifetime, so its white-dwarf formation time remains close to that of the progenitor.","The same physics would produce a population of stripped giants around intermediate-mass black holes in globular clusters, signaling the black hole's presence."],"supporting_citations":[{"why":"Provides the MESA stellar evolution code in which the giant stars and stripped remnants are evolved.","marker":"Paxton et al. 2011"},{"why":"Describes the MESA version (r23.05.1) and its updates, used for the simulations in this work.","marker":"Jermyn et al. 2023"},{"why":"Supplies the approximate relation between pericenter distance and bound stellar mass used to compute the stripped mass in each PTDE.","marker":"Ryu et al. 2020b"},{"why":"Establishes that giant TDEs are predominantly partial, providing the motivation for studying remnants.","marker":"MacLeod et al. 2012"},{"why":"Confirms that for SMBH masses above roughly 3×10^5 solar masses all giant TDEs are partial.","marker":"Rossi et al. 2021"},{"why":"Determines the SMBH mass threshold below which the dense giant core can itself be disrupted.","marker":"Krolik & Piran 2011"},{"why":"Argues that stars grow into the loss cone as they become giants, supporting the estimated giant TDE rate.","marker":"Syer & Ulmer 1999"},{"why":"Provides the TDE rate estimates used to predict the number of giant PTDE remnants in the Galactic center.","marker":"Magorrian & Tremaine 1999"},{"why":"Reports observed low-mass helium-burning giants, providing the observational counterpart that the paper proposes to search for in the Galactic center.","marker":"Li et al. 2022"}],"fun_headline_variants":["Giant stars survive black hole nibbling, shine twice as bright","Partial TDE survivors re-inflate and outshine normal giants","Light giants near black holes betray past near-death events","Recycled giants: black hole snacks leave brighter survivors","Once a giant, always a giant: partial TDE remnants persist"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that removing part of the envelope as a slow, spherical, thermally relaxed wind in MESA produces the same remnant structure as the real, impulsive, aspherical tidal stripping by the black hole, which the paper justifies by the envelope being pure hydrogen with negligible chemical gradient.","fun_headline_variants_meta":{"raw":{"variants":["Giant stars survive black hole nibbling, shine twice as bright","Partial TDE survivors re-inflate and outshine normal giants","Light giants near black holes betray past near-death events","Recycled giants: black hole snacks leave brighter survivors","Once a giant, always a giant: partial TDE remnants persist"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000506,"raw_usage":{"total_tokens":2486,"prompt_tokens":981,"completion_tokens":1505,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":597,"completion_tokens_details":{"reasoning_tokens":1418}},"tokens_in":597,"tokens_out":1505,"duration_ms":11613,"temperature":1.0,"reasoning_tokens":1418,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T14:23:48.213766+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"One could run a full 3D hydrodynamical simulation of a 1 solar-mass horizontal-branch giant on a parabolic orbit around a $10^{6}$ solar-mass black hole with pericenter about 130 gravitational radii, and follow the resulting remnant for roughly $10^{5}$ years; if the envelope does not re-expand to approximately the progenitor radius, or if the core's mass or burning structure is altered significantly, the spherical-wind approximation fails.","supporting_citations":[],"review_version":1}