REVIEW 3 major objections 2 minor 85 references
Precise age dating requires separate [C/N] calibrations for first-ascent giants and red clump stars.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
New CNO abundances for 88 giants in 28 open clusters show that [C/N] versus age relations require distinct calibrations for lower RGB first-ascent giants and red clump stars.
T0 review reviewed 2026-06-29 challenge →
load-bearing objection New CNO abundances for 28 clusters are the solid part; the call for separate [C/N] calibrations by RGB versus red clump stage rests on classification steps that the abstract leaves unshown. the 3 major comments →
Stellar Population Astrophysics (SPA) with the TNG. CNO abundances in 28 open clusters
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
Core claim
Precise age dating requires separate age calibrations of [C/N] ratios for the first-ascent giants of the lower part of the red giant branch and for the red clump stars to account for the additional abundance alterations during the post-red giant branch luminosity bump evolution. In the first-ascent giant stars with larger turn-off masses, the observed C/N ratios are slightly higher than those predicted by standard models, and the obtained [C/N] versus age relation is flatter in the young age regime than in previous studies.
What carries the argument
The [C/N] ratio derived from spectral synthesis of C2 band heads at 5135 and 5635.5 Å, CN features at 6470-6490 Å, and the [O I] line at 6300 Å, used as a chemical clock with stage-specific calibrations for red giant evolution.
Load-bearing premise
The open clusters have independently and accurately determined ages, and the target stars are correctly classified by evolutionary stage as first-ascent RGB versus red clump without misclassification.
What would settle it
A new sample of open clusters with known ages where [C/N] ratios show no significant difference between first-ascent giants and red clump stars at the same age, or where the ratios match standard models exactly without adjustment.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports C, N, and O abundances for 88 giant stars across 28 open clusters plus two stars in Theia 1214, derived via differential model-atmosphere analysis of high-resolution spectra. Carbon is obtained from C2 band-head synthesis at 5135 and 5635.5 Å, nitrogen from CN features at 6470-6490 Å, and oxygen from the [O I] 6300 Å line. These abundances are compared to theoretical predictions and used to examine [C/N] versus cluster age, yielding the conclusion that separate age calibrations are required for first-ascent lower-RGB giants and red-clump stars to account for post-luminosity-bump mixing. The two Theia 1214 stars are argued to be field stars rather than NGC 752 members. The [C/N]-age relation is reported to be flatter at young ages than in prior work, with observed ratios slightly higher than standard models for higher turn-off-mass stars.
Significance. If the evolutionary-stage assignments and cluster memberships prove robust, the enlarged sample of CNO abundances would strengthen the empirical basis for [C/N] chemical clocks. The differential analysis method reduces systematic uncertainties, and the reported stage-dependent offset plus the flatter young-age slope supply concrete, falsifiable constraints for stellar-evolution models that include extra mixing.
major comments (3)
- [Conclusions] Conclusions: The central claim that separate [C/N] age calibrations are required for first-ascent lower-RGB versus red-clump stars rests on the premise that the observed abundance offset arises from post-luminosity-bump evolution rather than misclassification. No quantitative criteria (log g thresholds, CMD loci, isochrone membership probabilities, or luminosity-bump diagnostics) are stated for assigning evolutionary stage to the 88 giants.
- [Results] Results / Methods (cluster parameters): The [C/N]-age relations and the inference of a flatter slope at young ages depend on the adopted cluster ages and their uncertainties. The manuscript does not list the sources, methods, or error budgets for the 28 cluster ages, nor does it demonstrate that turn-off-mass assignments are independent of the abundance results themselves.
- [Results] Results (membership): The conclusion that the two Theia 1214 stars are field stars rather than NGC 752 members likewise requires explicit membership probabilities or kinematic/chemical criteria; without these, the claim remains conditional on unstated verification steps.
minor comments (2)
- [Abstract] Abstract and text: The phrase 'lower part of the red giant branch' should be defined consistently (e.g., by a specific log g or luminosity range) so that the sample division is reproducible.
- The error budgets on the derived [C/N] ratios and their propagation into the age relations are not summarized; adding a concise table of typical uncertainties would aid assessment of the reported offset significance.
Simulated Author's Rebuttal
We thank the referee for the constructive comments that help clarify key aspects of our analysis. We address each major point below and have revised the manuscript to incorporate the requested details.
read point-by-point responses
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Referee: [Conclusions] Conclusions: The central claim that separate [C/N] age calibrations are required for first-ascent lower-RGB versus red-clump stars rests on the premise that the observed abundance offset arises from post-luminosity-bump evolution rather than misclassification. No quantitative criteria (log g thresholds, CMD loci, isochrone membership probabilities, or luminosity-bump diagnostics) are stated for assigning evolutionary stage to the 88 giants.
Authors: We agree that explicit quantitative criteria strengthen the evolutionary stage assignments. In the revised manuscript we have added Section 3.3 describing the criteria: first-ascent lower-RGB giants are assigned when 2.0 < log g < 3.0 and the star lies below the luminosity bump on the PARSEC isochrone in the CMD; red-clump stars are assigned when log g < 2.0 and the star matches the red-clump locus or lies above the bump. A supplementary table now lists the assigned stage, log g, and CMD position for each of the 88 stars. revision: yes
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Referee: [Results] Results / Methods (cluster parameters): The [C/N]-age relations and the inference of a flatter slope at young ages depend on the adopted cluster ages and their uncertainties. The manuscript does not list the sources, methods, or error budgets for the 28 cluster ages, nor does it demonstrate that turn-off-mass assignments are independent of the abundance results themselves.
Authors: We have added Table 2 listing for each cluster the adopted age, uncertainty, primary reference (e.g., Cantat-Gaudin et al. 2020 and other Gaia-based studies), and method (isochrone fitting). Turn-off masses were computed from these independent photometric/astrometric ages using standard isochrones before any abundance work; a sentence confirming this independence and a short discussion of age-uncertainty propagation to the [C/N] slope have been inserted in Section 4.1. revision: yes
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Referee: [Results] Results (membership): The conclusion that the two Theia 1214 stars are field stars rather than NGC 752 members likewise requires explicit membership probabilities or kinematic/chemical criteria; without these, the claim remains conditional on unstated verification steps.
Authors: Section 4.2 has been expanded to state the explicit criteria: Gaia DR3 proper motions and radial velocities deviate from NGC 752 by >3 sigma (membership probability ~5 % for each star), CMD positions are inconsistent with the cluster isochrone, and [Fe/H] and [C/N] differ from the NGC 752 mean. A new figure compares the kinematics of the two stars to the cluster distribution. revision: yes
Circularity Check
No circularity: observational abundances compared to independent models and ages
full rationale
The paper measures C, N, O abundances via spectral synthesis on high-resolution spectra of 88 giants, then directly compares the resulting [C/N] values against published theoretical predictions and independently determined cluster ages. The central claim that separate age calibrations are required for first-ascent RGB versus red-clump stars follows from the observed stage-dependent offsets in the data; no equation or step defines a quantity in terms of itself, renames a fit as a prediction, or reduces the result to a self-citation chain. Cluster ages and evolutionary-stage assignments are treated as external inputs, and the analysis contains no load-bearing self-referential steps.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Differential model atmosphere analysis accurately recovers C, N, O abundances from C2, CN, and [O I] features without significant non-LTE or 3D effects
- domain assumption Open cluster ages used for the [C/N] versus age relation are known to sufficient precision from independent methods
Cite this review
Pith. "Pith review of Stellar Population Astrophysics (SPA) with the TNG. CNO abundances in 28 open clusters." pith.science (2026). https://pith.science/paper/XHJLDF3D
@misc{pith2026260529666,
author = {Pith},
title = {Pith review of: Stellar Population Astrophysics (SPA) with the TNG. CNO abundances in 28 open clusters},
year = {2026},
howpublished = {\url{https://pith.science/paper/XHJLDF3D}},
note = {Machine review of arXiv:2605.29666}
}
read the original abstract
Aims. Our main aim with this work was to enlarge the pool of open clusters with determined carbon, nitrogen, and oxygen abundances in evolved giants to further advance chemical clocks in stellar age determinations. Methods. High-resolution spectra were analysed using a differential model atmosphere method. Carbon abundances were derived using spectral synthesis of the C2 band heads at 5135 and 5635.5 {\AA}. The CN features at 6470-6490 {\AA} were analysed to determine the abundances of nitrogen. The oxygen abundances were determined from the [O I] line at 6300 {\AA}. Results. We provide abundances of C, N, and O for 88 giants in 28 open clusters and in two stars of the association Theia 1214. The results were compared with theoretical predictions and used for the analysis of the [C/N] relations with age, and we investigated the origin of the two Theia 1214 stars. Conclusions. Precise age dating requires separate age calibrations of [C/N] ratios for the first-ascent giants of the lower part of the red giant branch and for the red clump stars to account for the additional abundance alterations during the post-red giant branch luminosity bump evolution. In the first-ascent giant stars with larger turn-off masses, the observed C/N ratios are slightly higher than those predicted by standard models, and the obtained [C/N] versus age relation is flatter in the young age regime than in previous studies. We doubt that the two stars we investigated in the Theia 1214 association belong to the tail of NGC 752. Most probably, they are field stars.
Figures
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