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arxiv: 2606.11006 · v1 · pith:KKKPNDI5new · submitted 2026-06-09 · 🌌 astro-ph.SR · astro-ph.GA

Spectroscopic analysis of RGB stars in nine open clusters

Saulo de Oliveira Cantanh\^ede , Alan Alves-Brito , Rodolfo Smiljanic , Beatriz Barbuy , Nad\`ege Lagarde , Corinne Charbonnel , Pierre North This is my paper

Pith reviewed 2026-06-27 11:47 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA
keywords open clustersred giant starsCNO abundancesisotopic ratiosthermohaline mixingstellar evolutionspectroscopy
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The pith

Spectroscopic measurements of carbon and oxygen isotopes in red giant stars from nine open clusters support the predicted mass dependence of thermohaline mixing on the red giant branch.

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

The paper measures CNO abundances and the isotopic ratios 12C/13C, 16O/17O and 16O/18O in 22 K-type giants across nine open clusters using high-resolution visible and near-infrared spectra. These data are compared with predictions from stellar evolution models that include extra-mixing processes. The observed patterns match the expected signatures of thermohaline mixing operating on the red giant branch and rotation-induced mixing on the main sequence, both showing the anticipated dependence on stellar mass. The work also notes that the spread in oxygen isotopic ratios may require adjustments to the assumed initial abundances of 17O and 18O. A sympathetic reader would care because these ratios serve as direct tracers of internal mixing that standard models otherwise underpredict, affecting how we interpret the chemical evolution of stars and the Galaxy.

Core claim

The values of radial velocities and chemical abundances match literature values for the clusters. The results obtained in both the visible and infrared spectral regions support the occurrence and predicted mass dependence of thermohaline mixing on the red giant branch together with rotation-induced mixing on the main sequence. Variations of the initial abundances of 17O and 18O may be required to account for the observed dispersion in the oxygen isotopic ratios among the red giant stars.

What carries the argument

High-resolution spectra yielding CNO abundances and the isotopic ratios 12C/13C, 16O/17O and 16O/18O, analysed with Turbospectrum and MOOG and compared to model predictions that incorporate thermohaline and rotation-induced mixing.

If this is right

  • Thermohaline mixing must operate on the red giant branch with a strength that increases with stellar mass.
  • Rotation-induced mixing must operate on the main sequence and leave a detectable imprint on the surface abundances of red giants.
  • Standard initial 17O and 18O abundances may need revision to reproduce the full range of observed oxygen isotopic ratios.
  • Extra mixing beyond first dredge-up is required in stellar models to match observations in the 1–2 solar-mass range.

Where Pith is reading between the lines

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

  • If the mass dependence holds, the same mixing physics should produce observable differences in the carbon and oxygen isotope ratios of field red giants at different metallicities.
  • The need for adjusted initial oxygen isotopes points to possible revisions in the yields from asymptotic giant branch stars or supernovae that set the starting composition of open clusters.
  • Extending the same analysis to clusters of different ages would test whether the mixing signatures accumulate predictably with time on the red giant branch.

Load-bearing premise

The isotopic ratios derived from the spectra can be compared directly to the mixing model predictions without systematic offsets arising from the choice of model atmospheres or line lists.

What would settle it

New high-resolution spectra of a comparable sample of red giants in open clusters that show no mass-dependent trend in the 12C/13C or oxygen isotopic ratios would falsify the central claim.

Figures

Figures reproduced from arXiv: 2606.11006 by Alan Alves-Brito, Beatriz Barbuy, Corinne Charbonnel, Nad\`ege Lagarde, Pierre North, Rodolfo Smiljanic, Saulo de Oliveira Cantanh\^ede.

Figure 1
Figure 1. Figure 1: Isochrone fitting using PARSEC (Bressan et al. 2012) isochrone to the CMD of the NGC188 cluster, with data from (Stetson et al. 2004), used to determine colour excess 𝐸 (𝐵 − 𝑉), distance modulus (𝑚 − 𝑀)𝑉 , age, [Fe/H], turn-off mass 𝑀TO, and the evolutionary stage of the stars. Filled squares indicate cluster members, while open squares indicate non-members. 3.2 Cluster parameters and evolutionary stages W… view at source ↗
Figure 2
Figure 2. Figure 2: Diagram with steps evolved in the transformations between photometric systems. spectra with relatively low S/N (∼ 15−42, see [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Top: iron abundance [Fe I/H] versus excitation potential 𝜒 of iron lines of Fe I (to determine the spectroscopic effective temperature when the trend of the graphic is null). Middle: iron abundance versusthe reduced equiv￾alent width log(𝐸𝑊/𝜆) (to determine the microturbulent velocity when the trend of the graphic is null). Bottom: iron abundance of Fe I (red dots) and Fe II (blue dots) versus the reduced … view at source ↗
Figure 4
Figure 4. Figure 4: Observed (black dots) and synthetic (blue, red, and green dashed lines) spectra used to determine the projected rotational velocity 𝑣 sin 𝑖 (top) and macroturbulence velocity 𝜁 (bottom) for object 8 (NGC188-2187). The best fit curve is the red one [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comparison between the observed (black dots) and synthetic (blue, red, and green lines) spectra in the regions considered for spectral synthesis of CNO and 12C/13C isotopic ratio. The best fit curve is the red one. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 46620 46640 46660 46680 46700 46720 ID 18 [C/Fe] = −0.25 : [N/Fe] = +0.34 [O/Fe] = −0.21 : 12C/13C = 12 Teff = 4905 K : log g = 2.20 dex [Fe/H] = +0.00 dex : ξ … view at source ↗
Figure 6
Figure 6. Figure 6: Spectral synthesis used to obtain 16O/17O and 16O/18O isotopic ratios. Black dots correspond to the observed spectrum, and coloured lines correspond to synthetic spectra. The best fit curve is the red one. Some lines of C17O and C18O used in the synthesis are labelled. MNRAS 000, 1–22 (2026) [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Top: Comparison between the results of heliocentric radial velocity Object 𝑉ℎ obtained in this work with the literature 𝑉ℎ,𝑙𝑖𝑡 for all the objects (see Table A1 in Appendix A for references). Bottom: Results of the heliocentric radial velocity for the NGC188 cluster. The mean velocity of the NGC188 cluster is −42.61 ± 1.23 km s−1 , ignoring in the calculation the radial velocity of binary stars (objects 3 … view at source ↗
Figure 8
Figure 8. Figure 8: Projected rotational velocity Photometric temperature (K) 𝑣 sin 𝑖 of RGB stars as a function of photometric temperature. Red symbols indicate stars analysed in this work, and gray symbols indicate stars from the sample of Carlberg et al. (2011). Numeric values can be seen in [PITH_FULL_IMAGE:figures/full_fig_p015_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Abundance ratio [X/Fe] as a function of metallicity for our NGC188 sample stars (red symbols) using their individual [Fe/H] spectroscopic values. [Fe/H] Yellow symbols represent giant field stars of Luck & Heiter (2007), green symbols represent clump giants of Mishenina et al. (2006, 2007) and black symbols represents the mean abundances of open clusters available in the literature (Gratton & Contarini 199… view at source ↗
Figure 10
Figure 10. Figure 10: 12C/13C isotopic ratio as a function of abundance ratio [N/C] (left) and turn-off mass (right), compared with open clusters data from the literature (Smiljanic et al. 2009; Mikolaitis et al. 2010, 2011a,b, 2012; Böcek Topcu et al. 2015, 2016; Drazdauskas et al. 2016a,b; Bagdonas et al. 2018; da Silveira et al. 2018; Peña Suárez et al. 2018; Szigeti et al. 2018; Martinez et al. 2020) and models of Charbonn… view at source ↗
Figure 11
Figure 11. Figure 11: 16O/17O (left) and 16O/18O (right) isotopic ratios as function of turn-off mass for analysed stars (blue symbols), compared with literature data. Green symbols represent giant field starts (Harris & Lambert 1984b; Harris et al. 1985, 1987, 1988; Tsuji 2008; Abia et al. 2012, all the lower limits for 16O/17O are from Tsuji 2008) and pink symbols represent open cluster members (Lebzelter et al. 2015). Liter… view at source ↗
Figure 12
Figure 12. Figure 12: 16O/17O (left) and 16O/18O (right) as a function of 12C/13C isotopic ratio for analysed stars (blue symbols), compared with literature data. Green symbols represent giant field starts (Harris & Lambert 1984b; Harris et al. 1985, 1987, 1988; Tsuji 2008; Abia et al. 2012) and pink symbols represent open cluster members (Lebzelter et al. 2015). Literature models presented in [PITH_FULL_IMAGE:figures/full_fi… view at source ↗
read the original abstract

Stellar clusters are crucial tools for studying the age, spatial distribution, dynamics, kinematics, and chemical composition of different Galactic stellar populations. In this work, we used red giant stars from open clusters to better understand the extra-mixing process through the CNO abundances and $^{12}$C/$^{13}$C, $^{16}$O/$^{17}$O and $^{16}$O/$^{18}$O isotopic ratios determined using high-quality spectra in the visible and near-infrared regions. We analysed the radial velocities and chemical composition of 22 K-type giant stars from nine open clusters (NGC188, NGC2682, NGC3680, NGC5822, IC4756, NGC6633, NGC3532, NGC6281, and NGC5460). High-resolution and high signal-to-noise spectra of stars in the NGC188 cluster were obtained with the ESPaDOnS spectrograph at the CFHT in the visible region. The stars in the other clusters were observed with the CRIRES spectrograph at the VLT. We used IRAF to compute radial velocities and Turbospectrum and MOOG for the chemical analysis. The values obtained for the radial velocities and abundances of the sample are similar to those found in the literature. The results in the visible and infrared support the occurrence and predicted mass dependence of thermohaline mixing on the red giant branch and of rotation-induced mixing on the main sequence. Variations of the initial abundances of $^{17}$O and $^{18}$O may be needed to explain the dispersion of the oxygen isotopic ratios in red giant stars.

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 / 1 minor

Summary. The manuscript analyzes high-resolution visible (ESPaDOnS) and near-IR (CRIRES) spectra of 22 K-type RGB stars across nine open clusters. Radial velocities are derived with IRAF; CNO abundances and the isotopic ratios 12C/13C, 16O/17O and 16O/18O are obtained with Turbospectrum and MOOG. The derived quantities are stated to be consistent with literature values. The authors conclude that the isotopic ratios confirm the occurrence and mass dependence of thermohaline mixing on the RGB and rotation-induced mixing on the main sequence; dispersion in the oxygen ratios is ascribed to possible variations in initial abundances.

Significance. If the isotopic ratios prove robust, the work supplies cluster-based constraints on extra-mixing mechanisms at known ages and masses, directly testing stellar-evolution predictions for thermohaline and rotational mixing. The use of both optical and IR diagnostics on the same stars is a strength, and the agreement with published radial velocities and abundances supports the basic reduction pipeline.

major comments (1)
  1. [Chemical analysis / abstract] Abstract and chemical-analysis description: the central claim that the measured isotopic ratios support the predicted mass dependence of thermohaline and rotation-induced mixing requires that the ratios can be compared at face value to model predictions. No systematic error budget, alternative line-list tests, NLTE corrections, or model-atmosphere grid swaps are reported for the isotopic ratios, so it is unclear whether the observed trends survive plausible changes in atomic data or atmospheric assumptions.
minor comments (1)
  1. [Abstract] The abstract asserts that the derived values 'are similar to those found in the literature' but supplies neither quantitative offsets nor the specific literature references used for the comparison.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. The major concern about the lack of a systematic error budget for the isotopic ratios is addressed point-by-point below. We agree that additional discussion is needed to support direct comparison with model predictions and will revise the manuscript accordingly.

read point-by-point responses

  1. Referee: [Chemical analysis / abstract] Abstract and chemical-analysis description: the central claim that the measured isotopic ratios support the predicted mass dependence of thermohaline and rotation-induced mixing requires that the ratios can be compared at face value to model predictions. No systematic error budget, alternative line-list tests, NLTE corrections, or model-atmosphere grid swaps are reported for the isotopic ratios, so it is unclear whether the observed trends survive plausible changes in atomic data or atmospheric assumptions.

    Authors: We agree that an explicit systematic error analysis would strengthen the comparison of our isotopic ratios to stellar evolution models. The submitted manuscript reports consistency with literature values for the same clusters and internal agreement between optical and near-IR diagnostics, but does not include the requested tests. In the revised version we will add a dedicated error-analysis subsection that (i) quantifies sensitivity of the 12C/13C, 16O/17O and 16O/18O ratios to the adopted line lists, (ii) discusses the expected magnitude of NLTE corrections for K-giant oxygen isotopes based on published calculations, and (iii) reports the effect of swapping between the two model-atmosphere grids already employed (MARCS and ATLAS). These additions will allow a clearer assessment of whether the observed mass-dependent trends remain robust. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational abundance analysis with external model comparisons

full rationale

The paper reports radial velocities and CNO abundances plus isotopic ratios derived from ESPaDOnS and CRIRES spectra using standard tools (IRAF, Turbospectrum, MOOG). These observed quantities are then compared to literature values and to pre-existing model predictions for thermohaline and rotation-induced mixing. No equations, fitted parameters, or self-citations are used to derive the target mixing predictions inside the paper; the central claim is a direct empirical comparison. The analysis contains no self-definitional loops, fitted-input predictions, or load-bearing self-citations that reduce the result to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard assumptions of local thermodynamic equilibrium in stellar atmospheres and on mixing prescriptions taken from prior stellar-evolution calculations; no new free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Local thermodynamic equilibrium holds for the spectral lines used in the abundance analysis
    Implicit in the use of Turbospectrum and MOOG for giant-star spectra
  • domain assumption Cluster ages and turn-off masses are known sufficiently well to test the predicted mass dependence of mixing
    Required to interpret the observed trends with mass

pith-pipeline@v0.9.1-grok · 5848 in / 1252 out tokens · 17351 ms · 2026-06-27T11:47:28.350782+00:00 · methodology

discussion (0)

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