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Caveats about measuring carbon abundances in stars using the CH band

T0 review · 4 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Carbon abundances measured from the CH 4300 Å band with the GAUGUIN spectrum-synthesis code can differ by up to ~0.8 dex for the same star depending only on which reference synthetic grid is used, even though all internal consistency…

desk verdict The grid-dependence of CH-based [C/Fe] is real and well demonstrated, but the paper overreaches in blaming the synthetic carbon models without ruling out its own grid-tied normalisation. read the letter →

arxiv 2507.11351 v1 pith:CSJJEXID submitted 2025-07-15 astro-ph.GA astro-ph.SR

Pablo Santos-Peral , Patricia Sánchez-Blázquez , Alejandro Vazdekis , Pedro Alonso Palicio , Adam Thomas Knowles , Alejandra Recio-Blanco , Carlos Allende Prieto This is my paper · ORCID
classification astro-ph.GAastro-ph.SR
keywords carbonabundancesCH4300ÅbandspectralsynthesisX-shooterLibrarymodeldependenceGAUGUINstellarG
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper measures carbon-to-iron ratios ([C/Fe]) for about 200 stars of the X-shooter Spectral Library by fitting two CH molecular bands near 4300 Å with the GAUGUIN spectrum-synthesis code. It runs the same stars and the same bands through two different grids of reference synthetic spectra — the theoretical library behind sMILES and the updated BOSZ library — each with the same [C/Fe] coverage. Every internal check looks healthy within each grid: the two CH bands agree, the values show no trend with stellar parameters, Monte Carlo uncertainties are small, and solar and Arcturus fits are good. Yet the same star can receive [C/Fe] values up to ~0.8 dex apart depending only on which grid is adopted, while [Mg/Fe] measured the same way agrees across grids. The paper's central claim is that CH-band carbon abundances from spectral synthesis are strongly model-dependent, so wrong answers can be delivered with high quoted precision and no warning.

What carries the argument

The machinery is GAUGUIN, an automated spectrum-synthesis abundance code, applied to two narrow CH windows at 4301.5–4303.4 Å and 4307.1–4308.8 Å, with a 5-dimensional grid of synthetic spectra in effective temperature, surface gravity, metallicity, alpha-enhancement, and [C/Fe] as the reference. Two such grids are used: the theoretical library computed for sMILES (ATLAS9 atmospheres, ASSET/SYNSPEC, Allende Prieto et al. 2018 line lists) and the updated BOSZ library (MARCS atmospheres, newer SYNSPEC, Masseron et al. 2014 CH line list). Because the CH region is crowded, GAUGUIN defines a pseudo-continuum per grid: it divides the observed spectrum by the interpolated synthetic one, fits the residual with a third-degree polynomial, and fits [C/Fe] relative to that normalisation. The two grids therefore carry different pseudo-continua and differ in atmospheres, microturbulence treatment, molecular opacities, and SYNSPEC version — the small model differences that, the paper argues, expand into the large [C/Fe] excursions.

What would settle it

Re-derive [C/Fe] for the same XSL stars from the same two CH windows while forcing both grids to share one grid-independent continuum (e.g., pseudo-continuum anchored to spectral windows outside CH absorption), and check whether the up-to-0.8 dex offsets persist; alternatively, measure the CI lines at 5052 and 5380 Å at R ≥ 50000 for the same stars and see which grid's CH-based values match.

Watch

Extended reading notes

Core claim

The central discovery is that the reference synthetic grid, not the star, can set the measured carbon abundance. For identical spectral windows, identical stellar parameters, and the same [C/Fe] coverage in the two grids, GAUGUIN returns internally consistent but mutually incompatible catalogues: comparing the [C/Fe] derived with the Knowles et al. (2021) grid to that derived with the Mészáros et al. (2024) BOSZ grid gives a star-to-star scatter of about 0.3 dex and individual discrepancies up to |∆[C/Fe]| ≈ 0.8 dex, with no obvious cause in the fits themselves; the authors do note a decreasing trend with [Fe/H] in the offset. The same comparison for [Mg/Fe] is flat and agrees to about 0.02 dex. The authors conclude that small intrinsic differences between synthetic models in the crowded, blended CH 4300 Å region are amplified by the abundance-estimation procedure and produce large, unnoticed inaccuracies in stellar carbon measurements.

Load-bearing premise

The load-bearing premise — stated in Sect. 4.4 — is that the grid-to-grid [C/Fe] offset is caused by hidden defects in how the synthetic spectra model carbon, and not by the GAUGUIN pseudo-continuum normalisation, which is defined separately from each grid (Sects. 3.2–3.3).

Editorial extensions

If this is right

  • Published [C/Fe] values derived from the CH 4300 Å band with any single synthetic grid may carry unrecognised grid-dependent offsets of order 0.3 dex, with outliers near 0.8 dex.
  • Stellar population models built on empirical libraries whose carbon abundances come from CH bands inherit that model dependence in their carbon-sensitive predictions.
  • The flat, dispersed [C/Fe] versus [Fe/H] trend seen in this work and in earlier CH-band studies is not a secure measurement of the Galactic carbon trend unless the grid dependence is understood.
  • Passing internal quality checks — band-to-band agreement, no parameter trends, small uncertainties — is not sufficient evidence that a CH-based [C/Fe] measurement is accurate.
  • Before CH-band carbon abundances are used as benchmarks, the same stars should be cross-checked with an independent grid or with high-resolution atomic CI lines.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: the grid-to-grid offset may be produced by the pseudo-continuum normalisation itself, since each grid defines its own pseudo-continuum and all internal consistency tests stay inside one grid; if so, the paper's attribution of the offset to carbon modeling is not the only reading.
  • Editorial inference: if carbon opacity is the culprit, the offset should grow where CH features are stronger (cooler stars, lower gravity, higher [C/Fe]); the paper's Fig. 9 suggests a [Fe/H]-dependent offset that could be mapped against CH strength to test this.
  • Editorial inference: measuring the same XSL stars through other carbon molecules, such as C2 or CN, or through the 8727 Å [CI] line at high resolution, would separate a carbon-physics problem from a CH-band crowding problem.
  • Editorial inference: a simple decisive test is to fit both grids with a common, grid-independent continuum; the discrepancy should collapse if normalisation is responsible and persist if it is not.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The paper measures [C/Fe] abundances for ~200 stars of the X-shooter Spectral Library from two CH bands near 4300 Å using the GAUGUIN spectral-synthesis code, adopting two different 5D reference synthetic grids (Knowles et al. 2021 and the updated BOSZ/MARCS library of Mészáros et al. 2024). Within each grid the results appear precise and internally consistent: band-to-band agreement of ~0.01–0.03 dex, small Monte Carlo parameter uncertainties (Table 2), no trends with Teff or log g (Fig. 4), and a clean [Mg/Fe] control test (Appendix B). However, the two grids give systematically offset [C/Fe] values for the same stars, up to |Δ[C/Fe]| ~ 0.8 dex, with the solar spectrum yielding +0.12 (Knowles+21) versus −0.52 (BOSZ) and Arcturus +0.28 versus +0.46. The paper's central conclusion is that this offset reflects hidden problems in the carbon modeling of the crowded CH 4300 Å region, so that CH-based carbon measurements can be inaccurate without being detected by internal quality tests.

Significance. If the result holds, the paper delivers a genuinely useful warning to the stellar-population community: XSL is a benchmark empirical library, carbon is a key missing ingredient in population models, and a demonstrated 0.6–0.8 dex grid-dependence of CH-band [C/Fe] that is invisible to standard internal checks would explain a large part of the scatter among literature carbon abundances. The comparative design is a real strength: the [Mg/Fe] control (Appendix B) is clean (|Δ[Mg/Fe]| ~ 0.02 dex between grids), the internal precision tests (Table 2; Figs. 5 and 8) are convincing, and the authors honestly state in Sect. 4.3 that no objective criterion decides which grid is more accurate. The main gap is that precision is repeatedly presented as if it implied accuracy: the solar and Arcturus benchmarks favour different grids, and the only atomic-line anchor (Fig. 12) was run with a single grid. With the causal attribution either tested or weakened to an explicit model-dependence warning, the revised paper would be a valuable cautionary contribution.

major comments (4)
  1. [§4.4 and §5, with §3.2] The conclusion that the grid-to-grid offset is caused by "hidden issues in the carbon modeling" of the CH 4300 Å region is not established, because GAUGUIN's normalisation step is reference-grid dependent: as described in Sect. 3.2, the observed spectrum is divided by a polynomial fit to the Synthetic/Observed residual computed against the reference grid, so broad-band differences between the grids (line wings, pseudo-continuum depression in this crowded region) can be absorbed into each grid's own continuum zero-point. All internal validation tests (band-to-band agreement, Monte Carlo parameter uncertainties, absence of Teff/log g trends, and the Appendix B [Mg/Fe] control) are performed within a single grid or on a region with a cleaner continuum; they establish precision, not zero-point accuracy. Two facts in the paper point directly to this ambiguity: the Sun favours Knowles+21 (+0.12 vs. −0.52) while Arcturus favours BOSZ (+0.46 vs. +0.28, against a literature value of +0.43), which is inconsistent with a single grid-wide carbon-model error; and the high-resolution CI-line anchor (Fig. 12) was computed only with the Knowles+21 grid, so it cannot show whether the BOSZ offset is specific to CH or common to all carbon lines. To support the stated conclusion the authors should either weaken it to a model-dependence warning or add a test that isolates the synthesis models from the normalisation, e.g., fitting synthetic spectra drawn from one grid after normalising them with the other grid's residual polynomial.
  2. [§3.1.3, item 3] The manuscript describes the BOSZ grid as providing spectra at four fixed microturbulence values (0, 1, 2, and 4 km/s) but never states which value was adopted for the 5D subset ingested into GAUGUIN. Given that the paper itself notes (Sect. 3.1.3) that microturbulence can affect line strengths in this region by 1–2 Å, an unspecified vmic is a free parameter that could plausibly contribute to the measured offsets, particularly for the cool giants that dominate the sample. The adopted value must be stated explicitly, and a sensitivity test (e.g., re-fitting a subsample with the 1 vs. 2 km/s grids) is needed before the offset can be attributed specifically to carbon modeling rather than to an input-parameter mismatch.
  3. [§4.4 and Fig. 9 (right panel)] The statement that the two grids produce "different and unpredictable [C/Fe] abundance results for the same star ... with no apparent reason" is contradicted by the paper's own Fig. 9: the grid-to-grid difference Δ[C/Fe] (BOSZ − Knowles+21) shows a coherent dependence on the Knowles-grid value, with the most negative Knowles values receiving the largest positive BOSZ offsets. A structured, monotonic pattern of this kind is a testable signature (a zero-point shift, a scale factor, or a parameter-dependent mapping), and characterising it would help discriminate between candidate causes such as normalisation versus synthesis differences. The text describing this panel ("a decreasing trend with [Fe/H]") also mismatches its axes (x-axis: Knowles [C/Fe]; colour: [Fe/H]) and should be corrected.
  4. [Abstract and §4.1] The catalogue is described as "large and precise unbiased" and the conclusions state that the method "leads to inaccurate [C/Fe] abundance estimates ... without significantly affecting the measured high-quality precision." The evidence supports parameter-unbiasedness (no trends with Teff or log g, Fig. 4) and internal precision, but it does not support absolute accuracy: the solar and Arcturus offsets in Figs. 2, 7, A.1, and A.2 leave the zero-point unanchored, and Sect. 4.3 concedes that no objective criterion selects one grid. Since a 0.6–0.8 dex zero-point ambiguity is exactly what users of the catalogue need to know, the abstract and conclusions should carry an explicit grid-dependence caveat rather than the unqualified word "unbiased."
minor comments (6)
  1. [§4.2 and Fig. 9] State the overlap sample size N for the grid-to-grid comparison, and confirm that the |Δ[C/Fe]| ~ 0.8 dex cases lie within the Teff coverage common to both grids (Knowles+21 covers 3500–6000 K, BOSZ 3500–6500 K); the two final catalogues in Figs. 3 and 9 have different Teff extents, so the intersection is not obvious.
  2. [§4.1] Explain why extending the [C/M] range of the Knowles+21 grid from [−0.25, +0.25] to [−0.75, +0.5] reduces the number of stars passing the quality criteria from 199 to 176.
  3. [§3.2, §4.2, and §5] The repeated sentence "The effect of the hydrogen atom seems to be negligible since [C/Fe] variations can be reproduced and measured at a given [Fe/H]" is unclear, because hydrogen is not a varied dimension in either grid; please reword or remove it.
  4. [§5] Provide a machine-readable table or access link for the final [C/Fe] catalogue; the conclusions present it as a deliverable, but the manuscript contains no table or data URL.
  5. [Figs. 5 and 8] Report the number of stars used in every panel of the band-to-band comparisons; N=151 and N=143 currently appear only in the first panels.
  6. [§3.1.3] The closing sentence "Therefore, we did not find significant discrepancies among the models for the studied stellar sample" directly contradicts the central result of the paper (offsets up to |Δ[C/Fe]| ~ 0.8 dex in Sect. 4.2); as written it appears to refer only to a visual comparison of synthetic spectra and should be reworded to avoid the appearance of internal inconsistency.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the grid-to-grid [C/Fe] offset is an internal comparison, not a fit renamed as a prediction.

full rationale

The central claim is that identical stars yield different [C/Fe] values (up to |∆[C/Fe]| ~ 0.8 dex) when the same GAUGUIN fitting procedure is run against two independent synthetic grids (Knowles et al. 2021 and BOSZ/Mészáros et al. 2024). This is a difference of two independent measurements, not a prediction derived from a fitted parameter. The grids are constructed with the same [C/Fe] abundance coverage and the same [M/H]=[Fe/H], [C/M]=[C/Fe] convention, so the comparison is not self-definitional: the outcome of the comparison (agreement for [Mg/Fe], disagreement for [C/Fe]) is not imposed by the definitions. The paper does not fit an offset and then present it as a result; instead it reports and quantifies the offset as the finding. The grid-dependent pseudo-continuum normalisation (Sect. 3.2, based on Santos-Peral et al. 2020) is a plausible alternative explanation for the offset, and the internal tests (band-to-band agreement, Monte Carlo uncertainties, solar and Arcturus fits) cannot fully separate that pipeline effect from genuine carbon-model differences. However, that is a correctness or robustness concern about the interpretation, not circularity: the conclusion is not equivalent to its inputs by construction. The self-citations to Santos-Peral et al. (2020, 2023) describe the GAUGUIN methodology and sample selection; they are standard method citations rather than load-bearing appeals to an unverified uniqueness result. External benchmarks are also present: the solar CI-line checks, Arcturus, the [Mg/Fe] control, and comparisons with APOGEE DR17 and other literature catalogues. The paper is candid that it cannot decide which grid is more accurate, which is the opposite of forcing a conclusion through a circular chain. No quoted reduction of Eq. X to Eq. Y, and no fitted parameter renamed as a prediction, can be exhibited. The correct circularity verdict is therefore no significant circularity.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The paper introduces no new physical entities. Its free parameters are methodological choices: the CH band selection, the normalisation window, and the extension of the grid's [C/M] dimension. The main axiomatic burden is that the fitting-and-normalisation method is not responsible for the grid offsets, and that the two grids differ only in legitimate physical input. The paper honestly flags that it cannot decide which grid is accurate, which is the main source of uncertainty.

free parameters (2)
  • CH band selection and normalisation windows = 4301.5-4303.4 A, 4307.1-4308.8 A; normalisation window chosen by hand
    The specific band limits are adapted from Suarez-Andres et al. (2017) to X-shooter resolution, and the normalisation window was chosen by hand after testing. Different windows change the results, so this is a methodological choice affecting the measurement, not a fitted physical parameter.
  • Grid range extension for [C/M] = Extended from -0.25..+0.25 dex to -0.75..+0.5 dex
    The sMILES grid was originally limited to [C/M] = -0.25..+0.25; the authors extended it to cover a wider range. This choice changes the sample of stars with high-quality measurements (199 to 176 stars) and influences the observed trend at low [C/Fe], so it is a freedom affecting the catalog.
assumptions (5)
  • domain assumption The XSL atmospheric parameters (Teff, logg, [Fe/H]) from Arentsen et al. (2019) and [alpha/Fe] from Santos-Peral et al. (2023) are accurate enough for abundance fitting.
    These parameters are fed into GAUGUIN as fixed inputs (Sect. 3). If they are biased, the derived [C/Fe] would be biased even though Monte Carlo uncertainties around them are small.
  • domain assumption The GAUGUIN pseudo-continuum normalisation procedure does not absorb real CH line-strength variations into the fitted continuum.
    Sect. 3.2 describes fitting a third-degree polynomial to a sigma-clipped residual over the line window. The paper notes the continuum is a pseudo-continuum in this crowded region. The claimed caveat that carbon modeling is the source of grid differences assumes the normalisation is not the culprit.
  • domain assumption Both synthetic grids are reliable representations of the true stellar spectra except for carbon-specific line data.
    The grids are treated as validated references, based on their agreement for Mg (Appendix B) and on prior literature. The paper states that the small differences in the synthetic models over the crowded CH region induce the disparity, which assumes the models differ mainly in carbon-related opacities.
  • standard math 1D LTE synthesis is adequate for the CH 4300 A region in the parameter range of the sample.
    The measured CH features are interpreted under LTE with the cited literature (Alexeeva & Mashonkina 2015) claiming minor 3D NLTE effects. The paper does not test this.
  • domain assumption The external stellar parameters used for the Sun and Arcturus benchmarks are correct.
    The solar fits use the Vesta/HARPS spectrum and the Arcturus fits use Ramírez & Allende Prieto (2011) parameters. These benchmarks are used to discuss solar offsets and grid differences, but only two stars are checked.

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Cite this review

Pith. "Pith review of Caveats about measuring carbon abundances in stars using the CH band." pith.science (2026). https://pith.science/paper/CSJJEXID

@misc{pith2026250711351,
  author       = {Pith},
  title        = {Pith review of: Caveats about measuring carbon abundances in stars using the CH band},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CSJJEXID}},
  note         = {Machine review of arXiv:2507.11351}
}
abstract

Deriving accurate carbon abundance estimates for a wide variety of stars is still complex due to the difficulties in properly measuring it from atomic and molecular lines. The aim of this paper is to analyse the carbon abundance determination for the large empirical X-shooter Spectral Library (XSL), commonly used as a benchmark for the development of stellar population models. The analysis was performed over strong molecular CH bands in the G-band region. We used the GAUGUIN automated spectrum synthesis code, and adopted two different grids of reference synthetic spectra separately, each with the same [C/Fe] abundance coverage. We carried out a detailed comparison between both grids to evaluate the accuracy and the model dependence of the measured [C/Fe] abundances. We obtained a large and precise unbiased [C/Fe] abundance catalogue from both theoretical grids, well distributed in the Hertzsprung-Russell (HR) diagram and with no trend with the stellar parameters. We also measured compatible values from each independent CH band, with a high-quality [C/Fe] abundance estimate for both dwarfs and giants indistinctly. We observed a dispersed flat trend around [C/Fe] = 0.0 dex all along the metallicity regime, in agreement with some literature studies. However, we reported variations up to 0.8 dex in the [C/Fe] composition of the star depending on the adopted grid. We did not find such differences in the $\alpha$-element measurements. This behaviour implies a strong model dependence in the [C/Fe] abundance estimate. Potential sources of error could be associated with the use of spectral synthesis methods to derive stellar carbon abundances in the CH4300A band. Intrinsic small differences in the synthetic models over this crowded and blended region may induce a large disparity in the precise abundance estimate for any stellar type, leading to inaccurate carbon measurements without being noticed

Figures

Figures reproduced from arXiv: 2507.11351 by the authors.

Figure 1
Figure 1. Synthetic Solar spectrum in the CH bands region around ∼4300 Å at the X-shooter resolution (R = 10000), from both employed reference grids: the one computed by Knowles et al. (2021, blue), and the updated BOSZ library (orange). The selected CH bands (see Ta￾ble 1), where the [C/Fe] abundance is measured, are delimited by the black dashed vertical lines. The flux difference at each wavelength be￾tween both synthetic … view at source ↗
Figure 2
Figure 2. Example of the fit carried out by the spectrum synthesis code GAUGUIN for the two CH analysed bands (showed in [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Top row: Stellar abundance ratios [C/Fe] vs. [Fe/H] of the X-shooter catalogue after applying the optimal methodology, with the original synthetic grid from Knowles et al. (2021) (top-left) and with the extended [C/Fe] dimension (top-right), with the estimated internal uncertainties as vertical error bars. Each red point and error bar corresponds to the measured [C/Fe] average and scatter in metallicity bins of 0.5 … view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4: Derived [C/Fe] abundance ratio from each analysed CH band separately (left and right columns, shown in [PITH_FULL_IMAGE:figures/full_fig_p007_4.png]
Figure 5
Figure 5. Figure 5: Comparison between the derived [C/Fe] abundances from the individual analysed CH bands, colour-coded according to the effective temperature (left), surface gravity (middle), and metallicity (right) of the star. The black dashed line reproduces the 1:1 relation. The mea…
Figure 6
Figure 6. Figure 6: [C/Fe] vs. [Fe/H] with the extended synthetic grid from Knowles et al. (2021), only those cases with high signal-to-noise (S/N > 50, top-left), hot stars (Teff > 5000 K, top-right), dwarfs (log(g) > 3.5 cm s−2 , bottom-left), and cool-giant stars (Teff < 4500 K and log…
Figure 7
Figure 7. Figure 7: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_7.png]
Figure 8
Figure 8. Figure 8: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_8.png]
Figure 9
Figure 9. Figure 9: Left: Obtained abundance ratios [C/Fe] vs. [Fe/H] of the X-shooter catalogue with the BOSZ synthetic spectra grid (Mészáros et al. 2024). The red points are defined as in [PITH_FULL_IMAGE:figures/full_fig_p009_9.png]
Figure 10
Figure 10. Figure 10: [C/Fe] vs. [Fe/H] abundance ratios of previous literature studies with different carbon lines selection, as indicated on the top. −0.4 −0.2 0.0 0.2 0.4 [C/Fe] - Literature (dex) −0.4 −0.2 0.0 0.2 0.4 [C/Fe] - Knowles et al. 2021 (dex) µ = -0.04 dex σ = 0.27 dex median…
Figure 11
Figure 11. Figure 11: Direct comparison between the derived stellar abundance ratio [C/Fe] in this work, using the synthetic grid from Knowles et al. (2021) (left) and the BOSZ library (Mészáros et al. 2024, right), and the abundance estimate for stars in common within the literature, with…
Figure 12
Figure 12. Figure 12 [PITH_FULL_IMAGE:figures/full_fig_p011_12.png]

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