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No adjustment to FeH gf-values gives the best match of M-dwarf metallicities to binaries, clusters, and interferometric temperatures.

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2026-07-31 05:08 UTC pith:A2Z3XUZU

load-bearing objection Solid multi-benchmark DR19 test that favors leaving Hargreaves FeH gf-values alone; diffusion on the G primaries is a real but modest soft spot, not a collapse of the claim. the 3 major comments →

arxiv 2607.24948 v1 pith:A2Z3XUZU submitted 2026-07-27 astro-ph.SR

A Test of FeH Line Parameters using DR19 APOGEE spectra of Benchmark M Dwarfs

Anderson Silva-Andrade , Diogo Souto , Katia Cunha , Verne V. Smith , Carlos Allende Prieto , Ricardo P. Schiavon , Ver\'onica Loaiza-Tacuri , Aida Behmard
show 4 more authors
Dmitry Bizyaev Ilija Medan Dan Qiu B\'arbara Rojas-Ayala
This is my paper
classification astro-ph.SR
keywords M dwarf starsspectral line listsfundamental parameters of starsmetallicityFeHAPOGEEbenchmark stars
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

Earlier H-band work on M dwarfs reported iron abundances from FeH lines up to 0.2 dex lower than those from atomic Fe I. This paper tests whether that offset is caused by wrong transition probabilities in the FeH line list. It analyzes APOGEE spectra of three independent benchmark sets: 18 wide binaries with a G primary and M secondary, four M dwarfs with interferometric diameters, and six M members of the Hyades and Coma Berenices. Using the same 1-D LTE models and the unaltered Hargreaves et al. FeH list, the authors recover metallicities that agree between binary components to 0.06 dex and that match published optical cluster values. Spectroscopic and interferometric temperatures also agree within the errors. Forcing the temperatures into exact coincidence by lowering all FeH gf-values by ~0.2 dex improves the temperature match but degrades the metallicity matches. The authors therefore conclude that the original FeH gf-values already provide the most consistent overall scale.

Core claim

The best simultaneous concordance among binary metallicities, open-cluster metallicities, and interferometric effective temperatures is obtained when the FeH gf-values taken from Hargreaves et al. (2010) are left unchanged; a systematic downward shift of those gf-values that forces exact Teff agreement worsens the metallicity comparisons.

What carries the argument

Fe I–FeH equilibrium: stellar parameters for each M dwarf are fixed by requiring that iron abundances derived from atomic Fe I lines and from selected E–A FeH lines are identical, using 1-D LTE MARCS atmospheres and the baseline APOGEE line list.

Load-bearing premise

That one-dimensional LTE atmospheres and a single fixed microturbulence fully capture how Fe I and FeH lines form in M-dwarf H-band spectra, so any remaining abundance tension can be blamed on the gf-values themselves.

What would settle it

A larger set of M dwarfs with both precise interferometric diameters and independent high-resolution optical metallicities that still show a systematic Fe I–FeH offset after the same analysis, or that require a non-zero mean gf correction to restore simultaneous Teff and [Fe/H] agreement.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • The unaltered Hargreaves FeH list can be used for large APOGEE M-dwarf abundance catalogs without a global gf zero-point shift.
  • M-dwarf metallicities placed on this scale are directly comparable to G-dwarf and open-cluster scales at the ~0.06 dex level.
  • Previously reported 0.1–0.2 dex Fe I–FeH offsets are more likely to reflect analysis systematics than laboratory line-strength errors.
  • Future line-list updates should preserve the present zero-point unless they demonstrably improve both temperature and metallicity benchmarks at once.

Where Pith is reading between the lines

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

  • If 3-D or non-LTE effects later prove important, they may absorb the residual ~70 K temperature offset without requiring gf changes.
  • The same benchmark logic can be applied to other hydride molecules (CrH, MgH) that appear in cooler M and L dwarfs.
  • Homogeneous re-reduction of the full APOGEE M-dwarf sample with these fixed gf-values would tighten Galactic chemical-evolution constraints at the lowest masses.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 10 minor

Summary. The paper tests the FeH E4Π–A4Π line list of Hargreaves et al. (2010), as implemented in the APOGEE DR19 line list, against three benchmark samples: 18 wide G+M binaries, 4 M dwarfs with interferometric angular diameters, and 6 M dwarfs in the Hyades and Coma Berenices. Using 1-D LTE MARCS/Turbospectrum synthesis via BACCHUS, with M-dwarf Teff and log g set by enforcing Fe I–FeH abundance equality, the authors find: G- and M-dwarf binary metallicities agree at 0.01±0.07 dex; Hyades and Coma Berenices M-dwarf means (+0.08±0.04, +0.02±0.08) match optical literature values; spectroscopic Teff agrees with interferometric Teff at +72±79 K. Line-by-line astrophysical gf corrections from the binaries average ⟨δ log gf⟩ = −0.016±0.055 with no trends in wavelength, χ, or log gf. Forcing the interferometric Teff scale via δ log gf ≈ −0.2 dex degrades the binary and cluster metallicity agreement. The authors conclude the baseline FeH gf-values require no adjustment.

Significance. FeH is one of the few diagnostics available for M-dwarf temperatures and metallicities in the APOGEE H-band, and the claimed 0.1–0.2 dex Fe I–FeH mismatch has propagated into several published analyses. An empirical, externally anchored validation of the Hargreaves et al. (2010) E–A4Π gf-values is therefore directly useful to the M-dwarf and galactic-archaeology communities. Strengths worth naming: three mutually independent external anchors (co-natal binaries, cluster metallicities, interferometric diameters) rather than internal consistency alone; falsifiable line-by-line astrophysical gf corrections (Table 5); machine-readable abundance tables; full use of public DR19 data and standard codes. If the conclusion survives the corrections requested below, the paper usefully rules out large (≳0.2 dex) FeH gf errors and bounds residual offsets at the ≲0.1 dex level.

major comments (3)
  1. [§5.2.2 / §6] The binary test compares present-day G-primary photospheric [Fe/H] against the M-secondary, but the correct benchmark is the pair's common initial composition. The manuscript itself cites Dotter et al. (2017) depletions of -0.03 to -0.10 dex for G dwarfs, yet does not propagate this. Since lowering FeH log gf raises the fitted M-dwarf abundance, a diffusion correction would shift the binary-preferred adjustment from <delta log gf> = -0.016 +/- 0.055 to roughly -0.05 to -0.12 dex — overlapping the Hyades formal minimum (-0.04), the Coma Berenices result (-0.120 +/- 0.11), and the direction favored by the interferometric Teff comparison. The headline conclusion ('best overall concordance... no adjustments', Abstract and Section 6) is therefore contingent on an assumption the paper itself flags as wrong in sign. Required: quantify the shift in <delta log gf> over the plausible diffusion ran
  2. [§5.2.1–5.2.3] With the diffusion correction applied, all four external anchors lean mildly negative: binaries (shifted), Hyades formal best at -0.04 (Fig. 6 bottom, §5.2.3), Coma Berenices at -0.120 +/- 0.11 (Table 5), and interferometry at -0.18 (with large uncertainty: propagating the +72 +/- 79 K offset through the stated ~50 K per 0.1 dex sensitivity gives delta log gf ≈ -0.14 +/- 0.16). The paper weights the binaries above the clusters ('to a lesser degree', §6) without justification and never presents a combined constraint. The interferometric anchor is n=4 and <1 sigma from zero, and the Coma Berenices gf test uses a single star; these limitations should be stated where the results are used, not only implied. Requested: a short joint analysis (even a simple weighted combination with the diffusion range treated as a systematic) showing the interval of delta log gf consistent with all anchors, an
  3. [§5.2.3 / Table 5] The single Coma Berenices M dwarf is calibrated against K-dwarf Fe I abundances from Souto et al. (2021). Yet §3.2 excludes K-dwarf primaries from the binary sample precisely because APOGEE K-dwarf abundances are systematically suspect (citing Grilo et al. 2024). The Souto et al. (2021) analysis is an independent Fe I line analysis rather than ASPCAP, so the two statements may be compatible, but the manuscript never reconciles them. Given that this single star yields the most negative preferred adjustment (-0.120 +/- 0.11), the reliability and systematics of its metallicity benchmark need explicit discussion, or the test should be presented as strictly weaker evidence than the binaries.
minor comments (10)
  1. [Abstract] Abstract states G- vs M-dwarf binary agreement of '0.06 dex', while §5.2.2 and Fig. 8 report <delta> = 0.01 +/- 0.07 dex. Reconcile the two numbers or specify which statistic (mean offset vs. scatter) the abstract quotes.
  2. [§5.2.1 vs §6] The adjustment required to force the interferometric Teff scale is quoted as -0.2 dex in §5.2.1 and -0.18 dex in §6. Use one value, with an uncertainty propagated from the +/-79 K scatter.
  3. [§5.2.1] Text says negative delta(log gf) 'systematically increase[s] the metallicity discrepancies', but Fig. 6 (bottom) and §5.2.3 put the formal minimum at -0.04 dex, not 0. The wording should state where the minimum lies and that it is within the uncertainties of zero.
  4. [§5.1] 'The lack of any discernible trend with Teff suggests... that non-LTE effects are likely small.' Absence of a trend across a mixed G+M sample does not constrain non-LTE in either subsample; soften or remove.
  5. [Figure 1] Caption says the left panels show log gf vs. wavelength, but the rendered left panels appear to show normalized flux (0.8–1.0) vs. wavelength, duplicating the right panels. Please check that the intended log gf panels are included; the log gf distributions and the E-A/F-X line-density argument are referenced from this figure.
  6. [Table 1] Column header 'D0' (constant 2.410) is never defined — presumably the dissociation energy in eV used in the gf conversion; state this. The '<delta(x-y)>' notation in §5.2.2/Fig. 8 is also undefined; define x and y explicitly.
  7. [§4] xi = 1.00 +/- 0.25 km/s is held fixed for all stars. A brief note on the sensitivity of the derived delta log gf adjustments to this choice would strengthen the systematic budget, since Fe I and FeH lines respond differently to xi.
  8. [Figure 5] Fig. 5 bottom-left labels give '[Fe/H] = 0.06 +/- 0.06' (giants) and '0.05 +/- 0.02' (dwarfs) while the text quotes differences of -0.06 and -0.05 dex; check sign conventions between figure and text.
  9. [§4.1] Since Teff and log g are set by enforcing Fe I = FeH, internal Fe I–FeH agreement is by construction; stating this explicitly in §4.1 would help readers see why the external anchors carry the full weight of the test.
  10. [§5.2.1 / Acknowledgments] Typo: 'uncertanties'. The acknowledgement thanking the referee is premature for a submitted draft. Citation 'S. Collaboration et al. 2025' needs proper author list formatting for the DR19 paper.

Circularity Check

1 steps flagged

Mild by-construction Fe I–FeH equality in the Teff/log g method; the load-bearing gf-value conclusion is tested on external binary, cluster, and interferometric anchors.

specific steps
  1. self definitional [Section 4.1 (Effective Temperatures and Surface Gravities); Figure 4]
    "We derived stellar parameters for the M dwarfs by enforcing consistency between the iron abundances obtained from Fe I and FeH lines. We follow the procedure of D. Souto et al. (2018). The method consists of iteratively varying first Teff and then log g, while deriving the corresponding iron abundances. ... In this approach, we determine a Teff that yields consistent abundances from both indicators (Fe I and FeH)."

    Teff and log g are defined as the intersection where A(Fe I)=A(FeH). Therefore internal equality of Fe I- and FeH-based iron abundances at the adopted parameters is true by construction and cannot be cited as independent evidence that the FeH gf-values are correct. External anchors (binaries, clusters, interferometry) carry the validation; this step only fixes the parameter scale under the baseline line list.

full rationale

The only structural circularity is methodological and limited: M-dwarf Teff and log g are chosen so that mean A(Fe) from Fe I equals mean A(Fe) from FeH, so internal Fe I–FeH abundance agreement at the adopted parameters is true by construction and cannot itself validate the Hargreaves et al. (2010) gf-values. That is standard ionization/molecular equilibrium practice, not a tautological proof of the paper’s claim. The actual claim—that zero global adjustment to those gf-values gives the best overall concordance—is supported by three external comparisons that are not forced by the fit: (i) M-secondary [Fe/H] vs G-primary Fe I [Fe/H] in 18 wide binaries (mean difference ~0.01±0.07 dex; line-by-line mean δlog gf ≈ −0.016±0.055), (ii) Hyades/Coma Berenices M-dwarf means vs independent optical/APOGEE literature, and (iii) spectroscopic vs interferometric Teff (offset +72±79 K, with forced Teff agreement via δlog gf ~ −0.2 worsening the metallicity matches). Prior Souto/Cunha/Smith papers supply the analysis pipeline and the original Fe I–FeH offset report, but they are not used as a uniqueness theorem that forbids alternatives; the present work re-tests the line list against new benchmarks. Atomic-diffusion caveats on G primaries affect correctness of the binary zero-point, not circularity of the derivation. Score 2 reflects one minor self-definitional step that is not load-bearing for the central conclusion.

Axiom & Free-Parameter Ledger

3 free parameters · 7 axioms · 0 invented entities

The claim rests on standard cool-star spectroscopic machinery (1-D LTE MARCS, Turbospectrum, APOGEE atomic+molecular lists), the Hargreaves et al. FeH E–A line data, fixed microturbulence, ASPCAP parameters for G primaries, and the physical assumptions that wide binaries and open-cluster members share bulk metallicity (modulo diffusion) and that interferometric diameters plus luminosities define Teff. No new particles or forces; free parameters are the usual analysis knobs (ξ, optional global/per-line δ log gf explored and then rejected).

free parameters (3)
  • microturbulence ξ = 1.00 ± 0.25 km s−1
    Fixed at 1.00±0.25 km s−1 for all synthetic spectra following Souto et al. 2017; not fitted per star but adopted by hand and affects line strengths.
  • optional global δ(log gf) for FeH E–A lines = 0 (preferred); −0.18 to −0.2 if Teff forced
    Explored in steps (−0.1, −0.2, …) to force spectroscopic–interferometric Teff agreement; best overall concordance is at δ=0, so the preferred model sets this parameter to zero rather than fitting it.
  • per-line astrophysical δ(log gf) for diagnostic FeH lines = ⟨δ log gf⟩ = −0.016 ± 0.055 (binaries)
    Adjusted line-by-line in binaries and one Coma star to match primary/cluster Fe I; mean near zero (−0.016±0.055) supports no net correction.
axioms (7)
  • domain assumption 1-D LTE plane-parallel MARCS model atmospheres and Turbospectrum radiative transfer suffice for H-band Fe I and FeH in M dwarfs.
    Stated in Analysis §4; no 3-D or non-LTE grids used. Underpins all abundance and parameter inferences.
  • domain assumption Wide binary components share the same bulk iron abundance at formation; residual G-dwarf diffusion is small compared with the tested offsets.
    Motivation in Introduction and §5.2.2; authors discuss Dotter et al. diffusion (Δ[Fe/H]~−0.03 to −0.10) but still treat G primaries as metallicity anchors.
  • domain assumption Open-cluster members are chemically homogeneous at the level of the abundance precision.
    Used in §5.2.3 when matching Hyades/Coma M dwarfs to literature FGK scales.
  • domain assumption Interferometric angular diameters plus Gaia parallaxes and Mann et al. luminosities define a reliable fundamental Teff scale for the four stars.
    §5.2.1; spectroscopic Teff is judged against this scale.
  • domain assumption APOGEE Fe I gf-values (tuned to Sun/Arcturus) are accurate enough that any Fe I–FeH offset should be ascribed to FeH data or analysis, not to atomic Fe I.
    Explicit in §5.2.1 when only FeH gf-values are varied.
  • domain assumption Hargreaves et al. (2010) wavelengths, lower-state energies, and intensity-derived gf-values are the correct physical baseline for E4Π–A4Π FeH in the APOGEE window.
    Section 2 and conclusion; paper tests zero-point offsets to this list rather than replacing the physics.
  • standard math Standard spectral synthesis, chi-square line fitting, and Gaussian-like uncertainty propagation from abundance scatter to Teff/log g.
    BACCHUS methodology §4; intersection of Fe I and FeH A(Fe) trends defines parameters.

pith-pipeline@v1.2.0-grok45-kimik3 · 33850 in / 4206 out tokens · 90421 ms · 2026-07-31T05:08:28.329519+00:00 · methodology

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read the original abstract

Recent studies have suggested a mismatch of up to 0.20 dex between iron abundances derived from Fe I and FeH lines in the H-band spectra of M dwarfs, and in this work we investigate the nature of this possible offset. We analyze near-infrared H-band APOGEE spectra of stars in 18 binaries composed of a G-dwarf primary and an M-dwarf secondary, together with four M-dwarf stars having measured angular diameters from the literature, and six M-dwarf members of the Hyades and Coma Berenices open clusters. These three families of benchmarks were used to evaluate the FeH line list and check for possible systematic uncertainties in the FeH $gf$-values. Our tests used 1-D LTE plane-parallel model atmospheres, a radiative transfer code, and the baseline APOGEE spectral line list to derive metallicities for the binary G-dwarf primaries using Fe I lines, while stellar parameters and metallicities for the M dwarfs used both FeH and Fe I lines. The mean metallicity obtained for the Hyades M-dwarfs was $\langle$[Fe/H]$\rangle$=+0.08$\pm$0.04, and for Coma Berenices $\langle$[Fe/H]$\rangle$=+0.02$\pm$0.08. The metallicities of the G- and M-dwarfs in binary systems showed excellent agreement (0.06 dex), and the mean metallicities for the open clusters were also consistent with literature values from high-resolution optical analyses. We investigated the consistency between the spectroscopic and interferometric $T_{\rm eff}$ scales, finding agreement within the uncertainties. Forcing full agreement between the spectroscopic and interferometric $T_{\rm eff}$ scales resulted in a poorer match for the metallicities in the binaries and the open clusters. We conclude that the best overall concordance is obtained when no adjustments are made to the FeH $gf$-values, which are based on the Hargreaves et al. (2010) line list.

Figures

Figures reproduced from arXiv: 2607.24948 by Aida Behmard, Anderson Silva-Andrade, B\'arbara Rojas-Ayala, Carlos Allende Prieto, Dan Qiu, Diogo Souto, Dmitry Bizyaev, Ilija Medan, Katia Cunha, Ricardo P. Schiavon, Verne V. Smith, Ver\'onica Loaiza-Tacuri.

Figure 1
Figure 1. Figure 1: Left panels: top and bottom panels display log gf as a function of wavelength for R. J. Hargreaves et al. (2010) and M. Dulick et al. (2003) line lists, respectively. The orange circles correspond to the FeH diagnostic lines measured in this study, which are from the R. J. Hargreaves et al. (2010) line list. The shaded area demarcates the interval covered by APOGEE spectra. Note that the overwhelming major… view at source ↗
Figure 2
Figure 2. Figure 2: Synthetic spectra for Teff = 3500 K log g = 5.0 dex and [Fe/H] = 0.0 dex, excluding the FeH line list from R. J. Hargreaves et al. (2010) (blue) or M. Dulick et al. (2003) (red). The impact of the R. J. Hargreaves et al. (2010) list for the E 4Π – A4Π electronic transition is far more important in the APOGEE region than that of the M. Dulick et al. (2003) list for the F4∆ – X4∆. lected because the APOGEE a… view at source ↗
Figure 3
Figure 3. Figure 3: Color–magnitude diagram of the sample studied in this work. The top/bottom panel shows the 2MASS/Gaia photometry. Primary stars are plotted as dark blue hexagons, and secondary stars as light blue circles, while the interferometric, Hyades, and Coma Berenices stars are shown as red triangles, green squares, and orange diamonds, respectively. Three MIST isochrones of solar age (4.5 Gyr) with different initi… view at source ↗
Figure 4
Figure 4. Figure 4: Left panels: portions of APOGEE spectra for the M dwarf 2M09025200-0040368 (green dotted line). The blue, gray, and red solid lines represent synthetic spectra computed for Teff = 3500, 3600, 3700 K (top panel) and log g = 4.6, 4.8, 5.0 dex (bottom panel). Right panels: illustration of the Teff (top panel) and log g (bottom panel) determinations. The solid and dashed black lines correspond to iron abundanc… view at source ↗
Figure 5
Figure 5. Figure 5: Kiel diagram displaying the stellar parameters of the M dwarfs obtained with the baseline APOGEE line list, along with adopted parameters from ASPCAP for the G dwarfs studied. Three MIST isochrones are shown for metallicities of –0.5, 0.0, and +0.5 dex, solar age (4.5 Gyr). Stellar parameters for dwarf stars, despite scattering due to uncertainties, are in general agreement with theoretical isochrones. Top… view at source ↗
Figure 6
Figure 6. Figure 6: Top panel: The difference between the mean effec￾tive temperature from iron lines and the mean effective tem￾perature calculated from angular diameters for benchmark M dwarfs as a function of systematic changes applied to the log gf of the FeH line list. Middle panel: the derived log g val￾ues for each realization in delta log gf. Bottom Panel: the difference between the average metallicity obtained in thi… view at source ↗
Figure 7
Figure 7. Figure 7: Line-by-line δ(log gf) obtained for the binary stars for FeH transitions as a function of wavelength (λ), excitation potential (χ), and log gf. The dashed horizontal line indicates no change in the FeH linelist. No significant trends are observed with any of the examined parameters. 0.350.300.200.100.000.100.200.300.350.40 [FeH/H] (Secondaries M dwarfs) 0.4 0.3 0.2 0.1 0.0 0.1 0.2 0.3 0.4 [Fe/H] (Primaries… view at source ↗
Figure 8
Figure 8. Figure 8: Comparison of [Fe/H] derived for G-dwarf pri￾maries and their corresponding M-dwarf secondaries. The color bar represents the M dwarfs Teff , and includes a resid￾ual diagram at the bottom. ical abundances provide fiducial benchmarks for stellar metallicity ([Fe/H]). Effective temperatures derived in the four interfero￾metric M-dwarf benchmarks using the Fe I - FeH tech￾nique were found to have marginally … view at source ↗

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