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arxiv: 2605.30733 · v1 · pith:HOXF3HW4new · submitted 2026-05-29 · 🌌 astro-ph.SR · astro-ph.GA

Weak-CN Stars Are Ordinary Cool Red Supergiants

Yuan-Sen Ting , Puragra Guhathakurta , Douglas Grion Filho , Evan N. Kirby , Elliot M. Kim This is my paper

Pith reviewed 2026-06-28 21:24 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA
keywords red supergiantsCN absorptionsynthetic spectrastellar atmospheresfirst dredge-upLocal Groupmolecular bandsequivalent widths
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The pith

Weak CN absorption in red supergiants matches predictions from ordinary cool-star models.

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

The paper examines weak-CN absorption near 8000 Å detected in evolved red supergiants across the LMC, M33, and M31. It compares pseudo-continuum equivalent widths from observed coadds against a grid of synthetic RSG atmospheres that vary effective temperature, alpha-element abundance, and surface carbon and nitrogen offsets. The models reproduce the data at the level of measurement uncertainties, identifying the combined surface abundance change in carbon plus nitrogen as the controlling factor. This places the weak-CN stars in a narrow regime cool enough for CN to appear but warm enough that TiO bands remain unsaturated. The result indicates that these features arise from standard first dredge-up in slowly rotating stars rather than any exotic pathway.

Core claim

Ordinary cool-RSG models reproduce the weak-CN coadds across all three hosts, with per-feature residuals at the level of the adopted EW systematic floors. The robust observable is the combined surface abundance Δ[C/H]+Δ[N/H] rather than each offset individually, because CN forms from the product of available C and N number densities. Mapping Δ[C/H]+Δ[N/H] to initial rotation through PARSEC v2.0 has modest leverage, and slow-rotation first dredge-up is consistent with LMC and M33, and with M31 once a single-feature CaT 8542 Å calibration anchor is allowed.

What carries the argument

Self-consistent grid of synthetic RSG atmospheres spanning T_eff, [α/Fe], and surface C and N offsets, evaluated through pseudo-continuum equivalent widths measured at matched resolution.

If this is right

  • The product Δ[C/H]+Δ[N/H] serves as the primary observable constraint rather than separate carbon or nitrogen measurements.
  • Slow initial rotation followed by standard first dredge-up accounts for the surface abundances in all three galaxies examined.
  • Weak-CN features mark a normal molecular-equilibrium stage in cool RSGs instead of a transitional state toward carbon stars.
  • Per-feature agreement within systematic floors validates the synthetic spectra for this molecular regime.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar weak-CN detections in more distant galaxies could be interpreted with the same combined-abundance metric without invoking unusual enrichment.
  • Targeted observations that sample a wider temperature range around the CN-visible window would test the sharpness of the TiO saturation boundary.
  • If the modest rotation leverage holds, population-level CN measurements could statistically constrain average initial spin rates in RSG progenitors.

Load-bearing premise

The synthetic atmosphere grid correctly describes the narrow temperature window where CN appears while TiO remains unsaturated, and the observed coadds reflect typical uncontaminated RSG properties.

What would settle it

New coadded spectra from additional Local Group galaxies at matched effective temperatures that show CN equivalent widths systematically outside the model grid predictions would falsify the ordinary cool-RSG explanation.

Figures

Figures reproduced from arXiv: 2605.30733 by Douglas Grion Filho, Elliot M. Kim, Evan N. Kirby, Puragra Guhathakurta, Yuan-Sen Ting.

Figure 1
Figure 1. Figure 1: Observed weak-CN and carbon-star coadded templates. The top panel shows the three weak-CN coadds on a common pseudo-continuum scale; the 3 × 3 zooms compare each galaxy’s weak-CN coadd (coloured) with its carbon-star coadd (grey) in the three diagnostic windows TiO γ 6149 ˚A, CN+TiO 7050 ˚A, and CN red. The weak-CN class is what the discovery papers noticed: a CN band that has the carbon-star morphology bu… view at source ↗
Figure 2
Figure 2. Figure 2: Atmospheric physics underlying the weak-CN diagnostic. Left: T(τR) for four representative cool-RSG models, with the TiO and CN dissociation transitions marked and a common Rosseland-depth reference band at log τR ≃ −0.4. Right: CN and TiO number densities at the same models on the same depth axis. The figure shows the orthogonal response of the two molecules: as Teff drops through the WCN range, TiO grows… view at source ↗
Figure 3
Figure 3. Figure 3: Where the diagnostic discriminates parameters in the EW plane. Symbols are the observed weak-CN coadds; coloured curves are model iso-Teff tracks computed at each host’s metallicity and [α/Fe], with each plotted point obtained by averaging the model EWs over the grid points whose ∆[C/H] + ∆[N/H] falls within ±0.05 dex of the fixed sum. Dotted curves connect fixed ∆[C/H] + ∆[N/H] across Teff by the same ave… view at source ↗
Figure 4
Figure 4. Figure 4: Observed weak-CN coadds (coloured) compared to the joint best-fit synthetic spectra (black) at the grid points of [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Profile χ 2 in the (∆[C/H], ∆[N/H]) plane, marginalised over Teff and [α/Fe], on the extended grid. White contours: ∆χ 2 = 2.3 (1σ, 2 dof; solid) and 6.17 (2σ; dashed). PARSEC first-dredge-up predictions are shown as a cyan star (ωi = 0, ∆C = −0.17, ∆N = +0.58) and an orange square (ωi = 0.6, ∆N = +0.75). Because the data constrain only the combined offset ∆C + ∆N, we plot the data argmin as a solid red is… view at source ↗
read the original abstract

Weak CN absorption near ~8000 A has recently been detected in evolved red supergiants (RSGs) of 5-10 $M_\odot$ across three Local Group galaxies. These weak-CN RSGs sit in a narrow molecular regime: cool enough for CN to be visible in a non-carbon, C/O<1 atmosphere, but warm enough that TiO is not saturated and changes in $T_{\rm eff}$ and in the surface C+N reservoir move CN and TiO in distinct directions. We test this picture with pseudo-continuum equivalent widths (EWs) measured from LMC, M33, and M31 weak-CN and carbon-star coadds, compared at matched resolution to a self-consistent grid of synthetic RSG atmospheres spanning $T_{\rm eff}$, $[\alpha/{\rm Fe}]$, and surface C and N offsets relative to each host's scaled-solar baseline. Ordinary cool-RSG models reproduce the weak-CN coadds across all three hosts, with per-feature residuals at the level of the adopted EW systematic floors. The robust observable is the combined surface abundance $\Delta$[C/H]+$\Delta$[N/H] rather than each offset individually, because CN forms from the product of available C and N number densities. Mapping $\Delta$[C/H]+$\Delta$[N/H] to initial rotation through PARSEC v2.0 has modest leverage -- the variable shifts by ~0.07 dex from $\omega_i$=0 to $\omega_i$=0.6 -- and within this resolution slow-rotation first dredge-up is consistent with LMC and M33, and with M31 once a single-feature CaT 8542 A calibration anchor is allowed. The straightforward resolution of the discovery puzzle is therefore that weak CN is not an exotic carbon-star intermediate but the expected molecular-equilibrium signature of ordinary cool RSGs.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 3 minor

Summary. The paper claims that weak-CN absorption near 8000 Å in 5-10 M⊙ RSGs across the LMC, M33, and M31 is the expected molecular-equilibrium signature of ordinary cool RSGs (C/O < 1, in the narrow T_eff window where CN is visible but TiO unsaturated), not an exotic carbon-star intermediate. This is demonstrated by matching pseudo-continuum EWs from host-specific weak-CN and carbon-star coadds to a self-consistent grid of synthetic RSG atmospheres varying T_eff, [α/Fe], and surface C/N offsets relative to each galaxy's scaled-solar baseline; residuals lie at the adopted EW systematic floors. The robust observable is the sum Δ[C/H] + Δ[N/H] (due to CN formation depending on the product of C and N densities), which maps via PARSEC v2.0 tracks to initial rotation with only modest leverage (~0.07 dex shift from ω_i = 0 to 0.6), consistent with slow rotation and standard first dredge-up (with a CaT 8542 Å anchor for M31).

Significance. If the result holds, it provides a straightforward resolution to the weak-CN discovery by showing consistency with standard cool-RSG models and molecular equilibrium, without new evolutionary channels. Strengths include the use of coadds for average properties, explicit identification of the sum of C+N offsets as the key observable, and the falsifiable test against an external synthetic grid; this strengthens abundance diagnostics in the CN/TiO regime and modestly constrains rotation via PARSEC tracks.

major comments (1)
  1. [Abstract] The central reproduction claim rests on the synthetic grid accurately capturing the narrow molecular regime (CN visible, TiO unsaturated) and the coadds being uncontaminated averages; while the abstract states residuals match systematic floors, explicit validation of grid construction (opacity treatment, T_eff sampling) against independent standards would be needed to confirm no hidden tuning affects the match.
minor comments (3)
  1. The abstract refers to 'adopted EW systematic floors' without quoting their numerical values or how they were derived; adding a short table or explicit statement of these floors would improve transparency of the residual comparison.
  2. The mapping of Δ[C/H]+Δ[N/H] to initial rotation is presented as secondary with modest leverage; a brief quantitative statement of the ~0.07 dex shift in the main text (rather than only abstract) would clarify its limited constraining power.
  3. Notation for abundance offsets is consistent, but briefly defining the host-specific scaled-solar baselines used for the grid would aid reproducibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive review and recommendation of minor revision. We address the single major comment below and will incorporate additional validation details as requested.

read point-by-point responses

  1. Referee: [Abstract] The central reproduction claim rests on the synthetic grid accurately capturing the narrow molecular regime (CN visible, TiO unsaturated) and the coadds being uncontaminated averages; while the abstract states residuals match systematic floors, explicit validation of grid construction (opacity treatment, T_eff sampling) against independent standards would be needed to confirm no hidden tuning affects the match.

    Authors: We agree that explicit validation of the grid strengthens the central claim. The manuscript already describes the grid in Section 3 as self-consistent synthetic RSG atmospheres with standard molecular opacities and T_eff sampling across the narrow regime (3400-4200 K in 100 K steps). In the revised version we will add a dedicated validation paragraph (or subsection) that (i) specifies the opacity sources (CN and TiO line lists from established databases with no ad-hoc scaling), (ii) confirms the T_eff grid resolves the CN-visible/TiO-unsaturated window, and (iii) compares selected models to independent MARCS atmospheres at matched parameters, showing agreement in CN and TiO band strengths to within the adopted EW floors. The coadd construction and contamination checks are already justified in Section 2 with membership and quality criteria; we will cross-reference this explicitly. These additions will demonstrate the match is not due to hidden tuning. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper derives that ordinary cool-RSG synthetic spectra reproduce the observed weak-CN coadd EWs by comparing pseudo-continuum measurements to an external self-consistent grid spanning T_eff, [α/Fe], and surface C/N offsets; the combined Δ[C/H]+Δ[N/H] is obtained from the molecular-equilibrium product of C and N number densities rather than being fitted directly to the weak-CN data. This comparison is anchored to PARSEC v2.0 tracks (external) and a single CaT calibration (secondary), with no load-bearing self-citation, no parameter fitted to a subset then renamed as prediction, and no ansatz or uniqueness theorem imported from prior author work. The result follows from standard molecular physics applied to an independent model grid and is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of stellar atmosphere modeling and molecular equilibrium; the paper introduces no new entities and uses external evolutionary tracks for the rotation mapping.

free parameters (1)
  • surface C and N abundance offsets
    Offsets relative to each host's scaled-solar baseline are varied in the grid to match observations; the sum is treated as the robust quantity.
axioms (1)
  • domain assumption CN forms from the product of available C and N number densities under molecular equilibrium in C/O < 1 atmospheres
    Invoked to explain why the combined Δ[C/H]+Δ[N/H] is the robust observable rather than individual offsets.

pith-pipeline@v0.9.1-grok · 5893 in / 1169 out tokens · 30168 ms · 2026-06-28T21:24:21.614906+00:00 · methodology

discussion (0)

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Works this paper leans on

25 extracted references · 23 canonical work pages

  1. [1]

    2016, MNRAS, 457, 3611, doi: 10.1093/mnras/stw222

    Bressan, A. 2016, MNRAS, 457, 3611, doi: 10.1093/mnras/stw222

    work page doi:10.1093/mnras/stw222 2016

  2. [2]

    L., Girardi, L., Marigo, P., et al

    Boyer, M. L., Girardi, L., Marigo, P., et al. 2013, ApJ, 774, 83, doi: 10.1088/0004-637X/774/1/83

    work page doi:10.1088/0004-637x/774/1/83 2013

  3. [3]

    Brooke, J. S. A., Ram, R. S., Western, C. M., et al. 2014, ApJS, 210, 23, doi: 10.1088/0067-0049/210/2/23

    work page doi:10.1088/0067-0049/210/2/23 2014

  4. [4]

    2010, A&A, 522, A10, doi: 10.1051/0004-6361/201014432

    Charbonnel, C., & Lagarde, N. 2010, A&A, 522, A10, doi: 10.1051/0004-6361/201014432

    work page doi:10.1051/0004-6361/201014432 2010

  5. [5]

    MNRAS , volume =

    Costa, G., Girardi, L., Bressan, A., et al. 2019, MNRAS, 485, 4641, doi: 10.1093/mnras/stz728

    work page doi:10.1093/mnras/stz728 2019

  6. [6]

    J., Williams, B

    Dalcanton, J. J., Williams, B. F., Lang, D., et al. 2012, ApJS, 200, 18, doi: 10.1088/0067-0049/200/2/18

    work page doi:10.1088/0067-0049/200/2/18 2012

  7. [7]

    2015, ApJ, 806, 21, doi: 10.1088/0004-637X/806/1/21 Grion Filho, D., Guhathakurta, P., Rinehart, S

    Davies, B., Kudritzki, R.-P., Gazak, Z., et al. 2015, ApJ, 806, 21, doi: 10.1088/0004-637X/806/1/21 Grion Filho, D., Guhathakurta, P., Rinehart, S. M., et al. 2025, ApJ, 981, 88, doi: 10.3847/1538-4357/adb030

    work page doi:10.1088/0004-637x/806/1/21 2015

  8. [8]

    R., et al

    Guhathakurta, P., Grion Filho, D., Bhattacharya, A. R., et al. 2025, ApJ, 987, 203, doi: 10.3847/1538-4357/adcc2c

    work page doi:10.3847/1538-4357/adcc2c 2025

  9. [9]

    2008, , 486, 951, 10.1051/0004-6361:200809724

    Gustafsson, B., Edvardsson, B., Eriksson, K., et al. 2008, A&A, 486, 951, doi: 10.1051/0004-6361:200809724

    work page doi:10.1051/0004-6361:200809724 2008

  10. [10]

    L., Guhathakurta, P., et al

    Hamren, K., Beaton, R. L., Guhathakurta, P., et al. 2016, ApJ, 828, 15, doi: 10.3847/0004-637X/828/1/15

    work page doi:10.3847/0004-637x/828/1/15 2016

  11. [11]

    2009, AJ, 138, 1243, doi: 10.1088/0004-6256/138/5/1243

    Harris, J., & Zaritsky, D. 2009, AJ, 138, 1243, doi: 10.1088/0004-6256/138/5/1243

    work page doi:10.1088/0004-6256/138/5/1243 2009

  12. [12]

    M., & Ting, Y.-S

    Kim, E. M., & Ting, Y.-S. 2026, Journal of Open Source Software. https://arxiv.org/abs/2603.11693

    Pith/arXiv arXiv 2026

  13. [13]

    Kurucz, R. L. 2005, Memorie della Societa Astronomica Italiana Supplementi, 8, 14

    2005

  14. [14]

    2012, A&A, 543, A108, doi: 10.1051/0004-6361/201118331

    Lagarde, N., Decressin, T., Charbonnel, C., et al. 2012, A&A, 543, A108, doi: 10.1051/0004-6361/201118331

    work page doi:10.1051/0004-6361/201118331 2012

  15. [15]

    F., Durbin, M

    Lazzarini, M., Williams, B. F., Durbin, M. J., et al. 2022, ApJ, 934, 76, doi: 10.3847/1538-4357/ac7568

    work page doi:10.3847/1538-4357/ac7568 2022

  16. [16]

    M., Massey, P., Olsen, K

    Levesque, E. M., Massey, P., Olsen, K. A. G., et al. 2005, ApJ, 628, 973, doi: 10.1086/430901

    work page doi:10.1086/430901 2005

  17. [17]

    R., Dolphin, A

    Lewis, A. R., Dolphin, A. E., Dalcanton, J. J., et al. 2015, ApJ, 805, 183, doi: 10.1088/0004-637X/805/2/183

    work page doi:10.1088/0004-637x/805/2/183 2015

  18. [18]

    2009, ApJ, 696, 729, doi: 10.1088/0004-637X/696/1/729

    Magrini, L., Stanghellini, L., & Villaver, E. 2009, ApJ, 696, 729, doi: 10.1088/0004-637X/696/1/729

    work page doi:10.1088/0004-637x/696/1/729 2009

  19. [19]

    P., Marino, A

    Milone, A. P., Marino, A. F., D’Antona, F., et al. 2017, MNRAS, 465, 4363, doi: 10.1093/mnras/stw2965

    work page doi:10.1093/mnras/stw2965 2017

  20. [20]

    T., Costa, G., Girardi, L., et al

    Nguyen, C. T., Costa, G., Girardi, L., et al. 2022, A&A, 665, A126, doi: 10.1051/0004-6361/202244166

    work page doi:10.1051/0004-6361/202244166 2022

  21. [21]

    Monthly Notices of the Royal Astronomical Society , author =

    Pastorelli, G., Marigo, P., Girardi, L., et al. 2020, MNRAS, 498, 3283, doi: 10.1093/mnras/staa2565

    work page doi:10.1093/mnras/staa2565 2020

  22. [22]

    Scalo, J. M. 1974, ApJ, 194, 361, doi: 10.1086/153253

    work page doi:10.1086/153253 1974

  23. [23]

    2018, ApJ, 860, 159, doi: 10.3847/1538-4357/aac6c9

    Ting, Y.-S., Conroy, C., Rix, H.-W., & Asplund, M. 2018, ApJ, 860, 159, doi: 10.3847/1538-4357/aac6c9

    work page doi:10.3847/1538-4357/aac6c9 2018

  24. [24]

    F., Durbin, M

    Williams, B. F., Durbin, M. J., Dalcanton, J. J., et al. 2021, ApJS, 253, 53, doi: 10.3847/1538-4365/abdf4e

    work page doi:10.3847/1538-4365/abdf4e 2021

  25. [25]

    B., & Grebel, E

    Zaritsky, D., Harris, J., Thompson, I. B., & Grebel, E. K. 2004, AJ, 128, 1606, doi: 10.1086/423910

    work page doi:10.1086/423910 2004