{"id":"4d800695-207c-40bd-9951-f8befbbf7234","arxiv_id":"2506.19788","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Emission line ratios such as [SII] 6731/[OIII] 5007 shift by up to a factor of 470 with distance from an OB star, offering an unresolved-spectroscopy route to identifying externally photoevaporating disks in clusters.","lead":"Astronomers built a simple model of the hot gas flowing off planet-forming disks that are being evaporated by nearby massive stars, and used it to find which emission line ratios best reveal this process in distant clusters. The paper provides a practical diagnostic to find evaporating disks even when telescopes cannot resolve them.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Observability tests omit nebular background although the ONC benchmark already shows a factor ~3 [SII] excess; the f>10 detectability threshold is therefore not established for unresolved cluster spectroscopy.","rationale":"The paper is an honest, well-scoped modeling study with real independent support: CLOUDY-based line emissivities, a benchmark against one MUSE proplyd, and public line-ratio tables. The internal physics of ionization fronts, critical densities, and temperature effects is clearly explained, and the Parker-wind check in Appendix A supports the constant-velocity density profile. However, the central observable claim is not yet secured. The reader's weakest assumption is exactly the one I consider load-bearing: the selected lines are assumed to come only from the ionized wind, and the observability simulations omit nebular background. I sharpen it with numbers from the paper itself: the single ONC benchmark underpredicts [SII] 6716 by a factor ~3.3, and the proposed method's best line ratio uses [SII] 6731 in the numerator. If the missing [SII] arises from the proplyd tail, PDR, or the cluster nebula, then the factor-470 dynamic range and the f>10 gradient threshold are not reliable predictions for unresolved observations. A quantitative nebular-background injection test, using the same public MUSE data the paper already uses, would settle this without new observations. Until such a test is done, the appropriate verdict is the same CONDITIONAL the reader gave; nothing in my read moves it to ACCEPT or REJECT, because the model could still be right and the test is straightforward.","tokens_in":33254,"tokens_out":6496,"duration_ms":74390,"concrete_test":"Use the public VLT/MUSE ONC cube of Aru et al. (2024a) to measure the nebular [SII] 6731/[OIII] 5007 and [NII] 6583/[SII] 6731 ratios as a function of projected distance from theta1 Ori C in regions free of proplyds. Then rerun the Section 4.3 Monte Carlo cluster, adding this measured nebular spectrum (scaled to the target cluster distance and convolved with a representative PSF) to each proplyd's unresolved line flux before computing ratios. If the recovered radial gradient no longer separates from a background-only cluster at >3 sigma, or if the scatter inflates beyond the f~10 threshold, the central claim is not supported; if the gradient survives, the concern is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires that unresolved line ratios are dominated by, or separable from, non-wind emission. The observability analysis in Section 4.3.1 adds projection and inclination scatter but assumes all [SII] 6731, [OIII] 5007, and [NII] 6583 flux comes from the ionized wind hemisphere; it includes no nebular background or interior-I-front emission. Section 1 itself notes that high-UV environments have substantial nebular emission, and Section 5.3 concedes that [OI] 6300 and other interior tracers are excluded. The benchmark in Table 2 shows the simulation underpredicts [SII] 6716 by a factor of about 3.3 (Halpha/[SII] observed 400 vs simulated 1300), and the observed [SII] peak is 3.8% off radius. Some or all of that excess is plausibly PDR/tail/nebular emission, all FUV-dependent and absent from the model. Since [SII] 6731 is the numerator of the headline ratio, adding any background with its own [SII]/[OIII] gradient (normal for an H II region) will alter the recovered ratio and the scatter. The f>=10 criterion in Section 4.3.1 is therefore an internal model threshold, not a measured detectability limit.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents a fast 1D model of emission lines from the ionized wind of externally photoevaporating protoplanetary discs. The wind density is assumed to follow n ∝ r^{-2} with a constant sound-speed velocity, the mass loss rate is taken from the FRIED grid, and CLOUDY is used to compute radial emissivity profiles. After benchmarking against VLT/MUSE observations of the ONC proplyd 177-341W, the authors run the model over FUV fields 10^3–10^6 G0, rank thousands of lines by a line-selection metric, and produce sensitivity tables for MUSE and broader wavelength ranges. They then perform Monte Carlo cluster simulations with projected distances, inclinations, and PDR obscuration, and argue that line ratios varying by f≳10 over the FUV range should show observable spatial gradients in unresolved cluster spectroscopy. The headline diagnostic is [SII] 6731 Å / [OIII] 5007 Å, which varies by a factor of 470 in the base model.","tokens_in":33490,"tokens_out":11017,"duration_ms":113945,"significance":"The proposed diagnostic addresses a real and timely observational need: external photoevaporation is currently studied mostly in resolved ONC proplyds, while more distant massive clusters require unresolved spectral diagnostics. The model is computationally efficient, and the sensitivity tables, data release, and use of public CLOUDY/FRIED codes make the work reproducible and immediately usable by the community. The physical interpretation in terms of ionization-front radii, critical densities, and temperature sensitivity provides a useful framework for understanding why certain ratios are sensitive to the FUV field. The predictions are falsifiable with existing and planned instruments. The paper is appropriately cautious about the simplicity of the wind model and the exclusion of interior-I-front emission in several places, although the observability analysis does not fully carry this caution through to the detectability claims.","major_comments":[{"comment":"The detectability simulations in Section 4.3.1 include projection and inclination effects but omit nebular background emission, even though Section 1 acknowledges that high-UV environments are associated with substantial nebular emission. The benchmark in Table 2 shows that the model underpredicts [SII] 6716 relative to Hα by a factor of about 3.3 (Hα/[SII] observed 400 vs simulated 1300). Since [SII] 6731 is the numerator of the headline ratio [SII]/[OIII], any unresolved background with its own [SII]/[OIII] gradient will dilute or modify the predicted trend. The f≳10 threshold stated in Section 4.3.1 is therefore an internal model threshold, not a demonstrated detectability limit for real unresolved cluster spectroscopy. Please add a contaminant model, or otherwise quantify how the predicted gradients survive the level of background implied by the benchmark discrepancy.","section":"Section 4.3.1, Table 2"},{"comment":"The benchmark forces agreement by scaling the mass loss rate by a factor of 1.79 to match the Hα peak radius and by deriving extinction from the observed Hα/Hβ ratio. The Hα peak-position match is therefore enforced by construction. The factor-of-3 discrepancies in [SII] 6716 and [NII] 6583 line ratios show that the model is not calibrated for absolute line ratio levels. This is acceptable for trend predictions, but the paper should state clearly that the benchmark validates the ionization structure rather than the line-ratio normalization, and it should explain why the [SII]/[OIII] predictions are expected to be robust to a factor-of-3 level of contamination from processes not included in the model.","section":"Section 3.1, Table 2"},{"comment":"The observational test of the [SII] 6731 / [OIII] 4959 trend uses only five proplyds and shows very large scatter: 177-341W at 0.049 pc has a ratio of 0.0095, while 173-236 at 0.095 pc has 19. Even after excluding the anomalous 170-337, the implied change over 0.05 pc is far steeper than the base-case model trend in Figure 5. The paper should either provide a quantitative comparison with the model, including the expected Monte Carlo spread in mass loss rates and distances, or describe this as an illustrative anecdote rather than a 'preliminary trend' that supports the model.","section":"Section 4.1.3, Table 5"}],"minor_comments":[{"comment":"The surface density normalization is listed as Σau = 100 g/cm3, but surface density should be in units of g/cm^2; please correct the unit.","section":"Table 1"},{"comment":"The statement that r_m/r_IF 'always' increases when moving to larger distance (Y>1, X<1) does not follow from Eq. (17) in general, since X^{1/6}Y^{1/3} can be less than 1 if the mass loss rate falls steeply with distance. The plotted base-case results support the trend, but the 'always' claim should be replaced by a statement about the FRIED-grid dependence.","section":"Section 4.2, Eq. (17)"},{"comment":"The notation for the optically thin limit, 'dτ(r)=0 for all r', is inconsistent with the definition of τ(x,y0) in Eq. (5); please clarify that the limit reduces the projected intensity integrals to volume integrals of j(r).","section":"Section 2.3, Eqs. (4)-(6)"},{"comment":"The caveats list jets, bow shocks, tail emission, abundance variations, and multiple OB stars as additional noise sources, but do not explicitly list nebular background emission from the H II region itself, despite its prominence in the Introduction and its likely contribution to the benchmark [SII] discrepancy.","section":"Section 5.4"},{"comment":"The line-selection measure RM uses an ad hoc weighting function h(x); the robustness of the selected line set to this choice of weighting is not tested. A brief sensitivity test, or at least a statement that the full tables are available for users to apply other metrics, would strengthen the methodology.","section":"Section 4.1.1, Eqs. (9)-(10)"},{"comment":"The sentence 'Störzer & Hollenbach (1998); Ballabio et al. (2023) found that G ≳ 5000 G0 the [Oi] 6300 Å line luminosity rises significantly' is missing a word; it should read 'found that for G ≳ 5000 G0 ...'.","section":"Section 5.3"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within scope for MNRAS and the data products are useful. The main load-bearing issue is the absence of nebular background emission in the observability simulations, which directly affects the central detectability claim. This is fixable either by adding a simple background model or by substantially softening the f≳10 threshold claim. The benchmark and preliminary observational test should also be framed more cautiously."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe short version: this is a genuinely useful paper that gives observers a systematic atlas of line ratios sensitive to external photoevaporation, plus a clear physical explanation for why they vary. The soft spot is that the observability predictions omit nebular background emission, so the paper's f>10 threshold for detectable gradients is an internal model number, not yet a measured limit. I'd send it to peer review.\n\nWhat's new: a fast CLOUDY-based 1D model of the ionized wind outside the H I-front, a line-selection procedure that ranks line ratios by sensitivity and luminosity, and sensitivity tables for the MUSE range and for 1150 Å to 28.5 µm. The claim that [SII] 6731/[OIII] 5007 changes by a factor of 470 over FUV 10^6 to 10^3 G0 is specific and testable. The physical decomposition into ionized volume, critical density, and temperature effects (Section 4.2) is genuinely clarifying and will help people interpret the tables. They benchmark against proplyd 177-341W: radial peak positions within 3 au, most H-alpha-to-line ratios within a factor of about 3. That is creditable for a simple model, and they are honest that Mdot is scaled by 1.79 and extinction derived from H-alpha/H-beta.\n\nThe soft spots, in proportion. The main one is the observability analysis in Section 4.3.1: it adds projection and inclination scatter but assumes all line flux comes from the ionized wind hemisphere, with no nebular background. The paper itself notes that high-UV environments have substantial nebular emission, and the benchmark Table 2 already shows the model underproduces [SII] 6716 by a factor of ~3.3. Since [SII] 6731 is the numerator of the headline ratio, adding an H II region background with its own [SII]/[OIII] gradient will change both the recovered ratio and the scatter. So the f>=10 criterion is an internal threshold, not an observed detectability limit. That does not kill the method - the trends might still survive - but it should be tested or caveated before using the threshold to design surveys. The exclusion of [OI] 6300 and other interior lines is acknowledged in Section 5.3, so this is an honest limitation, not a hidden one. The ONC empirical test of five proplyds is small and has an outlier, but it is a real first step.\n\nBottom line: the paper is clearly written, the data products are public, and the central claim is plausible. The missing nebular background is an important caveat rather than a fatal flaw. I would send to a referee and expect the authors to add background estimates or soften the observability claims. Useful for anyone planning IFU or MOS surveys of distant clusters.","headline":"A useful, honest new diagnostic atlas for external photoevaporation in unresolved proplyds, with the main caveat that the observability threshold omits nebular background; deserves peer review.","tokens_in":34102,"tokens_out":3336,"would_cite":true,"duration_ms":33554,"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":"Emission-line ratios can identify external photoevaporation in stellar clusters without resolving the discs.","keywords":["external photoevaporation","protoplanetary discs","proplyds","emission-line diagnostics","line ratios","photoionization modelling","stellar clusters","FUV radiation"],"falsifier":"Take spatially unresolved spectra of proplyd candidates in a distant massive cluster where the FUV field can be estimated, subtract the nebular background, and plot [SII] 6731 Å / [OIII] 5007 Å against projected distance from the O star; if the ratio does not rise monotonically by roughly two to three orders of magnitude as the FUV field drops from $10^{6}$ to $10^{3}$ G0, or the gradient disappears once [OI] 6300 Å and background emission are included, the central claim is falsified.","tokens_in":33005,"feed_emoji":"☄️","tokens_out":6569,"duration_ms":64214,"temperature":0.7,"pith_summary":"The paper tries to give observers a way to detect external photoevaporation of protoplanetary discs in distant clusters where proplyds cannot be spatially resolved. It builds a deliberately simple model of the ionized wind streaming off a disc, computes emission-line luminosities with a photoionization code, and ranks all line ratios by how strongly they change as the FUV field from a nearby O star drops from $10^{6}$ to $10^{3}$ G0. The central result is quantitative: the ratio [SII] 6731 Å / [OIII] 5007 Å changes by a factor of 470 over that range, and Monte Carlo cluster simulations show that ratios with f ≳ 10 retain a detectable spatial gradient after projection and inclination effects. If this is right, spatially unresolved spectroscopy alone can locate regions of ongoing external photoevaporation in massive, distant star-forming clusters.","feed_headline":"A single line ratio changes 470-fold with distance from a massive star","feed_subtitle":"Unresolved spectroscopy of faraway clusters could map external photoevaporation using [S II]/[O III] gradients.","key_machinery":"The load-bearing machinery is a one-dimensional model of the ionized proplyd wind: mass conservation with a constant sound-speed outflow gives n ∝ $r^{-2}$, a precomputed grid of mass-loss rates fixes the wind density from disc radius, host mass, FUV field, and surface density, and a photoionization code computes the radial emissivity profile of every line outside the hydrogen ionization front. Observables are obtained by integrating emissivity over a hemisphere, mimicking an unresolved proplyd. A line-selection metric combines the fractional slope of each line ratio with distance, its Spearman monotonicity, and its luminosity relative to Hα, producing sensitivity tables whose entries log10(f) give the factor by which a ratio changes between $10^{3}$ and $10^{6}$ G0. Three physical effects drive the strongest ratios: emission-volume shifts across the ionization fronts of different metals, critical-density effects that change whether emissivity scales as n or $n^{2}$, and temperature sensitivity of high-excitation lines.","core_discovery":"On the paper's own terms, the discovery is that external photoevaporation leaves a specific, monotonic fingerprint in spatially unresolved emission-line ratios. In the model, the ratio [SII] 6731 Å / [OIII] 5007 Å swings by a factor of 470 as the FUV field goes from $10^{6}$ to $10^{3}$ G0, because the volume where S II dominates shrinks roughly 25-fold and the volume where O III dominates shrinks roughly 7-fold relative to the hydrogen ionization front. Monte Carlo populations of proplyds with realistic stellar masses, disc radii, disc masses, and viewing geometries still show a clean spatial gradient for this ratio and for [NII] 6583 Å / [SII] 6731 Å (f = 8.5), while ratios with f ≈ 4 or 1.2 wash out. The paper therefore claims that line ratios with f ≳ 10 can identify ongoing external photoevaporation in stellar clusters even when individual proplyds are unresolvable, and that this conclusion barely depends on the spectral type of the ionizing star.","pith_inferences":["Beyond the paper, the same logic suggests a two-step survey strategy: use unresolved line-ratio gradients to identify clusters with ongoing external photoevaporation, then target those clusters with resolved follow-up spectroscopy.","Because the model excludes emission from inside the ionization front, the practical diagnostic may be a composite that combines an ionized-wind ratio like [SII]/[OIII] with a PDR tracer such as [OI] 6300 Å to suppress contamination.","A testable extension would be to search for these gradients in existing wide-field IFU data of known clusters, checking whether the ratio rises monotonically outward once nebular background lines are subtracted."],"forward_implications":["Line ratios with f ≳ 10, led by [SII] 6731 Å / [OIII] 5007 Å, should show observable spatial gradients in unresolved cluster spectroscopy.","The gradient shape is largely insensitive to whether the ionizing star is B0, O7, or O3, so the same diagnostic can be applied to clusters with different OB populations.","The best diagnostic lines are predicted to lie in the optical and UV, with [OII] 3726 Å particularly promising for future blue-optical and UV facilities.","The sensitivity tables provide a ready-made list of ratio pairs, letting observers choose the brightest and most sensitive tracers for their wavelength coverage.","An initial test against five ONC proplyds shows the expected trend of [SII] 6731 Å / [OIII] 4959 Å increasing with projected distance, with one anomalous proplyd attributed to a jet and line-of-sight geometry."],"supporting_citations":[{"why":"Supplies the mass-loss-rate grid that fixes the density normalization of the ionized wind for each proplyd.","marker":"Haworth et al. 2023b"},{"why":"Provides the photoionization code used to compute radial emissivity profiles for every emission line.","marker":"Ferland et al. 2017"},{"why":"Gives the VLT/MUSE observations of ONC proplyds used to benchmark the model and to make the preliminary line-ratio comparison.","marker":"Aru et al. 2024a"},{"why":"Supplies the earlier analytical Hα brightness profile model and the two-dimensional outflow geometry that the paper uses for comparison.","marker":"Henney & Arthur 1998"},{"why":"Proposes the [NII]/[SII] and related FUV-field diagnostics that this paper tests against its own line-ratio trends.","marker":"Maucó et al. 2025"},{"why":"Shows that [OI] 6300 Å emission increases with FUV field, which sets the key caveat about emission from inside the ionization front.","marker":"Ballabio et al. 2023"},{"why":"Provides the analytic proplyd-wind model and the tail-length relation used to interpret projected distances and proplyd geometry.","marker":"Johnstone et al. 1998"}],"fun_headline_variants":["One line ratio maps photoevaporation in distant star clusters","SII/OIII ratio reveals proplyd winds from far away","Spectral fingerprint identifies external photoevaporation","Line ratio gradient spots photoevaporation in unresolved clusters","470-fold ratio change traces massive star influence"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The results assume that all chosen diagnostic lines come only from the ionized wind outside the hydrogen ionization front, ignoring emission from the photodissociation region, the disc, and the surrounding nebula.","fun_headline_variants_meta":{"raw":{"variants":["One line ratio maps photoevaporation in distant star clusters","SII/OIII ratio reveals proplyd winds from far away","Spectral fingerprint identifies external photoevaporation","Line ratio gradient spots photoevaporation in unresolved clusters","470-fold ratio change traces massive star influence"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000277,"raw_usage":{"total_tokens":1716,"prompt_tokens":1077,"completion_tokens":639,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":693,"completion_tokens_details":{"reasoning_tokens":560}},"tokens_in":693,"tokens_out":639,"duration_ms":7052,"temperature":1.0,"reasoning_tokens":560,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T18:25:11.953564+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Take spatially unresolved spectra of proplyd candidates in a distant massive cluster where the FUV field can be estimated, subtract the nebular background, and plot [SII] 6731 Å / [OIII] 5007 Å against projected distance from the O star; if the ratio does not rise monotonically by roughly two to three orders of magnitude as the FUV field drops from $10^{6}$ to $10^{3}$ G0, or the gradient disappears once [OI] 6300 Å and background emission are included, the central claim is falsified.","supporting_citations":[{"cited_title":"J., Henney W","cited_arxiv_id":null,"evidence_quote":"Shows that [OI] 6300 Å emission increases with FUV field, which sets the key caveat about emission from inside the ionization front."}],"review_version":2}