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arxiv: 2604.22249 · v1 · submitted 2026-04-24 · 🌌 astro-ph.SR

Non-LTE corrections for determinations of europium abundances in F-G-K stars in a broad metallicity range

L. I. Mashonkina , S. A. Yakovleva This is my paper

Pith reviewed 2026-05-08 09:53 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords europium abundancesnon-LTE effectsEu II linesstellar spectroscopyr-process elementsF-G-K starsgalactic chemical evolution
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The pith

Non-LTE effects bring europium abundances from two spectral lines into agreement within error bars for the Sun and metal-poor stars.

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

The paper updates the model atom for singly ionized europium with recent collision rate data and calculates non-LTE abundance corrections for eleven Eu II lines across grids of F-G-K star atmospheres. When these corrections are applied, the europium abundances derived from the 4129 Å and 6645 Å lines match within uncertainties, whereas local thermodynamic equilibrium assumptions produce systematic differences of 0.07 to 0.18 dex. The non-LTE approach also lowers the statistical scatter in the derived abundances. The corrections are made available for direct use in studies of galactic chemical evolution that rely on europium as an r-process tracer.

Core claim

Abandoning the LTE assumption and using an updated Eu II model atom that incorporates recent rate coefficients for inelastic Eu II + H I collisions produces non-LTE abundance corrections that reconcile the two diagnostic lines within their error bars while reducing statistical uncertainties in the derived stellar europium abundances. The corrections were computed for a grid of plane-parallel MARCS model atmospheres spanning the effective temperatures, surface gravities, and metallicities typical of F-G-K stars and are provided for public application.

What carries the argument

Updated Eu II model atom incorporating recent rate coefficients for inelastic collisions with neutral hydrogen, used in non-LTE statistical equilibrium calculations on plane-parallel MARCS atmospheres.

If this is right

  • Europium abundances derived from either line become consistent, allowing use of the stronger line in cooler or more metal-poor stars without systematic offset.
  • Statistical uncertainties on individual stellar Eu abundances decrease, tightening constraints on r-process enrichment patterns.
  • The tabulated corrections can be applied directly to existing and future high-resolution spectra of F-G-K stars to revise galactic chemical evolution trends.

Where Pith is reading between the lines

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

  • Similar non-LTE treatments for other rare-earth ions could reduce line-to-line scatter in abundance surveys and improve r-process yield comparisons.
  • In very metal-poor stars the corrections may shift the inferred timing or site of early r-process events when Eu/Fe ratios are used as tracers.
  • Large spectroscopic surveys could incorporate these corrections to produce more uniform europium maps across the Milky Way disk and halo.

Load-bearing premise

The new collision rates with hydrogen atoms are accurate and complete enough across the full range of metallicities and stellar parameters to correctly capture the departures from LTE.

What would settle it

High-resolution spectra of the reference stars analyzed with an independent set of model atmospheres or different collision data would still show abundance differences between the 4129 Å and 6645 Å lines larger than the quoted errors after the published non-LTE corrections are applied.

Figures

Figures reproduced from arXiv: 2604.22249 by L. I. Mashonkina, S. A. Yakovleva.

Figure 1
Figure 1. Figure 1: Excitation rate coefficients log q (cm3 s −1 ) for the Eu II + H I collisions from calculations by S.A. Yakovleva (circles) and Storm et al. (2024, squares) as a function of the transition energy Eij. The data correspond to T = 5000 K. The second data set is taken from Storm et al. (2024). These are the rate coefficients for 934 b-b transitions and 23 b-f transitions. The collisions of europium ions with h… view at source ↗
Figure 2
Figure 2. Figure 2: Observed line profiles (circles) of Eu II 4129 and 6645 ˚A in spectra of the Sun (top row) and HD 8724 (bottom row) in comparison with synthetic spectra, calculated taking into account non-LTE effects (SBY24, solid curve) for lines of Eu II. The obtained non-LTE abundances are given in view at source ↗
Figure 3
Figure 3. Figure 3: b-factors of the Eu II selected levels as a function of log τ5000 in the atmospheric model with Teff = 4670 K, log g = 1.41, [Fe/H] = −1.58. The tick marks indicate the locations of line center optical depth unity for the Eu II 4129 ˚A (1) and 6645 ˚A (2) lines. -0.4 -0.2 0.0 0.2 Eu II 4129 -- 6645 (dex) Sun HD8724 HD83212 HD122956 HD165195 HD204543 view at source ↗
Figure 4
Figure 4. Figure 4: Abundance differences between Eu II 4129 view at source ↗
Figure 5
Figure 5. Figure 5: Non-LTE abundance corrections for Eu II 4129 view at source ↗
Figure 6
Figure 6. Figure 6: The same as in Fig. 5 for model atmospheres of giants with view at source ↗
read the original abstract

Europium plays a key role in studies of nucleosynthesis in the rapid (r-) process of neutron capture nuclear reactions and the evolution of the r-process element abundances in galaxies. We refine the method for analyzing the Eu~II lines in stellar spectra by updating the Eu~II model atom with recent data on the rate coefficients for inelastic processes in the Eu II + H I collisions. The method was tested by deriving abundances from the Eu~II 4129 A and 6645 A lines in the Sun and five reference stars with well determined atmospheric parameters and high-resolution spectra available. It was shown that abandoning the local thermodynamic equilibrium (LTE) assumption allows the abundances from the two lines to be matched within the error bars, while the difference in the LTE abundances amounts to -0.09 dex for the Sun and -0.07 dex to -0.18 dex for the metal-poor stars. Accounting for the departures from LTE (non-LTE effects) for Eu~II leads to reducing the statistical errors of the derived stellar Eu abundances. The non-LTE abundance corrections for 11 lines of Eu~II were calculated for a grid of the plane-parallel MARCS model atmospheres with effective temperatures, surface gravities, and metallicities relevant to F-G-K type stars. They are publicly available and can be applied for improving stellar Eu abundances in studies of the galactic chemical evolution.

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

2 major / 2 minor

Summary. The manuscript updates the Eu II model atom with recent theoretical rate coefficients for inelastic Eu II + H I collisions, tests the non-LTE approach on the Sun and five reference stars with known parameters, and shows that non-LTE abundances from the 4129 Å and 6645 Å Eu II lines agree within errors (unlike LTE offsets of -0.09 dex in the Sun and -0.07 to -0.18 dex in metal-poor stars). It computes and publicly releases non-LTE corrections for 11 Eu II lines on a grid of plane-parallel MARCS atmospheres spanning F-G-K stellar parameters and metallicities.

Significance. If robust, the work supplies a practical set of non-LTE corrections that can reduce line-to-line scatter and statistical uncertainties in stellar Eu abundances, directly benefiting r-process nucleosynthesis studies and galactic chemical evolution analyses across a wide metallicity range. The empirical validation on reference stars and the public release of the grid are clear strengths that increase the result's utility.

major comments (2)
  1. [Model atom construction and reference-star abundance tests] The central claim that non-LTE with the updated model atom reconciles the 4129 Å and 6645 Å abundances (and reduces errors) depends on the specific Eu II + H I inelastic collision rates adopted from recent theoretical calculations. No sensitivity tests—such as scaling the rates by factors of 0.1–10, swapping in older Drawin-type estimates, or comparing independent rate sets—are reported in the model-atom description or the reference-star results. Because these rates control the thermalization of Eu II levels and the sign/magnitude of the corrections, plausible uncertainties could reopen the line-to-line discrepancy or alter the quoted error reduction.
  2. [Grid of non-LTE corrections] The non-LTE corrections are computed exclusively on plane-parallel MARCS model atmospheres. For the most metal-poor stars in the grid, no assessment is provided of how 3D or spherical effects in the line-forming layers might modify the corrections, even though such effects are known to matter for resonance lines at low metallicity.
minor comments (2)
  1. [Abstract and conclusions] The abstract states that the corrections 'are publicly available,' but the manuscript should include an explicit repository link, DOI, or access instructions in the main text or data-availability statement.
  2. [Results for reference stars] Notation for the two diagnostic lines (4129 Å and 6645 Å) should be standardized throughout, and any table or figure presenting the LTE vs. non-LTE abundance differences for the reference stars should include the individual line abundances and their uncertainties for direct inspection.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and positive review, which highlights the practical utility of our non-LTE corrections for Eu abundances. We address each major comment below, providing the strongest honest response based on the manuscript as written.

read point-by-point responses

  1. Referee: [Model atom construction and reference-star abundance tests] The central claim that non-LTE with the updated model atom reconciles the 4129 Å and 6645 Å abundances (and reduces errors) depends on the specific Eu II + H I inelastic collision rates adopted from recent theoretical calculations. No sensitivity tests—such as scaling the rates by factors of 0.1–10, swapping in older Drawin-type estimates, or comparing independent rate sets—are reported in the model-atom description or the reference-star results. Because these rates control the thermalization of Eu II levels and the sign/magnitude of the corrections, plausible uncertainties could reopen the line-to-line discrepancy or alter the quoted error reduction.

    Authors: The inelastic collision rates incorporated in our updated Eu II model atom are taken directly from the most recent ab initio theoretical calculations published in the literature. These replace the approximate Drawin-type estimates used in earlier Eu non-LTE work. The empirical validation on the Sun and five reference stars shows that the chosen rates produce consistent abundances from the 4129 Å and 6645 Å lines within the quoted uncertainties, supporting their suitability. We did not include explicit sensitivity tests (scaling or substitution) because the focus of the paper is on the application of the best available rates rather than a comprehensive uncertainty analysis of the rates themselves. We agree that such tests would add robustness and will therefore add a short discussion paragraph in the revised manuscript that references the source papers for the rates and notes the typical uncertainties reported in the broader literature for similar ions. This addition does not alter the main results or the public grid. revision: partial

  2. Referee: [Grid of non-LTE corrections] The non-LTE corrections are computed exclusively on plane-parallel MARCS model atmospheres. For the most metal-poor stars in the grid, no assessment is provided of how 3D or spherical effects in the line-forming layers might modify the corrections, even though such effects are known to matter for resonance lines at low metallicity.

    Authors: The correction grid was computed on the standard 1D plane-parallel MARCS atmospheres to match the modeling framework used for the reference-star tests and to provide a practical, immediately usable dataset for the community. We recognize that 3D hydrodynamical and spherical effects can modify the temperature and density structure in the line-forming layers, particularly for resonance lines at low metallicity. A full 3D non-LTE re-calculation of the entire grid lies outside the scope of the present study owing to the prohibitive computational cost. In the revised manuscript we will insert an explicit limitations paragraph stating that the provided corrections are 1D-based and advising users to consider separate 3D corrections when applying the grid to the most metal-poor stars. revision: partial

Circularity Check

0 steps flagged

No significant circularity; results computed from external rates and models

full rationale

The paper's chain begins with an externally supplied set of Eu II + H I inelastic collision rates and standard plane-parallel MARCS atmospheres. These inputs are used to solve the statistical-equilibrium equations for a model atom, yielding non-LTE level populations and line-formation corrections. The subsequent test on the Sun and five reference stars compares the resulting abundances from the 4129 Å and 6645 Å lines against each other and against LTE values; the reported agreement within error bars is an output of that calculation, not a definitional identity or a parameter fitted to the target abundances. No equation or step equates a derived quantity to its own input by construction, and the central claim (reduction of line-to-line scatter and statistical errors) retains independent content once the external rates and atmospheres are granted. The grid of tabulated corrections is likewise a direct numerical product rather than a renaming or self-referential prediction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim depends on the accuracy of externally supplied collision rates and on standard assumptions of stellar atmosphere modeling; no new free parameters are fitted to the stellar data in the described procedure.

axioms (2)
  • domain assumption Plane-parallel geometry, hydrostatic equilibrium, and the validity of MARCS model atmospheres for F-G-K stars
    Invoked when computing the correction grid for the stated range of effective temperatures, surface gravities, and metallicities.
  • domain assumption The recent rate coefficients for Eu II + H I inelastic collisions are accurate inputs across the metallicity range
    The update to the model atom relies on these external data without re-derivation in the paper.

pith-pipeline@v0.9.0 · 5562 in / 1613 out tokens · 68550 ms · 2026-05-08T09:53:30.238350+00:00 · methodology

discussion (0)

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Reference graph

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