Beryllium enhancement in stars of the accreted Thamnos-2 system

Technical Summary: Beryllium Enhancement in Stars of the Accreted Thamnos-2 System

Problem and Context
Galactic halo surveys have identified numerous stellar streams and substructures, such as Gaia-Sausage-Enceladus (GSE) and Sequoia, which represent dwarf galaxies accreted by the Milky Way approximately 10 Gyr ago. While these structures offer unique opportunities to study extragalactic chemical evolution in situ, trace elements like beryllium ($^9$Be) are difficult to measure in external galaxies. $^9$Be is produced exclusively through spallation reactions involving energetic cosmic rays and CNO nuclei. Previous studies noted distinct $^9$Be enrichment histories in different populations, such as the GSE. Recently, Monaco et al. (2025) reported an anomalously high beryllium abundance in the star BPM 3066, a member of the Sequoia system, suggesting a rare enrichment event. However, the origin of this anomaly (e.g., planetary engulfment vs. hypernova) remained unconstrained, and the extent of this phenomenon within the associated Thamnos substructure was unknown.

Methodology
The authors analyzed a sample of nine stars associated with the Thamnos substructure (specifically the Thamnos-2 component), alongside two previously known Be-rich stars (HD 106038 and HD 132475) and the previously studied BPM 3066.

• Target Selection: Candidates were drawn from the GALAH value-added catalog using dynamical criteria (Feuillet et al. 2021) and kinematic constraints to isolate Thamnos members. Selection criteria included effective temperatures ($T_{\text{eff}} > 5700$ K) and surface gravities ($\log g > 3.65$) to avoid stars affected by Be depletion. High-quality astrometry (Gaia DR3) and reliable spectral flags were required.
• Observations: High-resolution spectra were obtained using the UVES spectrograph at the ESO VLT (Programme 111.24HT), focusing on the Be II resonance doublet at 313.0 nm. Archival UVES data were utilized for HD 106038 and HD 132475 to ensure homogeneity.
• Analysis: Stellar parameters were derived using Gaia photometry and parallaxes, refined via the MyGIsFOS code. Model atmospheres were computed with ATLAS9. Synthetic spectra were generated using turbospectrum and analyzed with MyGIsFOS in single-model mode. Direct line-profile fitting using the proprietary code abbofit was employed for Be II and Li I lines due to significant blending in the 313.0 nm region.

Key Results

1. Discovery of Be-Rich Population: Four new stars in the sample (TS 1, TS 2, TS 4, TS 6) exhibit significant beryllium overabundances (0.4–0.7 dex above the standard Be–Fe relation). Two additional stars (TS 3, TS 5) show moderate enhancements. In contrast, two metal-poor stars (TS 8, TS 9), likely members of the Thamnos-1 component, show Be abundances consistent with typical Galactic halo stars.
2. Kinematic Association: All Be-rich stars, including the re-analyzed HD 106038 and HD 132475, occupy the same low-energy, retrograde orbital region in the ($E$–$L_z$) plane, confirming their membership in the Thamnos-2 structure.
3. Correlations with Other Elements: The Be enhancement is accompanied by elevated Silicon (Si) abundances. A strong positive correlation is detected between Be abundance and neutron-capture elements (Ba, Sr, Y, Zr) and Si. Conversely, elements such as Cr, V, Ca, and O show no correlation with Be, with O specifically showing no enhancement despite being a primary spallation target.
4. Lithium and Spallation Signatures: In the most extreme cases (BPM 3066 and HD 106038), the ratio of excess Be to excess Li is approximately 8. This ratio matches the theoretical spallative cross-section ratio for energetic CNO nuclei colliding with protons and $\alpha$-particles, strongly indicating a spallation origin for both elements.
5. Stellar Ages: Derived ages for the Thamnos-2 stars are remarkably old (11.8–13.5 Gyr). Standard Galactic chemical evolution predicts negligible Be at these ages and metallicities, yet these stars exhibit high Be abundances, implying a prompt enrichment event rather than gradual accumulation.

Significance and Conclusions
The paper concludes that the Thamnos-2 structure hosts a distinct population of Be-rich stars originating from a single, highly energetic event, likely a hypernova.

• Hypernova Origin: The simultaneous enhancement of Be, Li, Si, and neutron-capture elements, combined with the specific Be/Li excess ratio, supports a scenario where a hypernova accelerated CNO nuclei to high energies. These nuclei fragmented via spallation upon colliding with the surrounding interstellar medium, producing Be and Li.
• Rapid Enrichment: The event rapidly enriched the surrounding gas to $[\text{Fe/H}] \simeq -1.5$ before the formation of the observed stars. The varying degrees of Be enhancement among the stars are attributed to different dilution factors of this enriched material with pristine gas.
• Chemical Tagging: The presence of Be enhancement serves as a powerful chemical tag to distinguish Thamnos-2 members from the more metal-poor, Be-normal Thamnos-1 population.
• Cosmological Context: The Li abundances in these accreted stars align with the Galactic halo "Spite plateau," reinforcing the universality of the cosmological lithium problem for old, metal-poor stars regardless of their extragalactic origin.

The authors emphasize that no comparable Be-rich population is known elsewhere in the Galaxy, highlighting this as a rare, localized enrichment event captured in the stellar record of the accreted Thamnos system.