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arxiv: 1906.08284 · v2 · pith:7XLABDYKnew · submitted 2019-06-19 · 🌌 astro-ph.SR · astro-ph.GA

Catalog of Chromium, Cobalt, and Nickel Abundances in Globular Clusters and Dwarf Galaxies

Evan N. Kirby (1) , Justin L. Xie (1 , 2) , Rachel Guo (1 , 3) , Mikhail Kovalev (4) , Maria Bergemann (4) ((1) Caltech , (2) The Harker School
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(3) Irvington High School (4) Max-Planck Institute for Astronomy)
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Pith reviewed 2026-05-25 19:45 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA
keywords chromium abundancescobalt abundancesnickel abundancesglobular clustersdwarf galaxiesred giant starsmedium-resolution spectroscopychemical abundances
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The pith

Measurements of chromium, cobalt, and nickel abundances are presented for 4113 red giant stars in globular clusters and dwarf galaxies.

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

This paper establishes a catalog of abundances for chromium, cobalt, and nickel across 4113 red giants, with 2277 in globular clusters and 1820 in Milky Way dwarf satellites. Abundances come from fitting absorption lines in medium-resolution Keck/DEIMOS spectra while using prior estimates of temperature, gravity, and metallicity. Systematic errors are gauged from abundance spreads inside mono-metallic clusters, and the work includes validation via repeated observations and high-resolution comparisons. A reader would care because the data set supplies a uniform resource for tracing iron-peak element patterns in these stellar systems.

Core claim

The paper claims to deliver abundance measurements of chromium, cobalt, and nickel for 4113 red giants drawn from globular clusters, dwarf satellite galaxies, and a few field stars. The values are extracted by fitting spectral regions that contain the relevant absorption lines in archival medium-resolution spectra covering roughly 6500-9000 Å. Stellar parameters previously derived from the same spectra serve as fixed inputs, and uncertainties are quantified both from internal cluster dispersions and from direct comparisons to high-resolution results.

What carries the argument

Spectral fitting of absorption-line regions in Keck/DEIMOS medium-resolution spectra (R ~ 6500), anchored to previously measured temperature, surface gravity, and metallicity values.

If this is right

  • The catalog supplies uniform [Cr/Fe], [Co/Fe], and [Ni/Fe] data that can be compared directly between globular clusters and dwarf galaxies.
  • Median uncertainties of 0.20 dex for chromium and cobalt and 0.13 dex for nickel set the precision limit for any follow-up analysis.
  • Dispersion within mono-metallic clusters provides an empirical floor on systematic error for the entire sample.
  • Duplicate observations and high-resolution cross-checks confirm that random and systematic errors are under control for the reported values.

Where Pith is reading between the lines

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

  • The measurements could be combined with existing alpha-element or neutron-capture abundances to isolate the relative roles of core-collapse and Type Ia supernovae in low-mass galaxies.
  • The same fitting approach might be applied to the much larger samples now available from multi-object surveys to map abundance gradients across entire satellite systems.
  • Differences in the new ratios between clusters and field stars in dwarfs could test whether cluster formation environments imprint distinct chemical signatures.

Load-bearing premise

The previously determined temperature, surface gravity, and metallicity values are accurate enough that the medium-resolution line fits recover the true abundances without large systematic biases.

What would settle it

A systematic offset larger than the stated uncertainties between these medium-resolution abundances and independent high-resolution measurements of the same stars would indicate the fitting procedure does not recover accurate values.

Figures

Figures reproduced from arXiv: 1906.08284 by 2), (2) The Harker School, 3), (3) Irvington High School, (4) Max-Planck Institute for Astronomy), Evan N. Kirby (1), Justin L. Xie (1, Maria Bergemann (4) ((1) Caltech, Mikhail Kovalev (4), Rachel Guo (1.

Figure 1
Figure 1. Figure 1: Small regions of the DEIMOS spectra showing absorption features of (a) Cr, (b) Co, and (c) Ni. The blue shaded regions show spectral regions used in the calculation of χ 2 (see Section 3.1). The black curves show the observed spectra, and the red curves show the best-fitting model spectra. The pink shaded regions show the changes in the model spectra for changes of ±0.3 dex in abundance. The same four star… view at source ↗
Figure 2
Figure 2. Figure 2: LTE and NLTE abundance ratios for [Cr/Fe] and [Co/Fe] as function of effective temperature in the globular cluster M79. The resulting spectra had 9 Cr and 12 Co transitions. These were the model spectra that were used in the χ 2 minimization. This procedure required us to sample NLTE corrections both between grid points and beyond the bounds of the NLTE corrections grid. We used linear interpolation and, w… view at source ↗
Figure 3
Figure 3. Figure 3: Distributions of ∆ (Equation 2) used in determining the systematic errors for the iron-peak abundance measurements. The dashed curves show Gaussians with a variance of unity. In the limit of perfectly Gaussian-distributed uncertainties, the histograms would conform to the dashed curves. The numbers of stars are indicated in parentheses. is no evidence that either cluster shows a dispersion in the ratios of… view at source ↗
Figure 4
Figure 4. Figure 4: Differences in measured abundance ratios between repeated observations of the same stars. The differences are normalized by the quadrature sum of the uncertainties (Equation 3). The dashed curves show Gaussians with zero mean and unit variance. Each panel gives the number of pairs of measurements in parentheses. single Co abundance. If it is variable, then δsys[Co/Fe] will encompass that variability. Alter… view at source ↗
Figure 5
Figure 5. Figure 5: Comparison between our MRS measurements and HRS measurements from the literature for the same stars for (a) [Cr/Fe], (b) [Co/Fe], and (c) [Ni/Fe]. Color-coding indicates the type of stellar system in which the stars reside. The dashed lines indicate one-to-one agreement. -4 -2 0 2 4 0 2 4 6 8 -4 -2 0 2 4 0 2 4 6 8 [Cr/Fe]MRS − [Cr/Fe]HRS (δ[Cr/Fe]2 MRS + δ[Cr/Fe]2 MRS) 1/2 (a) Cr N = 60 σ = 1.14 -4 -2 0 2 … view at source ↗
Figure 6
Figure 6. Figure 6: The differences between our abundance measurements from MRS and literature values from HRS for the same stars. The differences are normalized by the quadrature sum of the uncertainties. If the errors are Gaussian and properly estimated, then these histograms would be Gaussians with a standard deviation of 1, as shown in Equation 5. The dashed curves show Gaussians with σ = 1, whereas the dotted curves show… view at source ↗
read the original abstract

We present measurements of the abundances of chromium, cobalt, and nickel in 4113 red giants, including 2277 stars in globular clusters, 1820 stars in the Milky Way's dwarf satellite galaxies, and 16 field stars. We measured the abundances from mostly archival Keck/DEIMOS medium-resolution spectroscopy with a resolving power of R ~ 6500 and a wavelength range of approximately 6500-9000 A. The abundances were determined by fitting spectral regions that contain absorption lines of the elements under consideration. We used estimates of temperature, surface gravity, and metallicity that we previously determined from the same spectra. We estimated systematic error by examining the dispersion of abundances within mono-metallic globular clusters. The median uncertainties for [Cr/Fe], [Co/Fe], and [Ni/Fe] are 0.20, 0.20, and 0.13, respectively. Finally, we validated our estimations of uncertainty through duplicate measurements, and we evaluated the accuracy and precision of our measurements through comparison to high-resolution spectroscopic measurements of the same 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

0 major / 3 minor

Summary. The paper presents a catalog of [Cr/Fe], [Co/Fe], and [Ni/Fe] abundances measured for 4113 red giant stars (2277 in globular clusters, 1820 in Milky Way dwarf satellites, 16 field stars) from mostly archival Keck/DEIMOS medium-resolution spectra (R~6500, 6500-9000 Å). Abundances are obtained by fitting spectral regions containing the relevant absorption lines, using T_eff, log g, and [Fe/H] previously derived from the same spectra. Systematic errors are estimated from the internal abundance dispersion in mono-metallic globular clusters; median uncertainties are reported as 0.20, 0.20, and 0.13 dex. Validation is performed via duplicate measurements and direct comparison to high-resolution spectroscopy on a subset of stars.

Significance. If the reported values and uncertainties are reliable, the catalog supplies a substantial new homogeneous dataset on three iron-peak elements across a wide range of stellar populations and metallicities. The sample size and explicit validation steps (GC dispersion, duplicates, high-resolution overlap) make the catalog a useful resource for statistical studies of chemical evolution and nucleosynthesis, even at the stated precision level.

minor comments (3)
  1. [Abstract] The abstract states that abundances were determined by fitting spectral regions but does not specify which lines or wavelength intervals were used for each element; adding this information (or a reference to the relevant table/figure in the methods section) would improve reproducibility.
  2. While median uncertainties are given, the manuscript would benefit from reporting the full distribution (e.g., 16th/84th percentiles or a histogram) of the per-star uncertainties for each abundance ratio.
  3. The number of stars with high-resolution comparison data and the quantitative results of that comparison (mean offset, rms scatter) are mentioned but not quantified in the provided abstract; including these numbers in the main text would strengthen the accuracy claim.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our manuscript and for recommending acceptance. The referee's summary accurately captures the content and scope of the work.

Circularity Check

0 steps flagged

No significant circularity; catalog derived from standard spectral fitting with external validations

full rationale

The paper's central claim is a catalog of [Cr/Fe], [Co/Fe], and [Ni/Fe] abundances for 4113 stars obtained by fitting spectral regions in medium-resolution DEIMOS spectra, using prior T_eff, log g, and [Fe/H] estimates from the same spectra. This is a direct measurement procedure, not a derivation that reduces to its inputs by construction. Systematic errors are estimated from observed dispersion within mono-metallic globular clusters, and accuracy is checked via duplicate observations and high-resolution comparisons on a subset of stars. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations that lack independent verification appear in the described chain. Self-citation for the input parameters is normal and does not force the abundance results, as the validations are external to the fitting step itself.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is a purely observational paper reporting measured abundances from spectra. The central claim does not rest on free parameters, mathematical axioms, or newly invented physical entities.

pith-pipeline@v0.9.0 · 5787 in / 1264 out tokens · 42897 ms · 2026-05-25T19:45:13.318917+00:00 · methodology

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