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

¹ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano, 7100 Talcahuano, Chile
² INAF-OATs, Via G.B. Tiepolo 11, Trieste I 34143, Italy
³ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
⁴ Institute of Fundamental Physics of the Universe, IFPU, Via Beirut, 2, Trieste I-34151, Italy
⁵ Dipartimento di Fisica, Università degli Studi di Trieste, via Tiepolo 11, I-34143 Trieste, Italy
⁶ INFN, Sezione di Trieste, Via A. Valerio 2, 34127 Trieste, Italy

^⋆ Corresponding author: lorenzo.monaco@unab.cl

Received: 20 December 2024
Accepted: 8 March 2025

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.