A Sequoia stellar candidate with very high 7Li and 9Be

E. Caffau; G. Cescutti; L. Monaco; P. Bonifacio; P. Molaro

REVIEW 2 major objections 6 minor 108 references

A Sequoia stellar candidate with very high 7Li and 9Be

T0 review · 2 major / 6 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read BPM 3066, a metal-poor dwarf in the retrograde halo, shows a beryllium excess of 2.2 dex over the Galactic trend together with a lithium excess, and a lithium-to-beryllium ratio matching spallation predictions.

desk verdict A useful single-star report with a solid Li excess and a plausible but fragile Be measurement; deserves review if the referee pushes on the 313 nm systematics. read the letter →

arxiv 2504.21823 v1 pith:7CMZTPZ6 submitted 2025-04-30 astro-ph.GA

L. Monaco , E. Caffau , P. Molaro , P. Bonifacio , G. Cescutti This is my paper · ORCID

classification astro-ph.GA

keywords BPM3066lithium-richdwarfberylliumoverabundancespallationnucleosynthesismetal-poorhalostarSequoia/Thamnoschemicalabundancesstellarkinematics

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

BPM 3066, a main-sequence dwarf with $[\mathrm{Fe/H}] \approx -1.5$, has far more lithium and beryllium in its atmosphere than stars of its metallicity normally show. The paper derives $A(\mathrm{Li}) = 3.0$ from two $^7\mathrm{Li}$ lines and $A(\mathrm{Be}) = 2.1$ from the $^9\mathrm{Be}$ resonance lines, corresponding to excesses of about 0.8 dex for Li and 2.2 dex for Be relative to the Spite plateau and the Galactic Be-Fe relation. The measured ratio $^7\mathrm{Li}/{}^9\mathrm{Be} = 7.9$ is close to the value expected if both elements were made by spallation, so the paper argues the two excesses share a single production mechanism. Two origins are considered: spallation in hypernova ejecta, supported by the star's high $[\mathrm{Si/Fe}]$ but difficult to reconcile with other element abundances, and engulfment of rocky planets whose surfaces were enriched by irradiation, which could also explain the neutron-capture enhancements. The star's strongly retrograde, planar orbit places it among candidate members of the accreted Sequoia/Thamnos galaxy, so the unusual light-element pattern is now tied to that accreted population.

What carries the argument

The argument is carried by abundance measurements from high-resolution optical spectra combined with a kinematic classification. Lithium is measured by fitting the $^7\mathrm{Li}$ resonance doublet at 670.78 nm and the 610.36 nm subordinate line; beryllium is measured by fitting the $^9\mathrm{Be}$ II resonance doublet at 313 nm, where the adopted abundance comes from the more isolated 313.1067 nm line and the blended 313.0422 nm feature is checked for consistency. The lithium-to-beryllium ratio then acts as the interpretive pivot: 7.9 is close to the ratio predicted for spallation, and no known stellar or nova process makes beryllium, so the paper infers a common spallation origin. The kinematic membership uses orbit integration and the so-called action diamond, a plane that separates accreted halo populations by orbital actions, to place the star in the Sequoia/Thamnos region.

What would settle it

Take a much longer, higher-signal UVES spectrum of BPM 3066 covering the 313 nm region, refit the $^9\mathrm{Be}$ doublet with an independent line list and continuum model, and compare the derived $A(\mathrm{Be})$ with 2.1; if the refit yields a value below about 1 dex, the claimed excess and the spallation interpretation collapse. A second check would be to measure beryllium from a different spectral feature or with a different synthesis code.

Watch

Extended reading notes

Core claim

The central discovery is that BPM 3066 is simultaneously overabundant in $^7\mathrm{Li}$ and $^9\mathrm{Be}$, with $A(\mathrm{Li}) = 3.0$ and $A(\mathrm{Be}) = 2.1$ at $[\mathrm{Fe/H}] \approx -1.5$. Those values sit about 0.8 dex above the lithium plateau and 2.2 dex above the beryllium-iron relation, making this the largest beryllium excess reported in a metal-poor dwarf. The ratio of the two abundances, $^7\mathrm{Li}/{}^9\mathrm{Be} = 7.9$, is close to the spallation ratio expected from hypernova ejecta in one published model, and the paper interprets the pair as products of a common spallation episode rather than of stellar nucleosynthesis. A kinematic analysis of the star's orbit places it in the retrograde halo and in the region of action space occupied by candidate Sequoia/Thamnos members, so the star connects an unusual chemical signature to an accreted galactic component.

Load-bearing premise

The whole case for the 2.2 dex beryllium overabundance rests on one beryllium line at 313.1067 nm, fit in a spectrum with signal-to-noise near 15, with a blended companion line used only as a consistency check; if the continuum placement or the adopted contaminating lines are wrong, the excess and the spallation ratio could largely disappear.

Editorial extensions

If this is right

BPM 3066 becomes the largest known beryllium excess in a metal-poor dwarf, and its lithium-to-beryllium ratio points to spallation as the common source of both elements.
If the hypernova interpretation is correct, the explosion would have enriched and diluted roughly 8,000 solar masses of interstellar medium, producing the measured oxygen and iron abundances while leaving some element anomalies, such as the high N, Na, Al, Sc, V, and Cu, unexplained.
If the planet-engulfment interpretation is correct, then rocky planets enriched in lithium and beryllium by irradiation can form around a star with metallicity below [Fe/H] = -1, and their engulfment can alter the surface composition of a main-sequence star.
The star's membership among candidate Sequoia/Thamnos stars implies that such light-element overabundances may be a chemical signature worth looking for in other stars of that accreted galaxy.
The paper rules out nova or AGB contamination as the sole explanation, since those sources make lithium but not beryllium.

Reading between the lines

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

If the beryllium measurement is later confirmed with a higher-signal spectrum and an independent treatment of the 313 nm blend, the 2.2 dex excess would establish a new class of light-element-enriched metal-poor stars; if not, the case rests on a single noisy line.
A straightforward test of the two proposed origins would be to survey other candidate Sequoia/Thamnos members for lithium and beryllium: a hypernova origin would predict a population-level correlation between light-element excesses and [Si/O] or neutron-capture abundances, whereas engulfment would predict that such excesses are rare and uncorrelated with the host star's birth environment.
The planet-engulfment channel has a testable analogue in white-dwarf pollution, where lithium and beryllium have been detected in accreted planetesimals; searching for similar enrichment in low-metallicity stars known to host close-in rocky planets could indicate whether such enriched material is common.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

You should know about this one: it is a single-star abundance paper claiming the most extreme Be overabundance in a metal-poor dwarf and the first Li+Be spallation pattern tied to the Sequoia/Thamnos merger. The lithium half is on solid ground; the beryllium half is where the paper lives or dies.

What is actually new: BPM 3066 had GALAH lithium and retrograde kinematics, but this is the first UVES beryllium measurement. The abundance analysis is standard and careful — Li from two lines with GALAH agreement, Be re-reduced manually, and a full 20-element pattern. The paper gives generous credit to the precedent, HD 106038, which shows the same qualitative Li+Be excess, and it is honest about the hypernova model's problems — the required dilution mass is not compatible with expected swept-up mass, and several observed element enhancements contradict the yields. That honesty is worth something.

The soft spot is exactly where the stress-test puts it: A(Be)=2.1 comes from one line, the redder doublet member at 313.1067 nm, at S/N around 15. The bluer line is heavily blended, and the quoted 0.20 dex uncertainty is just the difference between the two feature fits, not a propagated error budget. Continuum placement alone could shift the abundance by several tenths. If A(Be) drops by 0.5 dex, the 2.2 dex excess becomes a more modest 1.7 dex, and the 7Li/9Be=7.9 ratio — the linchpin of the spallation claim — weakens. The paper flags the blend but does not quantify the systematics.

That said, the concern is proportionate. The Li excess is independently confirmed by GALAH, the kinematics are solid, and the paper does not oversell the Sequoia association. This is a report of an extreme object, not a sweeping claim. The strongest conclusion — that BPM 3066 shows simultaneous Li and Be enhancement — likely survives even if the exact magnitude of the Be excess is revised.

Who is this for? Stellar abundance specialists, spallation nucleosynthesis people, and anyone tracking unusual light-element patterns in accreted halo stars. It deserves a serious referee, but the referee should demand a sensitivity analysis of the Be line: continuum placement, gf-value uncertainties, and a full line-list test. I would send it out rather than desk reject.

Referee Report

2 major / 6 minor

Summary. The paper reports a high-resolution UVES/VLT abundance analysis of the metal-poor dwarf BPM 3066, finding A(Li)=3.0 and A(Be)=2.1, which are respectively about 0.8 and 2.2 dex above the Spite plateau and the expected Be-Fe relation at [Fe/H]≈-1.5. The authors also derive abundances for 20 additional elements, finding enhancements in Si, Al, and the neutron-capture elements Sr, Y, Zr, and Ba. Using Gaia astrometry and radial velocities, they characterize the orbit as eccentric, strongly retrograde, and confined to the Galactic plane, and classify the star as a candidate member of the Sequoia/Thamnos accreted galaxy. They discuss two possible origins for the Li and Be overabundances: spallation in a hypernova environment and engulfment of rocky planets rich in spallated Li and Be, noting inconsistencies with the hypernova model for several elements.

Significance. If the abundance measurements are reliable, this is the largest Be excess ever observed in a metal-poor dwarf (2.2 dex above the Galactic Be-Fe relation) and the first case of a combined Li+Be overabundance in a candidate Sequoia/Thamnos member. The observed 7Li/9Be ratio of 7.9 is close to the predictions of spallation synthesis in hypernova ejecta, which would provide a rare observational constraint on light-element production processes. The paper uses a standard, well-documented abundance-analysis pipeline, derives Li from two independent lines with agreement from GALAH, and checks the Be spectral region with a manual re-reduction. The kinematic analysis is careful and makes appropriate use of the Feuillet et al. (2021) selection criteria, including a caveat about the Sequoia/Thamnos nomenclature. However, the central claim of an unprecedented Be excess rests on a single blended line at low signal-to-noise, which is the main weakness that needs to be addressed.

major comments (2)

[Sect. 3.1, Fig. 4, Table 1] The central claim of a 2.2 dex Be excess and the resulting 7Li/9Be ratio of 7.9 rests entirely on A(Be)=2.1 derived from the single Be II 313.1067 nm line at S/N ~ 15. The paper does not test the sensitivity of this abundance to continuum placement, the adopted blend line list (V ii 313.0269, Fe ii 313.0565, Ti ii 313.0798, and weaker lines), or the oscillator strengths, and the quoted 0.20 dex uncertainty in Table 1 is not a propagated error budget. At S/N ~ 15, continuum normalization alone can shift the inferred abundance by several tenths of a dex, and a downward shift of 0.5 dex would reduce the claimed excess to about 1.7 dex and weaken the spallation-ratio argument. The authors should provide a systematics test (for example, varying the continuum placement, using an alternative line list, or fitting the 313.0422 nm blend as the primary diagnostic) and report a realistic error budget, or explicitly state the limitations that this places on the central claim.
[Table 1] The text states that for Be 'we report the difference between the value measured from the two features,' but the actual difference is not given in the table or in the text. If the two Be II lines do not agree within the adopted 0.20 dex uncertainty, the decision to adopt the abundance from the redder line requires additional justification. Please provide the abundance derived from the 313.0422 nm feature, state the line-to-line difference, and discuss whether a single abundance can simultaneously reproduce both the isolated 313.1067 nm line and the blended 313.0422 nm region.

minor comments (6)

[Sect. 3.1, Fig. 4 caption] The text says the Be region is blended with 'V ii and Fe i' lines, while the figure caption lists 'V ii 313.0269, Fe ii 313.0565, Ti ii 313.0798 nm'; please correct the inconsistency between Fe i and Fe ii.
[Sect. 5.2] The phrase 'the mass swapped by the explosion' and 'the mass swapped by an HN' should read 'the mass swept up by the explosion' and 'the mass swept up by an HN' (see also the similar use in the same paragraph).
[Sect. 4, first paragraph] There is a typo: 'Fig, 6' should be 'Fig. 6'.
[Sect. 4] The potential name 'MWPontential2014' should be 'MWPotential2014'.
[Sect. 3.1] The Li abundance is presented as an LTE value with no NLTE correction; since the Li i 670.78 nm line is known to be affected by NLTE in metal-poor dwarfs, a brief statement of the expected NLTE correction (or a reference for its magnitude) would strengthen the comparison with the Spite plateau.
[Sect. 5.2] The hypernova scenario is presented in the abstract as a possible origin, but the discussion itself concludes that the required dilution mass is incompatible with the expected swept-up mass of an HN and that several observed element enhancements (N, Na, Al, Sc, V, Cu) are at odds with the HN predictions; the abstract and conclusions should either temper this scenario to a more explicitly speculative level or quantify its viability relative to the planet-engulfment alternative.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the Li and Be excesses are measured against external empirical baselines, and the only fitted quantity (the hypernova dilution mass) is explicitly shown to be inconsistent with expectations.

full rationale

We walked the paper's full derivation chain. The Li and Be abundances are derived from line-profile fits to the UVES spectra (Sect. 3.1), with independent supports: two Li lines agree, the GALAH Li abundance agrees to within 0.02 dex, and the Be reduction was manually rechecked. The claimed excesses are defined against external baselines: the Spite plateau for Li and the Molaro et al. (2020) Be-Fe relation for Be. Although Molaro and Cescutti are authors of both the present paper and Molaro et al. (2020), that relation is a published empirical calibration based on other stars; it does not include BPM 3066 and is not refitted here, so citing it is independent support rather than circularity. The 7Li/9Be ratio is formed from the measured abundances and compared with theoretical spallation ratios from Fields et al. (2002) and Nakamura & Shigeyama (2004); the theoretical ratio is not derived from the observed ratio. In the hypernova scenario, the dilution mass is indeed fitted to reproduce the Li and Be overabundances, but the paper does not present this as a prediction; it explicitly states that the required dilution is incompatible with the mass expected to be swept up by a hypernova and that the scenario fails for N, Na, Al, Sc, V, and Cu. The O and Fe consistency check uses published HN yields and independently measured stellar abundances. The Sequoia/Thamnos classification uses external kinematic criteria from Feuillet et al. (2021) and Gaia astrometry. No equation or claim reduces to its own input. The single-blended-line Be measurement at S/N about 15 is a real systematic vulnerability, but that is an observational and correctness concern, not a circularity concern.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central measurement rests on standard spectroscopic modeling, external abundance baselines, and public astrometry. The main fitted inputs are the adopted stellar parameters and, in the hypernova scenario, a dilution mass chosen to reproduce the Li/Be excess; that dilution is an interpretation parameter, not evidence for the scenario. No new particles, forces, or conserved quantities are introduced.

free parameters (2)

Stellar atmospheric parameters (Teff, log g, microturbulence) = Teff = 5910 K, log g = 4.29 dex, xi = 1.15 km/s
Derived via photometric colour, Stefan-Boltzmann, and the Mashonkina calibration in Sect. 3; they are inputs to all abundance syntheses and carry systematic uncertainty that is not fully propagated into A(Li) or A(Be).
Hypernova dilution mass = 8 x 10^3 solar masses
Chosen in Sect. 5.2 so that the Fields et al. (2002) spallation yields reproduce the observed Li and Be overabundances; the same mass is used only as a consistency check for O and Fe and is not independently determined.

assumptions (5)

domain assumption LTE, 1D model atmospheres (ATLAS9/ATLAS12) and the adopted line lists reproduce the stellar flux in the Li and Be regions.
Used throughout Sect. 3; no NLTE corrections for Li or Be and no 3D effects are computed, so systematic offsets in A(Li) and A(Be) are possible.
domain assumption The expected Li and Be at [Fe/H] = -1.5 are given by the Spite plateau and the Molaro et al. (2020) Be-Fe relation.
The quoted 0.8 and 2.2 dex excesses are differences from these external baselines in Sect. 5; if the baselines do not apply to Sequoia stars, the excess values change.
domain assumption The Gaia DR3 parallax with the Lindegren zero-point correction and the MWPotential2014 galactic potential give reliable orbital integrals and Sequoia membership.
Used in Sect. 4; the membership assignment depends on the adopted potential, solar peculiar motion, and the Feuillet et al. (2021) action-selection box.
domain assumption Hypernova spallation yields from Fields et al. (2002) and Nakamura & Shigeyama (2004) describe 7Li/9Be production.
The 7Li/9Be = 7.9 comparison in Sect. 5.2 uses these yield models; the alternative planet-engulfment scenario does not rely on this assumption.
domain assumption BPM 3066 is an unevolved main-sequence star, so internal mixing cannot explain the Li excess.
Sect. 5 excludes stellar production on this basis; Fig. 1 shows a main-sequence position but leaves some ambiguity with the sub-giant branch.

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Cite this review

Pith. "Pith review of A Sequoia stellar candidate with very high 7Li and 9Be." pith.science (2026). https://pith.science/paper/7CMZTPZ6

@misc{pith2026250421823,
  author       = {Pith},
  title        = {Pith review of: A Sequoia stellar candidate with very high 7Li and 9Be},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7CMZTPZ6}},
  note         = {Machine review of arXiv:2504.21823}
}

read the original abstract

Aims. The metal-poor star BPM 3066 belongs to the retrograde halo and presents unexpectedly strong spectral features of lithium. To gain insight into the origin of this peculiar abundance, we investigate the chemistry and kinematic properties of this star. Methods. We performed a local thermodynamic equilibrium chemical abundance analysis of UVES/VLT high-resolution spectra of BPM 3066 using ATLAS9 and ATLAS12 model atmospheres and the MyGIsFOS code. We further characterised the orbital properties of the star by integrating its orbit and analysing its integrals of motion using the galpy code. Results. The star BPM 3066 shows an exceptional overabundance of both lithium and beryllium. The abundances are A(Li) = 3.0 and A(Be) = 2.1, which are respectively about 0.8 and 2.2 dex higher than the Li and Be abundances expected at [Fe/H] = -1.5, the metallicity of the star. The observed ratio 7Li/9Be is 7.9, which is close to that expected from a synthesis by spallation processes. Overabundances of Si, Al, and of the neutron capture elements Sr,Y, Zr, and Ba are also measured. Kinematically, BPM 3066 has an eccentric, strongly retrograde orbit, confined to a height lower than 1 kpc from the galactic plane, and it is a candidate member of the Sequoia/Thamnos accreted galaxy. Conclusions. The processes leading to the 7Li and 9Be synthesis could have occurred in the environment of a hypernova. This is supported by some abundance anomalies like the high value of Si, [Si/Fe]=1.2 and [Si/O]=1.1. However, the simultaneous high values of N, Na, Al, Sc, Ti, and Cu are at odds with the expectations from a hypernova. Alternatively, the abundances of BPM 3066 could result from the engulfing of rocky planets that were rich in spallated Li and Be. In both cases, it is remarkable that such an extreme abundance pattern has been found in a star belonging to the Sequoia/Thamnos accreted galaxy.

Figures

Figures reproduced from arXiv: 2504.21823 by the authors.

**Figure 1.** Left and middle panels: V vs (B-V) and Gaia G vs (GBP − GRP) colour-magnitude diagrams (CMDs). Right panel: Kiel diagram, log g vs Teff. Star BPM 3066 is indicated by the filled star. In the right panel, the orange star is for the parameters we adopt here, while the blue one is for the GALAH BSTEP parameters. In all panels, two PARSEC isochrones of metallicity and ages ([M/H], Age)=(-0.71, 11.7) and (- 1.26, 13.0) a… view at source ↗

**Figure 3.** Observed spectrum (solid black) compared to the best fit (solid red) with A(Li) = 3.0. The UVES Li i 670.78 nm region of BPM 3066 is shown in the top panel. The Li i λλ 610.36 nm 22P-32D subordinate transition is shown in the bottom panel; the strong line on the blue side is the Ca i 610.2723 nm line [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 4.** UVES spectrum of BPM 3066 around the 9Be ii λλ 313.0422, 313.1067 nm resonance doublet region. The observed spectrum (solid black) is compared to the synthesis (solid red) on both Be ii 313.0422 and 313.1067 nm resonance lines, with an abundance of A(9Be) = 2.10, the best fit value of the redder line. The 9Be region is a complex one and the stronger line of the Be ii doublet is blended with V ii 313.0269, Fe ii 313.… view at source ↗

Figures from the paper (2 more)

**Figure 6.** Figure 6: Upper panels: Orbit of BPM 3066 in the plane of the Galaxy (Y vs X, right panel) and in the meridional plane (Z vs R = √ X2 + Y 2 , left panel). Galactocentric cartesian coordinates are designated by X, Y, and Z, Z being the height over the Galactic plane. The circle w…

**Figure 7.** Figure 7: Observed abundances of BPM 3066 in red filled squares compared with the hypernova yields from Nomoto et al. (2013). Alexander (1967) proposed that the engulfing of a planet or a brown dwarf could be responsible for the Li-rich giants and this idea was then revised by …

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