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arxiv: 1907.10476 · v1 · pith:2WJJATPJnew · submitted 2019-07-24 · 🌌 astro-ph.GA

On the APOGEE DR14 sodium spread in the Galactic open cluster NGC 188

Andr\'es E. Piatti This is my paper

Pith reviewed 2026-05-24 16:41 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords APOGEENGC 188open clustersodium abundancemultiple populationsspectroscopic surveysabundance uncertaintiesred giants
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The pith

The sodium abundance spread seen in APOGEE DR14 data for NGC 188 red giants is probably not real but an effect of measurement uncertainties.

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

The paper examines chemical abundances from APOGEE DR14 for 15 red giant stars in the old open cluster NGC 188. It identifies a spread in [Na/Fe] of 0.16 dex split into two groups with means differing by 0.30 dex, along with hints of anti-correlations with Al and TiII. However, this spread matches the typical uncertainty level of 0.16 dex, and spectra of stars with similar parameters appear visually identical. The conclusion is a caution that automated analysis of large spectroscopic surveys can produce unreliable peculiar abundances that go unnoticed.

Core claim

Measurements of 17 chemical species in NGC 188 red giants from APOGEE DR14 reveal an apparent [Na/Fe] spread of ±0.16 dex from two groups differing by ~0.30 dex, with possible anti-correlations in Al and TiII. Since this matches the typical 0.16 dex uncertainty and spectra look similar for stars with comparable parameters, the spread is attributed to analysis issues rather than true abundance variations, warning against trusting unsupervised pipeline results in surveys.

What carries the argument

Direct comparison of reported abundances to their uncertainties combined with visual inspection of spectra for stars with similar atmospheric parameters.

If this is right

  • The [Na/Fe] spread does not support the presence of multiple populations in NGC 188.
  • Apparent spreads in other elements or clusters from APOGEE may also be artifacts.
  • Users should verify peculiar abundances from large surveys with additional checks.
  • Analysis pipelines need better safeguards against producing unnoticed bad results.

Where Pith is reading between the lines

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

  • Independent high-resolution spectroscopy would be needed to confirm if any real spreads exist.
  • The caution applies broadly to light element abundances in open clusters studied via surveys.
  • Automated methods may miss subtle issues that manual spectrum comparison reveals.

Load-bearing premise

That the similarity in appearance of spectra for stars with similar atmospheric parameters is enough to rule out real abundance differences, and that the quoted uncertainty captures the full error.

What would settle it

Re-measuring [Na/Fe] for the same stars using independent high-resolution spectra with uncertainties smaller than 0.1 dex to test if the two groups persist as distinct.

Figures

Figures reproduced from arXiv: 1907.10476 by Andr\'es E. Piatti.

Figure 1
Figure 1. Figure 1: RV versus [Fe/H] diagnostic diagram. Error bars are also drawn. Red filled circles represent the selected clus￾ter members [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Left: Relationships between different [X/Fe] ratios. The chemical (X) species are indicated at the top-left margin of each panel. Error bars are also drawn. Right: 2MASS K mag (mag), log(g) and Teff ( ◦K) versus [Na/Fe] are drawn from top to bottom panels, respectively [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: [X/Fe] distributions for different chemical species. Blue and red lines correspond to Na-poor and Na-rich stars (see text for details), respectively; black lines being the sum of them [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comparison of the spectra of two stars with sim￾ilar atmospheric parameters, respectively. Gaia. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Founda￾tion, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowl￾edges support and resources from the Center for High￾Performance Computing at the University of Utah. The SDSS we… view at source ↗
Figure 4
Figure 4. Figure 4: Top, middle and bottom panels are the same as Figs. 1-3, for NGC 6791, respectively [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
read the original abstract

Since several years ago, the search for multiple populations (MPs) in Galactic open clusters (OCs) has become a field of increasing interest, particularly to the light of the general knowledge that MPs are observed in relatively massive clusters (>= 5 10^4 Mo). I report here measurements of stellar parameters and abundances of 17 different chemical species, including light and iron-peak elements of the old open cluster NGC188, from the APOGEE DR14 database. I selected 15 bonafide cluster red giants that reveal an spread in [Na/Fe] of +-0.16 dex, caused by two different groups of measurements whose mean [Na/Fe] values differ by ~ 0.30 dex. A hint for a subtle anti-correlation with Al and TiII, which also show relative large [X/Fe] dispersions, is also seen in the data. However, [Na/Fe] abundances have been found to be typically affected by uncertainties of 0.16 dex. Furthermore, I directly compared the available spectra for stars with similar atmospheric parameters and it would seem that they look similar. Therefore, I warn users of large spectroscopic surveys to be extra careful when finding peculiar abundance results. Analysis pipelines that run in an unsupervised fashion may produce bad results which may not be noticed before publication.

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 analyzes APOGEE DR14 data for the open cluster NGC 188, deriving stellar parameters and abundances for 17 species in 15 bona fide red giant members. It reports a ±0.16 dex spread in [Na/Fe] arising from two groups whose means differ by ~0.30 dex, with hints of anti-correlations involving Al and Ti II. The central conclusion is that this spread is likely spurious because typical [Na/Fe] uncertainties are 0.16 dex and spectra of stars with similar parameters appear visually similar; the author therefore cautions users of large spectroscopic surveys against overinterpreting results from unsupervised pipelines.

Significance. If the conclusion that the spread is spurious is substantiated with quantitative evidence, the work would provide a concrete cautionary example for users of automated abundance pipelines in large surveys such as APOGEE. The use of publicly available catalog data is a strength that supports reproducibility. However, the current support for the claim rests on qualitative inspection rather than rigorous tests.

major comments (2)
  1. [text discussing spectral comparison] The assertion that the observed [Na/Fe] spread is spurious rests on the statement that spectra for stars with similar atmospheric parameters 'look similar' (text immediately following the abundance results). No quantitative metrics—such as residuals between normalized spectra, differences in Na I line depths or equivalent widths, or formal model comparison—are reported to demonstrate that the features are consistent within the noise at the level needed to exclude a 0.3 dex offset.
  2. [uncertainty discussion] The paper equates the reported ±0.16 dex [Na/Fe] spread with the typical uncertainty value of 0.16 dex without additional validation that this uncertainty fully captures the error budget for these specific stars or that parameter covariances cannot produce the apparent grouping into two populations (uncertainty discussion following the abundance table).
minor comments (2)
  1. [abstract] The abstract contains the grammatical error 'an spread' (should be 'a spread').
  2. [text discussing spectral comparison] The phrasing 'it would seem that they look similar' is informal and could be replaced with a more precise description of the comparison performed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive report. The comments highlight areas where our qualitative arguments can be strengthened with quantitative support, and we address each point below with plans for revision.

read point-by-point responses

  1. Referee: The assertion that the observed [Na/Fe] spread is spurious rests on the statement that spectra for stars with similar atmospheric parameters 'look similar' (text immediately following the abundance results). No quantitative metrics—such as residuals between normalized spectra, differences in Na I line depths or equivalent widths, or formal model comparison—are reported to demonstrate that the features are consistent within the noise at the level needed to exclude a 0.3 dex offset.

    Authors: We agree that the original comparison was qualitative and that quantitative metrics would provide stronger support. In the revised manuscript we will add measurements of Na I line equivalent widths for the relevant stars, compute residuals between normalized spectra of stars with similar parameters, and report the standard deviation of those residuals to demonstrate consistency with the noise level. revision: yes

  2. Referee: The paper equates the reported ±0.16 dex [Na/Fe] spread with the typical uncertainty value of 0.16 dex without additional validation that this uncertainty fully captures the error budget for these specific stars or that parameter covariances cannot produce the apparent grouping into two populations (uncertainty discussion following the abundance table).

    Authors: The 0.16 dex figure is taken directly from the APOGEE DR14 pipeline documentation for [Na/Fe]. We acknowledge that a fuller discussion of the error budget and possible covariances is warranted. The revision will expand this section to note the quoted uncertainties, discuss potential covariances with atmospheric parameters, and clarify that while such effects could contribute to apparent groupings, the spread matching the typical uncertainty still supports a cautionary interpretation. revision: partial

Circularity Check

0 steps flagged

No circularity: purely observational report on public catalog data

full rationale

The paper selects 15 stars from APOGEE DR14, reports an observed [Na/Fe] spread of ±0.16 dex, notes the quoted typical uncertainty of 0.16 dex, and states that spectra of stars with similar parameters appear visually similar. No equations, fitted parameters, predictions, ansatzes, or self-citations are used to derive any result; the central claim is a direct data report plus a caution based on external uncertainty values. No load-bearing step reduces to its own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Analysis rests on the APOGEE DR14 public database and the survey's standard stellar parameter and abundance pipeline; no additional free parameters, axioms, or invented entities are introduced.

pith-pipeline@v0.9.0 · 5767 in / 1081 out tokens · 21849 ms · 2026-05-24T16:41:12.552363+00:00 · methodology

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