{"id":"dcb1d82d-f526-4a86-bd11-2979563311bf","arxiv_id":"1908.05747","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Simulated TESS 2-minute transit light curves show starspot anomalies should be detectable, with predicted spot-size limits of roughly 1,900 to 15,900 km depending on host star and conditions.","lead":"This paper runs 20,573 simulated TESS transits of spotted stars to find when a starspot's brightness 'blip' rises above the noise. It reports the smallest detectable spot sizes for M4V, M1V, and K5V host stars and a relation between the flux dip of the spot and the transit.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Eq. 7 normalizes starspot flux by hemisphere area rather than projected disk area, making the paper's DeltaF_spot values and the abstract's 0.00015 minimum low by roughly a factor of 2.","rationale":"The paper has two intertwined claims: qualitative observability and quantitative detection limits. The qualitative claim is robust and well supported by the large PRISM simulation campaign, the model-recovery tests for the 2-sigma threshold, and the spot-free approximation cross-check (~1 ppm divergence). I am not objecting to that conclusion. However, the specific quantitative claim that the smallest detectable DeltaF_spot is 0.00015 depends on Eq. 7, and Eq. 7 has a normalization error: it uses spherical-cap surface area divided by hemisphere area rather than projected spot area divided by full-disk area. The corrected values are about a factor of 2 larger. This does not overturn the paper, because the radius-based detection limits and the k dependence survive, but it means the abstract and Section 3.6 must be revised. The reader's identified weakest assumption (single-transit mean-amplitude > 2 sigma) is a reasonable secondary caveat, but it is a methodological choice that the authors explicitly defend; the Eq. 7 error is an objective algebraic mistake in a headline number, so it is the more load-bearing concern.","tokens_in":67039,"tokens_out":19859,"duration_ms":193476,"concrete_test":"Recompute the DeltaF_spot values in Section 3.6 and Fig. 12 from the same simulated rspot limits and mean rho_spot values, replacing Eq. 7 with DeltaF_spot = sin^2(rspot)(1 - rho_spot), then refit Eq. 9. If the minimum DeltaF_spot moves from 0.00015 to approximately 0.00030, the abstract's flux-deficit claim and all Section 3.6 flux conversions should be corrected.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Equation 7 in Section 3.6 defines DeltaF_spot = (1 - cos rspot)(1 - rho_spot), with rspot an angular radius in radians (Eq. 1). Equation 5 computes the surface area of a spherical cap, and Eq. 6 normalizes by the area of the stellar hemisphere (2*pi*R*^2). But the flux occulted when a planet covers a spot near disk center is set by the spot's projected area on the sky, pi*R*^2 * sin^2(rspot), divided by the full-disk area pi*R*^2. The correct expression is therefore DeltaF_spot = sin^2(rspot)(1 - rho_spot) ~ rspot^2 (1 - rho_spot), which is twice the value of Eq. 7 for the small rspot values used throughout this paper (for rspot <= 0.35 rad, sin^2(rspot) ~ 2(1 - cos rspot) to within a few percent). Since Section 3.6 converts every simulated rspot limit to DeltaF_spot via Eq. 7 before fitting Eq. 9, the quoted 'smallest change in flux of the starspot (DeltaF_spot = 0.00015 +/- 0.00001)' and the coefficients of Eq. 9 are all too low by about a factor of 2. The location k = 0.082 is nearly unaffected because the factor is essentially constant across the fitted range. This is a concrete normalization error in a headline quantitative result, not a matter of detection-statistic taste. The radius-based detection limits (e.g., 4900 km for M4V) come directly from the PRISM simulations and are not affected.","agreement_with_reader":"disagree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents a large grid of forward-model simulations, using the PRISM transit-starspot code, of starspot anomalies in TESS 2-minute-cadence transit light curves of M4V, M1V, and K5V host stars. A starspot is deemed detected when the mean amplitude of the anomaly, averaged over the in-transit points describing the blip, exceeds 2.0 sigma of the injected Gaussian photometric noise. The authors report per-scenario minimum detectable spot radii, global detection limits of 4900 +/- 1700 km (M4V), 13800 +/- 6000 km (M1V), and 15900 +/- 6800 km (K5V), and a fitted quadratic relation between the spot flux deficit and the planet transit depth (Eq. 9), from which they quote a minimum detectable DeltaF_spot = 0.00015 +/- 0.00001 at k = 0.082 +/- 0.004. The central qualitative conclusion is that starspot anomalies will be observable in many TESS 2-minute transit light curves of K and M dwarfs.","tokens_in":67402,"tokens_out":9664,"duration_ms":94643,"significance":"If the quantitative calibration is corrected, the paper is a useful, transparent reference point for the TESS community: it gives a concrete expectation that starspot 'blips' will appear in 2-minute-cadence transits, and it quantifies how the detectability depends on spot temperature, wavelength, noise, orbital period, and planet size. The strengths include a large and clearly described simulation grid, a forward model benchmarked against JKTEBOP to about 10 ppm, explicit model-recovery tests motivating the detection threshold, and the promise to release the simulated light curves. However, the flux-deficit normalization in Eq. (7) is incorrect by approximately a factor of two, which propagates into Eq. (9), Figs. 11-12, and the abstract's DeltaF_spot value; this is a load-bearing quantitative error, although the radius-based detection limits from the PRISM simulations themselves are not affected. The numeric limits also depend on the chosen 2-sigma mean-amplitude detection statistic. With those caveats, the broad conclusion that starspot anomalies will be observable in TESS transit light curves is defensible.","major_comments":[{"comment":"The definition of DeltaF_spot is normalized incorrectly. For a spot at the centre of the stellar disc occulted by a planet, the flux deficit relative to the unspotted disc is set by the spot's projected area on the sky, pi R*^2 sin^2(rspot), divided by the full stellar disc area pi R*^2, i.e. DeltaF_spot = sin^2(rspot)(1 - rho_spot). Equation (7) instead uses the spherical-cap area normalized by the hemisphere area, (1 - cos rspot)(1 - rho_spot). For the small angular radii used throughout the paper (rspot <= 0.35 rad), sin^2(rspot) ~ 2(1 - cos rspot), so every DeltaF_spot value, the fitted coefficients in Eq. (9), the abstract's headline DeltaF_spot = 0.00015 +/- 0.00001, and the Section 3.6 contrast-extrapolation examples are too low by roughly this factor. In particular, the example claiming that a rspot = 1 degree spot cannot be detected for a 1 R_Jup planet around a 1 R_sun star would likely be reversed under the correct normalization. The optimum location k = 0.082 is nearly unaffected because the factor is approximately constant across the fitted range, and the radius detection limits computed directly from PRISM are unaffected. The authors should recompute all flux-deficit results with the projected-area normalization.","section":"Section 3.6, Table 4 and Eq. (9)"},{"comment":"Section 3.6, Table 4 and Eq. (9): The conversion from per-scenario rspot limits to DeltaF_spot uses a single mean contrast, rho_bar_spot, per spectral class (Table 4) rather than the actual rho_spot for each of the 1296 scenarios. Because rspot and rho_spot are correlated in the simulations, the product of the means is not the mean of the products, which introduces a systematic bias in the DeltaF_spot points used to fit Eq. (9). When correcting Eq. (7), the authors should recompute Fig. 12 and Eq. (9) using the per-scenario rho_spot values, or demonstrate quantitatively that the averaging is unbiased.","section":"Section 3, detection criterion"}],"minor_comments":[{"comment":"The number of M4V simulations is given as 6870 in Section 3.1 but 6718 in the Table A.1 caption; analogous discrepancies appear for M1V (7769 in the text vs. 7812 in Table A.2) and for K5V (5934 in the text vs. 6737 in Table A.3). These should be made consistent.","section":"Section 2.3"},{"comment":"The injected noise levels are described as the 'photometric sensitivity' of TESS; please clarify whether the 60-200 ppm values are point-to-point rms per 2-minute cadence sample, and how the cited 60 ppm hr^-1/2 noise floor is converted to the 2-minute cadence used in the simulations.","section":"Section 2.3"},{"comment":"Both DeltaF_p = k^2 and DeltaF_spot ignore limb darkening, even though PRISM applies quadratic limb darkening and the spot is placed at disc centre. For the limb-darkening coefficients used here, the centre-to-mean intensity ratio is about 1.15, which would alter the absolute flux-deficit values beyond the factor-of-two issue in Eq. (7); the approximations should be stated explicitly and ideally corrected.","section":"Section 3.6"},{"comment":"The use of three monochromatic wavelengths (600, 785, 1000 nm) to represent the TESS passband is a simplification, since rho_spot is wavelength-dependent and the TESS band is broad; a sentence quantifying the expected systematic uncertainty from not integrating over the full passband would be helpful.","section":"Section 2.1"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within the scope of A&A. The fact that PRISM is authored by the first author is not a concern given the JKTEBOP benchmarking and external use of the code. The central qualitative conclusion is defensible, but the Eq. (7) normalization error affects a headline quantitative result and must be corrected before publication; the detection-statistic dependence also deserves a robustness test."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nBottom line: this is a useful, transparent simulation campaign that makes a solid qualitative case that starspot 'blips' will be visible in TESS 2-min transit light curves of K and M dwarfs. But the paper has a concrete normalization error in its flux comparison section, and the abstract overstates the headline 1900 km number.\n\nWhat's new and good: It is the first systematic grid of starspot detection limits specifically for TESS 2-min cadence: 3888 scenarios, 20,573 simulated transits, three spectral types. The PRISM forward model is benchmarked against JKTEBOP at ~10 ppm, and the full tables are in the appendix with data at CDS. The authors also honestly explain and demonstrate the detection bias introduced by their mean-amplitude statistic.\n\nSoft spots, in order:\n\n1. Eq. 7 is wrong. The paper defines ΔF_spot = (1 - cos rspot)(1 - ρ_spot). That uses the spherical-cap surface area normalized by the hemisphere. But the occulted flux is the spot's projected area on the sky, π R*^2 sin^2(rspot), divided by the full disc area π R*^2. So the correct expression is ΔF_spot = sin^2(rspot)(1 - ρ_spot). For the small angles used here, sin^2(rspot) ≈ 2(1 - cos rspot). So all ΔF_spot values and the fitted Eq. 9 are low by roughly a factor of two. The abstract's ΔF_spot = 0.00015 should be about 0.00030. The location k = 0.082 barely moves, and the radius detection limits (4900 km, etc.) come directly from PRISM and are unaffected. This is not a matter of detection-statistic taste; it's a normalization error in a headline number.\n\n2. The abstract omits the caveat that 1900 km is the optimal-condition best case, seen in 7 of 1296 M4V scenarios. The body says it, the abstract doesn't. That's an overstatement, fixed by adding 'under optimal conditions'.\n\n3. Minor internal inconsistencies: text says 6870/7769/5934 simulations per star but table captions say 6718/7812/6737. Also the mean detection limits appear to exclude unresolved (NaN) scenarios, but that's not stated.\n\n4. The 2-σ mean-amplitude detection threshold is a hand-chosen definition. They justify it with recovery tests, and they openly discuss its consequences. It is a defensible choice, but the numeric limits are tied to it; readers should treat them as conditional on this definition.\n\nThe central conclusion is robust. The paper deserves a serious referee, but the flux-normalization error and the abstract caveat need fixing before the numbers go into the literature. I'd cite the radius limits after revision.","headline":"Solid TESS starspot detectability grid with a real factor-of-two flux normalization error in Eq. 7 and an overstated abstract; radius limits survive.","tokens_in":67945,"tokens_out":5314,"would_cite":true,"duration_ms":45041,"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":"Starspot anomalies will be observable in TESS 2-minute transit light curves, with detection limits of 4,900 ± 1,700 km for M4V, 13,800 ± 6,000 km for M1V, and 15,900 ± 6,800 km for K5V host stars.","keywords":["starspot anomalies","TESS","transit light curves","PRISM","detection limits","M dwarf stars","starspot contrast","photometric precision"],"falsifier":"Inject starspot anomalies of the predicted threshold sizes into real TESS 2-minute light curves of active M4V, M1V, and K5V dwarfs, add the same Gaussian noise levels, and measure the recovery rate under the $2.0\\sigma$ mean-amplitude rule; if recovery falls well below expectation, the claimed detection limits are optimistic. Conversely, a systematic search of TESS archival single transits that never finds blips at the predicted scales would falsify the observability conclusion.","tokens_in":66818,"feed_emoji":"⭐","tokens_out":5664,"duration_ms":48292,"temperature":0.7,"pith_summary":"This paper aims to establish whether the brief brightness 'blips' produced when a transiting planet eclipses a starspot will be visible in TESS 2-minute cadence data, and to quantify the smallest spot that can be detected around the K and M dwarf stars TESS targets. Using 20,573 simulated transits with the PRISM transit-starspot model across 3,888 scenarios, the authors find that spot anomalies will indeed be observable. The smallest spot seen under optimal conditions has radius about 1,900 km, while the mean detection limits are 4,900 ± 1,700 km (M4V), 13,800 ± 6,000 km (M1V), and 15,900 ± 6,800 km (K5V). The work also characterises how the smallest detectable spot-induced flux change depends on the planet-to-star radius ratio, reaching a minimum $\\Delta F_\\mathrm{spot} = 0.00015 \\pm 0.00001$ at $k = 0.082 \\pm 0.004$. If right, TESS light curves of active K and M dwarfs will routinely contain measurable spot anomalies, which is both a modelling complication and an opportunity for starspot tracking.","feed_headline":"TESS transits can unmask starspots as small as 1,900 km","feed_subtitle":"Simulated 2-minute light curves of K and M dwarfs predict measurable spot blips down to 4,900 km in typical cases.","key_machinery":"The central object is PRISM, a pixellation-based transit-starspot model that tiles the stellar disc into two-dimensional elements, assigns each element an intensity (with quadratic limb darkening and a blackbody-based spot contrast $\\rho_\\mathrm{spot}$), and integrates the received flux as the planet crosses. The load-bearing detection procedure compares the mean amplitude of the simulated spot anomaly, averaged over the in-transit data points that describe it, with the rms scatter of the light curve; a spot is counted as detected only when that mean exceeds $2.0\\sigma$. This statistic, not the spot physics alone, sets every numerical limit, and it is the reason the paper finds a planet-size-dependent detection bias.","core_discovery":"The central claim is that a starspot occulted by a transiting planet will produce a detectable brightening 'blip' in TESS 2-minute data for realistic M4V, M1V, and K5V hosts, with specific size limits. The authors define detection by the mean amplitude of the anomaly exceeding $2.0\\sigma$ of the photometric noise within a single transit, and use this to derive $r_\\mathrm{spot} = 0.045 \\pm 0.016\\,R_*$ for M4V, $0.040 \\pm 0.017\\,R_*$ for M1V, and $0.038 \\pm 0.016\\,R_*$ for K5V, corresponding to $4{,}900 \\pm 1{,}700$ km, $13{,}800 \\pm 6{,}000$ km, and $15{,}900 \\pm 6{,}800$ km. Under the most favourable combinations (cool spots, 600 nm, 60 ppm noise), the smallest detected spot is $r_\\mathrm{spot} = 0.017\\,R_*$, or about 1,900 km on an M4V star. They also find a universal relation between the spot flux change $\\Delta F_\\mathrm{spot}$ and the planetary transit flux change $\\Delta F_p$, with the minimum detectable $\\Delta F_\\mathrm{spot} = 0.00015 \\pm 0.00001$ at $k = 0.082 \\pm 0.004$. An additional result is the unexpected trend that small, hot spots are only detected by larger planets, which the authors attribute to the fixed 2-minute cadence and the mean-amplitude detection statistic.","pith_inferences":["An obvious extension the authors leave implicit is that a full likelihood-ratio or joint multi-transit analysis, rather than a single-transit mean-amplitude threshold, would likely push the effective detection limits somewhat smaller; their numbers should be read as conservative for dedicated analyses.","The predicted abundance of detectable blips implies that archival TESS light curves can be mined for single-transit spot occultations; counting their occurrence as a function of stellar activity would test the underlying spot-coverage assumptions.","The same simulation pipeline could be rerun for the 20-second cadence TESS mode, which should substantially improve the small-planet/small-hot-spot cases that the 2-minute cadence punishes.","The $\\Delta F_\\mathrm{spot}$–$\\Delta F_p$ relation could be turned into an estimator for spot contrast from a measured blip and known transit depth, effectively giving a spot-temperature diagnostic in the TESS band."],"forward_implications":["Many TESS transit light curves of active K and M dwarfs will contain measurable starspot blips, so analyses of those systems should include transit-starspot modelling rather than treating the transit shape as spot-free.","The quoted radius limits give a concrete target scale: spots larger than roughly 4,900 km on an M4V dwarf should generally be recoverable in single 2-minute transits, with the best-case floor near 1,900 km.","The quadratic $\\Delta F_\\mathrm{spot}$–$\\Delta F_p$ relation lets observers predict, for a given transit depth, the smallest spot flux deficit and lowest contrast that TESS can reveal, which can guide target selection for spot studies.","Unmodelled spot anomalies can bias measured planetary radii, limb-darkening coefficients, and transit timings; the paper's detection limits therefore bracket the severity of this bias for TESS systems.","Detectable single-transit spot anomalies enable the starspot-tracking technique for measuring stellar rotation and sky-projected spin-orbit obliquity in systems where the Rossiter-McLaughlin effect is impractical."],"supporting_citations":[{"why":"Supplies the PRISM model used to generate all simulated transits and the proportionalities between anomaly amplitude, spot area, and planet-to-star radius ratio.","marker":"Tregloan-Reed et al. 2013"},{"why":"Provides the TESS photometric precision, 60 ppm noise floor, M-dwarf target parameters, and planet-yield estimates that anchor the simulation setup.","marker":"Sullivan et al. 2015"},{"why":"Gives the blackbody contrast equation used to assign starspot intensities across the TESS passband.","marker":"Silva 2003"},{"why":"Provides the interferometric radius-mass-temperature relations used to set host-star parameters for M4V, M1V, and K5V dwarfs.","marker":"Boyajian et al. 2012"},{"why":"Supplies the quadratic limb-darkening coefficients for the TESS passband used by PRISM in the simulations.","marker":"Claret 2017"},{"why":"Documents the TESS 2-minute cadence and the target sample of 200,000+ stars that motivates the fixed cadence and noise assumptions.","marker":"Stassun et al. 2018"},{"why":"Supplies the 200–300 K spot temperature offsets for M dwarfs used to select the simulated starspot temperatures.","marker":"Barnes et al. 2015"}],"fun_headline_variants":["Starspot blips visible in TESS transits, down to ~1,900 km","M-dwarf starspots as small as 1,900 km seen in simulated TESS transits","TESS 2-min data can detect starspot anomalies in exoplanet transits","Simulations reveal TESS starspot detection limits: 1,900–15,900 km","TESS transit simulations set starspot size limits for M and K dwarfs"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The results stand on the choice that a spot is 'detected' only when the mean amplitude of its in-transit blip exceeds $2.0\\sigma$ of the photometric noise in a single transit; a different detection statistic or the use of phase-folded transits would move every reported limit.","fun_headline_variants_meta":{"raw":{"variants":["Starspot blips visible in TESS transits, down to ~1,900 km","M-dwarf starspots as small as 1,900 km seen in simulated TESS transits","TESS 2-min data can detect starspot anomalies in exoplanet transits","Simulations reveal TESS starspot detection limits: 1,900–15,900 km","TESS transit simulations set starspot size limits for M and K dwarfs"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000352,"raw_usage":{"total_tokens":2055,"prompt_tokens":1217,"completion_tokens":838,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":833,"completion_tokens_details":{"reasoning_tokens":723}},"tokens_in":833,"tokens_out":838,"duration_ms":8875,"temperature":1.0,"reasoning_tokens":723,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:06:15.876778+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Inject starspot anomalies of the predicted threshold sizes into real TESS 2-minute light curves of active M4V, M1V, and K5V dwarfs, add the same Gaussian noise levels, and measure the recovery rate under the $2.0\\sigma$ mean-amplitude rule; if recovery falls well below expectation, the claimed detection limits are optimistic. Conversely, a systematic search of TESS archival single transits that never finds blips at the predicted scales would falsify the observability conclusion.","supporting_citations":[{"cited_title":"2013, MNRA S, 428, 3671 V alio, A., Estrela, R., Netto, Y ., Bravo, J","cited_arxiv_id":null,"evidence_quote":"Supplies the PRISM model used to generate all simulated transits and the proportionalities between anomaly amplitude, spot area, and planet-to-star radius ratio."},{"cited_title":"W., Winn, J","cited_arxiv_id":null,"evidence_quote":"Provides the TESS photometric precision, 60 ppm noise floor, M-dwarf target parameters, and planet-yield estimates that anchor the simulation setup."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the blackbody contrast equation used to assign starspot intensities across the TESS passband."},{"cited_title":"S., von Braun, K., van Belle, G., et al","cited_arxiv_id":null,"evidence_quote":"Provides the interferometric radius-mass-temperature relations used to set host-star parameters for M4V, M1V, and K5V dwarfs."},{"cited_title":"G., Oelkers, R","cited_arxiv_id":null,"evidence_quote":"Documents the TESS 2-minute cadence and the target sample of 200,000+ stars that motivates the fixed cadence and noise assumptions."},{"cited_title":"R., Je ffers, S","cited_arxiv_id":null,"evidence_quote":"Supplies the 200–300 K spot temperature offsets for M dwarfs used to select the simulated starspot temperatures."}],"review_version":1}