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REVIEW 4 major objections 6 minor 78 references

NLTE abundances of Eu for a sample of metal-poor stars in the Galactic Halo and Metal-poor Disk with 1D and <3D> models

T0 review · 4 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash

Pith's one-line read This paper claims that accounting for non-LTE effects shifts europium abundances in metal-poor stars by small, line-dependent amounts, and that the corrected [Eu/Fe] trend still fits Galactic chemical evolution models with only a marginal…

desk verdict Useful NLTE Eu dataset for metal-poor stars, but the GCE conclusion needs a per-line treatment and the abstract's solar numbers don't match Table 2. read the letter →

arxiv 2412.06277 v2 pith:62FGDEA2 submitted 2024-12-09 astro-ph.SR astro-ph.GA

classification astro-ph.SRastro-ph.GA
keywords europiumabundancesNLTElineformationmetal-poorstarsGalacticchemicalevolutionr-processneutronstarmergersmagneto-rotatingsupernovaemodelatmospheres
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

Europium is the cleanest tracer of rapid neutron-capture (r-process) nucleosynthesis, but its measured abundance depends on how the spectral lines are modeled. This paper applies non-local thermodynamic equilibrium (NLTE) line formation, with both 1D and averaged-3D model atmospheres, to the two main Eu II lines in the Sun and 164 metal-poor halo and disk stars with [Fe/H] from -2.4 to -0.5. It finds that NLTE corrections are small and line-dependent: positive for the 4129 Å line and negative or nearly zero for the 6645 Å line, with magnitudes up to about 0.1 dex in the most metal-poor giants. The corrections make the two lines agree better, and when the corrected [Eu/Fe] trend is compared with Galactic chemical evolution models, the required fraction of magneto-rotating supernovae among core-collapse supernovae rises only from 0.01% to 0.013-0.015%. The paper's central claim is that NLTE treatment of Eu does not overturn the standard picture in which neutron-star mergers plus a small magneto-rotating supernova component explain the europium enrichment of the Galaxy.

What carries the argument

The load-bearing object is the Eu model atom taken from Storm et al. (2024): 662 energy levels (498 of Eu I, 163 of Eu II), three ionization stages closed by Eu III, with collision rates, photoionization cross-sections, and hyperfine structure. Departure coefficients $b_i = n_i^{\rm NLTE}/n_i^{\rm LTE}$ are computed with the MULTI1D code for grids of 1D MARCS and <3D> Stagger atmospheres, and Turbospectrum/TSFitPy uses them to synthesize NLTE line profiles. The mechanism that fixes the sign of the correction is the ratio $b_{\rm upper}/b_{\rm lower}$ at the line-formation height: when it exceeds unity the line source function beats the Planck function and the line weakens, producing a positive abundance correction, while when the upper and lower departure coefficients converge the enhanced line opacity strengthens the line, producing a negative correction. For the 6645 Å line the solar atmosphere falls in the first regime and a metal-poor giant ($T_{\rm eff}=4500$ K, $\log g=2.0$, [Fe/H] $=-1$) falls in the second, which is why the NLTE corrections point in opposite directions.

What would settle it

Recompute the departure coefficients for the Eu II 6645 Å line in a metal-poor giant (for example, $T_{\rm eff}=4500$ K, $\log g=1.5$ dex, [Fe/H] $=-2$) with a model atom whose electron-impact excitation rates are varied by a factor of two; if the NLTE correction changes sign or exceeds about 0.1 dex, the paper's conclusion that NLTE corrections leave the GCE parameters essentially unchanged would not hold. Alternatively, measure Eu in the same stars from an independent Eu II line or from Eu III and check whether the two standard lines still converge after the adopted corrections are applied.

Watch

Extended reading notes

Core claim

On its own terms, the paper establishes that the NLTE corrections for the two defining Eu II lines are opposite in sign, positive for the resonance line at 4129 Å and negative or near-zero for the 6645 Å line, and that applying them brings the two line-based abundances into closer agreement. In the solar spectrum, the 1D NLTE abundance is 0.61-0.62 dex from 4129 Å and 0.55-0.58 dex from 6645 Å, while the <3D> NLTE values are 0.64 dex and 0.58-0.60 dex respectively; the LTE values straddle these. In the metal-poor sample the corrections grow toward low metallicity, reaching about -0.1 dex for the 6645 Å line in red giants at [Fe/H] near -2, yet the resulting [Eu/Fe] versus [Fe/H] trend stays essentially flat in the metal-poor regime. Comparing that trend with OMEGA+ Galactic chemical evolution models shows that only a marginal increase in the magneto-rotating supernova fraction $f_{\rm MRSN}$ from 0.01% to 0.013-0.015% is needed to match the NLTE-corrected data. The paper concludes that the amount of NLTE correction does not require significant changes to the parameters of europium production in Galactic chemical evolution models.

Load-bearing premise

The paper adopts the 662-level europium model atom of Storm et al. (2024) as-is, including its collision rates, photoionization cross-sections, and hyperfine data, and tests it only against solar spectra; if those atomic rates are wrong for metal-poor FGK stars, the derived NLTE corrections and the Galactic chemical evolution conclusion would change.

Editorial extensions

If this is right

  • NLTE corrections reduce the line-to-line discrepancy between the 4129 Å and 6645 Å Eu abundances, so abundance determinations that ignore NLTE overstate the internal inconsistency of Eu measurements.
  • Because the 4129 Å correction is positive while the 6645 Å correction is negative at low metallicity, LTE-based [Eu/Fe] values are not uniformly biased; the size and direction of the bias depend on which line is used.
  • The required fraction of magneto-rotating supernovae among core-collapse supernovae rises only from 0.01% to 0.013-0.015%, so the standard mixture of neutron-star mergers plus a small MRSN fraction remains a viable description of Galactic europium enrichment.
  • The flat [Eu/Fe] trend in the metal-poor halo is robust to NLTE corrections, meaning the early Galaxy's europium production was already in place at [Fe/H] near -2.
  • Because <3D> NLTE solar abundances differ from 1D NLTE values, future full-3D NLTE analyses could shift absolute Eu abundances by a few hundredths of a dex, but the paper's relative trend and GCE conclusion are expected to resist such shifts.

Reading between the lines

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

  • The paper's conclusion that only a marginal $f_{\rm MRSN}$ increase is needed rests on the absolute size of the NLTE corrections; if the adopted atom model under-predicts departures in metal-poor giants, the same data could require a larger MRSN fraction or an additional prompt r-process source.
  • The opposite signs of the two lines' corrections offer a built-in consistency check: a future atom model that makes both corrections positive or both negative at low metallicity would signal that the current rate assumptions are wrong.
  • Because the paper validates the atom model only on the Sun, applying the same NLTE grid to metal-poor benchmark stars with independently known Eu abundances from other transitions would provide a sharp test the authors did not perform.
  • The <3D> results being higher than full-3D results in Storm et al. (2024) suggests that spatial averaging washes out some 3D NLTE effects; a full 3D NLTE analysis of a subsample could tighten or shift the GCE parameter constraints.
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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. This paper presents NLTE abundance measurements of europium for a sample of 164 metal-poor halo and disk stars, using the Eu II 4129 Å and 6645 Å lines, with 1D MARCS and <3D> Stagger model atmospheres. The authors determine solar Eu abundances and corrections, characterize NLTE corrections as a function of stellar parameters, and compare the derived [Eu/Fe] trend with GCE models. Their central claim is that NLTE corrections do not require a significant change to the GCE parameters for Eu production, specifically only a marginal increase in the magneto-rotating supernova fraction fMRSN from 0.01% to 0.013-0.015%.

Significance. If the result is robust, the paper provides an important test of whether previous LTE-based GCE conclusions survive NLTE treatment, and it extends Eu abundance analysis to a metal-poor sample with state-of-the-art model atoms and codes. The work builds on the recently published Storm et al. (2024) Eu model atom, and the paper uses standard tools (Turbospectrum, MULTI1D, TSFitPy) and publishes machine-readable tables. The main weakness is that the headline GCE conclusion is based on a combined sample of two lines with opposite NLTE corrections, and the paper does not demonstrate that the combined result is not an artifact of the mixing. There are also internal inconsistencies between the abstract and Table 2 that need correction.

major comments (4)
  1. [Section 4.4, Fig. 8] The binned average [Eu/Fe] shown in Fig. 8 mixes 141 stars from Eu II 4129 Å and 35 stars from Eu II 6645 Å. Figure 4 shows that at low metallicity the NLTE correction for 4129 Å is positive while that for 6645 Å is negative or close to zero, reaching about -0.1 dex for red giants at [Fe/H] = -2. Because the two lines' corrections pull in opposite directions, the NLTE_average in Fig. 8 is a weighted mixture of upward and downward shifts. The paper does not report per-line binned averages, per-line GCE fits, or the line composition of each bin, so the conclusion in Section 4.4 that 'the required change in fMRSN is not substantial' is not established for the full sample. Please provide the GCE comparison separately for each line, or justify a combination scheme, and give the fitted fMRSN with uncertainties.
  2. [Abstract vs. Table 2] The abstract's solar NLTE correction values do not match Table 2. The abstract states for Eu II 4129 Å that NLTE gives 'higher (0.04 dex) solar Eu abundance in 1D and higher (0.07 dex) in <3D>', but Table 2 gives +0.07 dex for 1D (0.54 to 0.61 for IAG, 0.55 to 0.62 for KPNO) and +0.05 dex for <3D> (0.59 to 0.64 for both spectra). For Eu II 6645 Å, the abstract says a negative <3D> correction of -0.03 dex, but only the KPNO spectrum shows -0.03 (0.63 to 0.60); the IAG spectrum shows -0.02 (0.60 to 0.58). The abstract should be corrected to match the values in Table 2 and the text of Section 4.2.
  3. [Section 4.4, star counts] The text states that the sample contains 'a total of 164 stars' and then says the plotted [Eu/Fe] values are 'a total of 141 stars based on the λ 4129 Å line and 35 stars based on the λ 6645 Å line combined.' These numbers sum to 176, not 164. The manuscript should clarify how many stars have both lines measured and how the binned averages are constructed (e.g., per-star averaging or a primary-line list). Without this information, the reader cannot judge whether the combined average is dominated by one line in specific metallicity bins.
  4. [Section 4.4, Fig. 8, error bars] The GCE comparison is based on binned averages plotted without uncertainties and with no quantitative bin definition. The green squares and blue dots are described as averages over bins with 'approximately equal number of stars', but no standard errors or bin boundaries are given. Since the central claim depends on the visual agreement of these averages with the fMRSN tracks, please add error bars to the binned data and specify bin boundaries and the per-line composition of each bin.
minor comments (6)
  1. [Abstract] In the phrase 'the distribution of elements in galactic provides', 'galactic' should be 'galaxies' (or similar grammatical correction).
  2. [Section 4.2] The text mentions 'Eu II λ 6645.70 Å' but the line is at 6645.10 Å; this appears to be a typo.
  3. [Section 4.4] The sentence 'The wavelength range for some of our stars does not include' should be 'do not include' for grammatical agreement.
  4. [Figure 7] The caption states that the figure shows 'differences between the λ 4129 Å and λ 6645 Å lines' but does not state the sign convention; please clarify whether the plot shows [Eu/Fe]4129 minus [Eu/Fe]6645.
  5. [Section 4.1] The sentence 'the NLTE effect weakens the Eu II 4129Å line' could be misinterpreted because a weaker line corresponds to a positive abundance correction; consider rephrasing to explain the relation between line strength and derived abundance.
  6. [Section 3.3] The description of the NLTE calculations would benefit from explicitly stating the trace-element approximation, i.e., that departures from LTE do not affect the model atmosphere structure; this is implied but not stated.

Circularity Check

0 steps flagged · score 2.0 of 10

No substantive circularity: the paper is a measurement plus a parameter fit, with only a minor, non-load-bearing self-citation of the Storm et al. (2024) model atom and solar reference.

full rationale

The central result is an abundance measurement, not a derivation from a model that contains the target conclusion. The paper adopts the NLTE model atom of Eu from Storm et al. (2024) in Section 3.3 ("We adopted the NLTE model of Eu from Storm et al. (2024)") and the solar reference abundance in Section 3.4 ("For the solar Eu abundance, we adopted A(Eu)=0.57 from Storm et al. (2024)"). These are self-citations from the same group, and they do supply the size and sign of the NLTE corrections, which are load-bearing for the final GCE statement. However, they are not circular in the sense of reducing the result to its own input: the gf-values come from independent laboratory measurements by Lawler et al. (2001), the prior model is an externally tested atomic calculation with data from NIST and Kurucz, and the paper validates its solar fits against two independent atlases (IAG and KPNO). No equation in the paper defines the derived stellar abundances or the fMRSN parameter in terms of the model's own outputs. The GCE comparison is a parameter fit: fMRSN is adjusted from 0.01% to 0.013-0.015% to match the NLTE average [Eu/Fe], and the conclusion that this change is not substantial is a postdiction, not a prediction claimed from first principles. The concern that the sample mixes the Eu II 4129 and 6645 lines, whose NLTE corrections have opposite signs, is a robustness or internal-consistency issue about how the combined average is formed, not a circularity. Therefore no step reduces by construction, and the central claim retains independent content beyond the cited inputs.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central claim rests on the adopted Eu model atom, atmospheric grids, stellar parameters, and GCE assumptions from prior work; no new particles or mechanisms are introduced. The only parameter fitted for the GCE conclusion is fMRSN, and microturbulence is fitted per star in the abundance analysis.

free parameters (2)
  • fMRSN (fraction of core-collapse supernovae replaced by magneto-rotating supernovae) = 0.013% to 0.015%
    Selected in Section 4.4 so the GCE models match the binned NLTE [Eu/Fe] data; no uncertainty is quoted, so the 'no significant change' conclusion rests on this hand-adjusted value.
  • Per-star microturbulence (xi_t) = 1.1 to 2.1 km/s in the excerpted rows
    Fitted per star by TSFitPy during abundance determination; the derived [Eu/Fe] values depend on this fitted quantity.
assumptions (6)
  • domain assumption Eu is a trace element, so NLTE departures do not modify the model atmosphere structure
    Invoked in Section 3.3 via MULTI1D, which assumes deviations from LTE do not influence the input model atmosphere; if Eu back-reaction on the atmosphere mattered, the departure coefficients would be inconsistent.
  • domain assumption The adopted Eu atomic data (log gf, hyperfine structure, isotope ratio) are accurate
    The log gf values come from Lawler et al. (2001), HFS from Mashonkina & Gehren (2000) and Storm et al. (2024), and the isotope ratio 47.8:52.2 from Lodders et al. (2009); these external inputs are not tested in this paper.
  • domain assumption 1D MARCS and <3D> Stagger grids represent the stellar atmospheres of the sample
    Atmospheres are taken from Gustafsson et al. (2008) and Magic et al. (2013a,b); the authors use the <3D> average as a proxy for full 3D and note it cannot capture all 3D effects.
  • standard math The radiative transfer and statistical equilibrium solvers are numerically correct
    The paper relies on Turbospectrum, TSFitPy, and MULTI1D without formal proof of the solvers; prior code validation is assumed.
  • domain assumption The GCE model parameterization (NSM delay time, ejecta mass, yields) is correct
    Section 4.4 adopts NSM delay time distribution t^-1, ejecta mass 2.5e-2 solar masses, and yields from Arnould et al. (2007) and Nishimura et al. (2015); the fMRSN conclusion depends on these choices.
  • domain assumption Stellar parameters (Teff, log g, [Fe/H]) from Bergemann et al. (2017b) are accurate
    The paper adopts these parameters without re-derivation; systematic errors would propagate into [Eu/Fe] and the NLTE corrections.

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Cite this review

Pith. "Pith review of NLTE abundances of Eu for a sample of metal-poor stars in the Galactic Halo and Metal-poor Disk with 1D and <3D> models." pith.science (2026). https://pith.science/paper/62FGDEA2

@misc{pith2026241206277,
  author       = {Pith},
  title        = {Pith review of: NLTE abundances of Eu for a sample of metal-poor stars in the Galactic Halo and Metal-poor Disk with 1D and <3D> models},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/62FGDEA2}},
  note         = {Machine review of arXiv:2412.06277}
}
abstract

Accurate measurements of europium abundances in cool stars are essential for an enhanced understanding of the r-process mechanisms. We measure the abundance of Eu in solar spectra and a sample of metal-poor stars in the Galactic halo and metal-poor disk, with the metallicities ranging from \GG{$-2.4$} to $-0.5$ dex, using non-local thermodynamic equilibrium (NLTE) line formation. We compare these measurements with Galactic Chemical Evolution (GCE) models to \GG{explore the impact of the NLTE corrections on the contribution of r-process site in Galactic chemical evolution. In this work, we use NLTE line formation, as well as one-dimensional (1D) hydrostatic and spatial averages of 3D hydrodynamical ($<$3D$>$) model atmospheres to measure the abundance of Eu based on both the Eu II 4129 \AA\ and Eu II 6645 \AA\ lines for solar spectra and metal-poor stars. We find that \GG{for Eu II 4129 \AA\ line the NLTE modelling leads to higher (0.04 dex) solar Eu abundance in 1D and higher (0.07 dex) in \GG{$<$3D$>$} NLTE while} NLTE modelling leads to higher (0.01 dex) solar Eu abundance in 1D and lower (0.03 dex) in \GG{$<$3D$>$} NLTE for Eu II 6645 \AA\ line. Although the NLTE corrections for the Eu II $\lambda$ 4129 \AA\ and Eu II $\lambda$ 6645 \AA\ lines are opposite, the discrepancy between the abundances derived from these individual lines reduces after applying NLTE corrections, highlighting the critical role of NLTE abundance determinations. By comparing these measurements with Galactic chemical evolution (GCE) models, we find that the \G{amount of NLTE correction does not require significant change of the parameters for Eu production} in the GCE models.

Figures

Figures reproduced from arXiv: 2412.06277 by the authors.

Figure 1
Figure 1. Grotrian diagram of the Eu II model atom. The model atom are taken from the Storm et al. (2024). The blue and red lines represent the transitions giving rise to the Eu II 4129 Å and 6645 Å lines, respec￾tively. of NLTE departure coefficients to calculate the NLTE line pro￾files by correcting the line source functions and the line opacity of all lines (further details described in Gerber et al. 2023). A Python wrappe… view at source ↗
Figure 2
Figure 2. Synthetic spectra for Eu II 4129Å (left) and Eu II 6645Å (right) generated from TSFitPy base on solar parameter with A(Eu)=0.57 dex. The red solid lines represent the line profile generated from 1D LTE and the blue solid lines are from 1D NLTE, while the red dash lines represent the line profile generated from <3D> LTE and the blue dash lines are from <3D> NLTE. Eu II line feature. The atomic parameters of these two… view at source ↗
Figure 4
Figure 4. NLTE corrections for the Eu II λ 4129Å line (left panel) and λ 6645Å line (right panel) are plotted against the metallicity [Fe/H] of the model atmospheres. The red line represents the NLTE correction of red giant with parameters of Teff=4500 K/log g=1.5 dex, the blue line rep￾resents the NLTE correction of subgiant with parameters of Teff=5500 K/log g=3.5 dex, and the black line represents the NLTE correction of ma… view at source ↗
Figures from the paper (2 more)
Figure 5
Figure 5. Figure 5: Departure coefficients for the Eu II λ 6645 Å line as a function of optical depth. The solar model atmosphere are depicted in red and Teff =4500 K/log g =2.0 dex/[Fe/H]= −1 dex in black, with solid and dashed lines representing lower and upper levels, respectively. Wav…
Figure 6
Figure 6. Figure 6: Examples of the fitting based on 1D LTE (red line) and NLTE (blue line) model. The black dots in each panel represent the observed spectrum with four different stars. We present the fitting results of four stars as examples in [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

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