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REVIEW 3 major objections 5 minor 32 references

The Gaia RVS Benchmark Stars. III . Confirmed and New High Radial Velocity Stars From Gaia DR3

T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Follow-up spectra confirm 25 of 26 extreme radial velocities from Gaia DR3.

desk verdict A solid confirmation of extreme Gaia RVs with useful new abundance data, despite a transparency gap on the UVES RVs and generally soft abundance systematics. read the letter →

arxiv 2507.07558 v1 pith:LJKRXYGS submitted 2025-07-10 astro-ph.GA

Elisabetta Caffau (LIRA) , David Katz (LIRA) , Piercarlo Bonifacio (LIRA) , Ana Gómez (LIRA) , Rosine Lallement (LIRA) , Paola Sartoretti (UNIDIA) , Frédéric Royer (LIRA) , Pasquale Panuzzo (UNIDIA
show 8 more authors
PISI) Monique Spite (LIRA) Patrick François (LIRA) Nicolas Leclerc (UNIDIA) Luca Sbordone (ESO) Frédéric Thévenin Hans-günter Ludwig (LSW) Laurent Chemin (ObAS)
This is my paper · ORCID
classification astro-ph.GA
keywords GaiaDR3high-velocitystarsradialvelocitymetal-poorstellarabundancesneutron-captureelementsGalactichaloUVESspectroscopy
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

The paper asks whether Gaia DR3's most extreme radial velocities, those above 500 km/s in absolute value, are real or artifacts of low signal-to-noise measurements. To answer it, the authors took high-resolution UVES spectra of 26 such stars and found that 25 of them do have the high radial velocity that Gaia reported; the lone exception, RVS929, shows a complex multi-component spectrum. The confirmed stars are all metal-poor and alpha-enhanced, as expected for a halo population, and their chemistry includes a rich set of neutron-capture elements. On this evidence, extreme Gaia radial velocities in this regime are overwhelmingly reliable, and kinematic selection is an efficient route to finding metal-poor halo stars.

What carries the argument

The load-bearing technique is template matching of the UVES spectra in the 840--880 nm region, the same wavelength range as Gaia's RVS, which avoids systematic differences between the two velocity measurements. The observed spectrum is cross-correlated against synthetic templates computed with the SYNTHE code and ATLAS9 model atmospheres, and the resulting radial velocity is compared directly with the Gaia DR3 value. For RVS929, the cross-correlation function shows at least two peaks rather than one, identifying it as a composite system and explaining why the Gaia RVS value is spurious.

What would settle it

Take a second-epoch UVES spectrum of the 25 confirmed stars, with special attention to RVS918, RVS1280, and RVS928, which show pulsational or shell signatures; a large radial-velocity shift between epochs, or the appearance of a companion's spectral lines, would show that the single-epoch confirmation was coincidental.

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Extended reading notes

Core claim

The paper's central claim is that the extreme absolute radial velocities reported in Gaia DR3 for this sample are genuine: 25 of the 26 stars observed with UVES have radial velocities matching Gaia's values above 500 km/s, and the only failure, RVS929, is a composite system whose emission lines corrupt the Gaia RVS measurement. The confirmed stars are metal-poor and $\alpha$-enhanced, spanning roughly $[\mathrm{Fe}/\mathrm{H}] = -1$ to $-2.4$, and they display a large star-to-star scatter in the abundances of neutron-capture elements. For samarium, the paper finds a statistically significant correlation with europium and no correlation with yttrium, which it interprets as evidence that Sm is produced only by the r-process at these metallicities, unlike in the solar system.

Load-bearing premise

The single-epoch template-matching radial velocity is assumed to be the star's true line-of-sight motion, so a confirmed star that is actually an unresolved binary or a pulsating variable could show an extreme velocity at only one epoch.

Editorial extensions

If this is right

  • Stars selected by Gaia DR3 radial velocities above 500 km/s can be treated as a reliable sample of genuine high-velocity objects rather than spurious measurements.
  • The confirmed stars enlarge the known census of metal-poor, alpha-enhanced halo stars in the metallicity range about $-1$ to $-2.4$.
  • The newly measured neutron-capture abundances, including rare elements like Rb, Ce, Sm, Dy, and Os, give new constraints on r- and s-process nucleosynthesis in metal-poor giants.
  • The correlation analysis implies that samarium is produced only through the r-process in this metallicity regime, a departure from the solar-system mix.
  • RVS929 becomes a cautionary case showing that a composite emission-line system can mimic an extreme radial velocity in the Gaia RVS.

Reading between the lines

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

  • If the same confirmation rate held across the whole of Gaia DR3, the global false-positive rate for $|V_r| > 500$ km/s would be a few percent, and the catalogue could be mined for hundreds of genuine extreme-velocity stars; the paper does not quantify this global rate.
  • Because the confirmation rests on a single epoch, binarity remains an open channel: a second-epoch spectrum with a baseline of months to years could reveal orbital motion that lowers a star's systemic velocity.
  • The paper's suggestion that the two high-sodium, low-oxygen stars may have escaped from a globular cluster is testable by looking for the parent cluster or by measuring additional light-element anticorrelations such as Mg-Al.
  • The r-only interpretation of samarium is a prediction that could be checked by comparing larger samples of metal-poor giants with updated nucleosynthesis yields.
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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

3 major / 5 minor

Summary. The paper reports UVES follow-up spectroscopy of 26 stars from Gaia DR3 with absolute radial velocities above 500 km/s. The authors confirm the extreme radial velocity for 25 of the 26 stars and identify RVS929 as a complex, likely SB2 system whose Gaia velocity is unreliable. For the 25 confirmed stars, they derive stellar parameters and chemical abundances, finding a sample of metal-poor, alpha-enhanced evolved stars that also shows a large star-to-star scatter in neutron-capture element abundances, including measurements of Rb, Ce, Sm, Dy and Os. The paper presents the chemical results as consistent with a mixture of s- and r-process nucleosynthesis and highlights two stars with possible globular-cluster-like Na-O anticorrelation. The central claim is that Gaia DR3's extreme radial velocities are overwhelmingly reliable and that the confirmed stars constitute a valuable set of metal-poor halo stars.

Significance. If the radial-velocity confirmation is sound, the paper provides a valuable empirical validation of Gaia DR3's extreme-velocity measurements, a regime where false positives are expected to be common. The result also adds a well-characterised sample of 25 metal-poor, alpha-enhanced halo giants with a rich neutron-capture inventory, including rare elements such as Os; the CDS tables of abundances and line data are a useful resource. The authors are transparent about many limitations, including low signal-to-noise spectra and variable stars, and they carefully exclude RVS929 from the confirmed sample. However, the central claim rests on a single-epoch UVES velocity measurement that is not reported numerically and that is deliberately derived from the same 840-880 nm wavelength range as the Gaia RVS, which weakens the independence of the confirmation.

major comments (3)
  1. [Section 3.1, Fig. 2] The UVES radial velocities are not reported in any table, only compared graphically as absolute values. Because the central claim is that 25 of 26 Gaia extreme radial velocities are confirmed, the numerical UVES values and their uncertainties must be made available. Without them the confirmation is not reproducible, and the size of any discrepancy between the UVES and Gaia measurements cannot be assessed.
  2. [Section 3.1] Using the 840-880 nm range, which the authors state is 'roughly equal to the RVS wavelength range', makes the UVES confirmation non-independent. A systematic error in the Ca II triplet rest wavelengths, in the template synthesis, or in the continuum placement would shift both the Gaia and UVES velocities by the same amount, so the agreement shown in Fig. 2 cannot by itself rule out a common bias. The authors should validate the UVES radial velocities with the blue-arm spectra (373-499 nm) or with lines outside the Ca II triplet and report the result; this is a load-bearing check for the paper's central claim.
  3. [Section 3.2, Table 1] Several stars have very low signal-to-noise ratios at 450 nm (RVS926 has S/N=3, RVS923 S/N=8, RVS932 S/N=6, RVS930 S/N=11, RVS924 S/N=13), yet their [Fe/H] values are listed with formal uncertainties of 0.12-0.30 dex. For these stars the Fe I lines used by MyGIsFOS are at or below the detection limit, so the reported metallicities and the blue-sensitive abundances cannot be reliable. The paper should state explicitly how the abundances were derived in these cases, indicate which stars are excluded from the averaged ratios in Section 5, and ensure that the correlations in Section 4 are not dominated by the least reliable measurements.
minor comments (5)
  1. [Section 5] In the sentence about Cu, the notation reads 'Cu (⟨[Mn∕Fe]⟩= −0.45±0 .16)'; this should be '⟨[Cu/Fe]⟩' rather than '⟨[Mn/Fe]⟩'.
  2. [Section 2] The phrase 'on gasgano version1' should read 'on gasgano version 1'.
  3. [Section 3.3.9] The parameters '𝑇eff= 4327 and log 𝑔=1.20' are missing their units (K and cgs, respectively); this should be stated for consistency with Table 1.
  4. [Table 2] The declination of RVS1281, '–50 08 56.77', contains a space and should follow the same sexagesimal format as the other entries.
  5. [Section 3.3.1, 3.3.7] For the variable stars RVS918 and RVS1280, a single-epoch UVES velocity agreeing with Gaia does not demonstrate that the systemic velocity is extreme if the velocity varies on the pulsation timescale; the paper should state this caveat explicitly.

Circularity Check

0 steps flagged · score 1.0 of 10

Independent UVES confirmation; no circularity beyond minor self-citations.

full rationale

The paper's central claim is that 25 of 26 high-|Vr| Gaia DR3 candidates are confirmed by independent UVES spectroscopy. The UVES radial velocities are derived by the authors' own template-matching code, using synthetic spectra computed with SYNTHE and ATLAS9, applied to the UVES observations alone. No parameter is fitted to the Gaia radial velocities, and the UVES velocities are not defined in terms of the Gaia values. The explicit choice to use the 840-880 nm region (Section 3.1) is motivated by a desire to avoid line-set-dependent systematic differences between the two measurements, not to force agreement; the comparison in Fig. 2 is still an empirical test. The abundance analysis reuses the authors' own pipeline (MyGIsFOS, ATLAS9, Turbospectrum) and references Caffau et al. (2024c) for systematics, but these references are not load-bearing for the kinematic confirmation. No uniqueness theorem or ansatz is imported via self-citation. The paper is self-contained in its verification against Gaia data and its benchmarks against literature abundances (e.g., comparison with SAGA for Os). The only minor issue is that the UVES radial velocities are not tabulated, but that affects reproducibility, not circularity. Therefore, no circular step reducing the result to its inputs is present.

Assumptions & free parameters 4 free parameters · 6 assumptions · 0 invented entities

The central RV confirmation rests on the stability of the targets and the fidelity of the template matching. The chemical analysis depends on several adopted parameters (mass, microturbulence, extinction) and standard assumptions (LTE, 1D models, pure s/r tracers). The mixing-length explanation for the CMD offset is an ad hoc premise introduced to reconcile the data with the models.

free parameters (4)
  • Assumed stellar mass = 0.8 Msun
    Used in the Stefan-Boltzmann equation with the Gaia parallax to derive log g (Section 3.2). The gravity directly affects the abundance analysis; a different mass would change [X/Fe].
  • Microturbulence xi = Mashonkina et al. (2017) calibration for most stars; 2.0 km/s for RVS1383
    Adopted because weak Fe I lines were often too weak to derive xi from line-to-line flatness (Section 3.2). Microturbulence affects derived abundances, especially of strong lines.
  • First-guess metallicity = -1.5
    Initial guess used to derive stellar parameters, then updated with MyGIsFOS; the final parameters depend on the converged value but the initialization is a choice.
  • Extinction A0 = From Vergely et al. (2022) 3D maps, per star
    Used to de-redden photometry for Teff. For stars with |Z|>400 pc, extinction beyond 400 pc is set to zero, which is an assumption that could bias Teff and abundances.
assumptions (6)
  • domain assumption LTE and 1D model atmospheres (ATLAS9) are adequate for the abundance analysis
    The analysis uses synthetic spectra computed with ATLAS9/Turbospectrum under LTE (Section 3.2); NLTE corrections are only partially applied and are propagated from previous papers.
  • domain assumption Y is a pure s-process element and Eu is a pure r-process element
    Used in Section 4 to interpret correlations of [X/Fe] with [Y/Fe] and [Eu/Fe] as s- or r-process signatures.
  • domain assumption The extinction beyond |Z|=400 pc is negligible
    Section 3.2 states this for stars farther from the Galactic plane; for RVS1281 with A0=3.5 this could be significant and is not tested.
  • domain assumption A single-epoch UVES radial velocity confirms the stellar radial velocity; the stars are assumed non-variable in RV except RVS929
    Several stars are long-period variables or show shell/wind signatures (RVS918, RVS1280, RVS928); the confirmation rests on one epoch.
  • ad hoc to paper The BASTI isochrones with alpha-enhanced composition and mixing length alpha_ML=2.006 are appropriate; the offset with spectroscopic metallicity is due to a lower mixing length
    Section 3 and Figure 1: the isochrones imply higher metallicity than spectroscopy; the paper proposes a lower mixing length without a quantitative test.
  • domain assumption Solar abundances used in the analysis match those used for the isochrones
    Stated in Section 3; this is required for a consistent comparison.

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Cite this review

Pith. "Pith review of The Gaia RVS Benchmark Stars. III . Confirmed and New High Radial Velocity Stars From Gaia DR3." pith.science (2026). https://pith.science/paper/LJKRXYGS

@misc{pith2026250707558,
  author       = {Pith},
  title        = {Pith review of: The Gaia RVS Benchmark Stars. III . Confirmed and New High Radial Velocity Stars From Gaia DR3},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/LJKRXYGS}},
  note         = {Machine review of arXiv:2507.07558}
}
read the original abstract

High-velocity stars are interesting targets to unveil the formation of the Milky Way. In fact they can be recently accreted from an infalling dwarf galaxies or they can be the result of a turbulent merging of galaxies. Gaia is providing the community a way to select stars for their kinematics and the radial velocity, one of the speed components, is derived from the Gaia RVS spectrum. High absolute radial velocity values are sensitive to be false positive. They are rare and as such they are more easily impacted than lower velocity stars, by contamination by very low SNR spurious measures. We here investigate a sample of 26 stars with Gaia absolute radial velocity in excess of 500___km/s with spectroscopic follow-up observations with UVES. For all but one star the extreme radial velocity is confirmed and these stars are all metal-poor and, as expected, enhanced in the elements. The complete chemical inventory confirm the large star-to-star scatter for the heavy elements and their on average larger ratio over iron with respect to the Sun, in agreement with the literature.

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Works this paper leans on

32 extracted references · 18 canonical work pages

  1. [1]

    Aguado,D.S.,Belokurov,V.,Myeong,G.C.,etal.2021,ApJ, 908, L8

    work page 2021

  2. [2]

    & Plez, B

    Alvarez, R. & Plez, B. 1998, A&A, 330, 1109

    1998

  3. [3]

    & Thielemann, F.-K

    Arcones, A. & Thielemann, F.-K. 2023, The Astronomy and Astrophysics Review, 31, 1 Arlandini,C.,Käppeler,F.,Wisshak,K.,etal.1999,ApJ,525, 886

    work page 2023

  4. [4]

    & Lardo, C

    Bastian, N. & Lardo, C. 2018, ARA&A, 56, 83

    2018

  5. [5]

    Deason, A. J. 2018, MNRAS, 478, 611 Bergemann,M.,Gallagher,A.J.,Eitner,P.,etal.2019,A&A, 631, A80 Billi,A.,Ferraro,F.R.,Mucciarelli,A.,etal.2024,A&A,690, A156 Bonifacio,P.,Caffau,E.,François,P.,&Spite,M.2025,arXiv e-prints, arXiv:2504.06335

    arXiv 2018

  6. [6]

    Bonifacio, P., Caffau, E., Monaco, L., et al. 2024, A&A, 684, A91 Boubert,D.,Guillochon,J.,Hawkins,K.,etal.2018,MNRAS, 479, 2789 Caffau,E.,Bonifacio,P.,Monaco,L.,etal.2024a,A&A,684, L4 Caffau,E.,Bonifacio,P.,Monaco,L.,etal.2024b,A&A,691, A245

    work page 2024

  7. [7]

    2020, A&A, 638, A122

    Caffau, E., Monaco, L., Bonifacio, P., et al. 2020, A&A, 638, A122

    work page 2020

  8. [8]

    1999, MNRAS, 303, 265

    Castellani, V., degl’Innocenti, S., & Marconi, M. 1999, MNRAS, 303, 265

    work page 1999

Show all 32 references
  1. [9]

    J., Sneden, C., Lawler, J

    Cowan, J. J., Sneden, C., Lawler, J. E., et al. 2021, Reviews of ModernPhysics,93,015002,publisher:APSADSBibcode: 2021RvMP...93a5002C

    2021

  2. [10]

    Culpan, R., Pelisoli, I., & Geier, S. 2021, A&A, 654, A107 François,P.,Cescutti,G.,Bonifacio,P.,etal.2024,A&A,686, A295 GaiaCollaboration,Brown,A.G.A.,Vallenari,A.,etal.2018, A&A, 616, A1 GaiaCollaboration,Vallenari,A.,Brown,A.G.A.,etal.2022, arXiv e-prints, arXiv:2208.00211 G...

    2021 arXiv

  3. [11]

    A., et al

    Hammer, F., Wang, J., Mamon, G. A., et al. 2024, MNRAS, 527, 2718

    2024

  4. [12]

    D., et al

    Haywood, M., Di Matteo, P., Lehnert, M. D., et al. 2018, ApJ, 863, 113

    2018

  5. [13]

    2021, A&A, 645, A106

    Heiter, U., Lind, K., Bergemann, M., et al. 2021, A&A, 645, A106

    2021

  6. [14]

    H., et al

    Helmi, A., Babusiaux, C., Koppelman, H. H., et al. 2018, Nature, 563, 85

    2018

  7. [15]

    2024, ApJ, 967, 89

    Ibata, R., Malhan, K., Tenachi, W., et al. 2024, ApJ, 967, 89

    2024

  8. [16]

    Kurucz, R. L. 2005, Memorie della Societa Astronomica Ital- iana Supplementi, 8, 14 Lebzelter,T.,Mowlavi,N.,Lecoeur-Taibi,I.,etal.2023,A&A, 674, A15

    2005

  9. [17]

    2021, A&A, 649, A4

    Lindegren, L., Bastian, U., Biermann, M., et al. 2021, A&A, 649, A4

    2021

  10. [18]

    2021, A&A, 656, A155 Lucertini,F., Sbordone,L., Caffau,E., etal

    Lombardo, L., François, P., Bonifacio, P., et al. 2021, A&A, 656, A155 Lucertini,F., Sbordone,L., Caffau,E., etal. 2025,A&A, 695, A36

    2021

  11. [19]

    J., et al

    Manchon, L., Deal, M., Goupil, M. J., et al. 2024, A&A, 687, A146

    2024

  12. [20]

    F., Venn, K

    Martin, N. F., Venn, K. A., Aguado, D. S., et al. 2022, Nature, 601, 45

    2022

  13. [21]

    2017, A&A, 604, A129 Matas Pinto, A

    North, P. 2017, A&A, 604, A129 Matas Pinto, A. d. M., Caffau, E., François, P., et al. 2022, Astronomische Nachrichten, 343, e210032

    2017

  14. [22]

    2021, A&A, 653, A90

    Mucciarelli, A., Bellazzini, M., & Massari, D. 2021, A&A, 653, A90

    2021

  15. [23]

    2017, A&A, 601, A112

    Pancino, E., Romano, D., Tang, B., et al. 2017, A&A, 601, A112

    2017

  16. [24]

    2024, Research Notes of the American Astronomical Society, 8, 111

    Parthasarathy, M. 2024, Research Notes of the American Astronomical Society, 8, 111

    2024

  17. [25]

    2021, ApJ, 908, 102

    Pietrinferni, A., Hidalgo, S., Cassisi, S., et al. 2021, ApJ, 908, 102

    2021

  18. [26]

    2012, Turbospectrum: Code for spectral synthesis, Astrophysics Source Code Library, record ascl:1205.004

    Plez, B. 2012, Turbospectrum: Code for spectral synthesis, Astrophysics Source Code Library, record ascl:1205.004

    2012

  19. [27]

    Preston, G. W. & Sneden, C. 2000, AJ, 120, 1014

    2000

  20. [28]

    Roman, N. G. 1950, ApJ, 112, 554

    1950

  21. [29]

    S., et al

    Ruiz-Lara, T., Matsuno, T., Lövdal, S. S., et al. 2022, A&A, 665, A58 Sartoretti,P.,Katz,D.,Cropper,M.,etal.2018,A&A,616,A6

    2022

  22. [30]

    2014, A&A, 564, A109

    Sbordone, L., Caffau, E., Bonifacio, P., & Duffau, S. 2014, A&A, 564, A109

    2014

  23. [31]

    1986, PASP, 98, 867

    Slettebak, A. 1986, PASP, 98, 867

    1986

  24. [32]

    2008, PASJ, 60, 1159 Vergely,J.L.,Lallement,R.,&Cox,N.L.J.2022,A&A,664, A174 6 APPENDIX: ON-LINE TABLES The detailed abundances are available in a csv table at CDS

    Suda, T., Katsuta, Y., Yamada, S., et al. 2008, PASJ, 60, 1159 Vergely,J.L.,Lallement,R.,&Cox,N.L.J.2022,A&A,664, A174 6 APPENDIX: ON-LINE TABLES The detailed abundances are available in a csv table at CDS. The atomic lines used in the analysis are presented in an ASCII table ...

    2021

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