Beryllium enhancement in stars of the accreted Thamnos-2 system

Abstract: Surveys of Galactic halo stars have revealed numerous streams and substructures tracing stellar populations accreted by the Milky Way. Among these, Gaia-Sausage-Enceladus (GSE) and Sequoia are the most prominent, both associated with dwarf galaxies accreted about 10 Gyr ago. We aim to measure beryllium abundances in nine stars associated with Thamnos, a substructure possibly linked to Sequoia, following the discovery of a Be-rich star BPM3066 by Monaco et al (2025). We used Gaia photometry and parallaxes to compute ATLAS9 model atmospheres. Synthetic spectra were generated with Turbospectrum and used with MyGIsFOS in single-model mode to analyze UVES high resolution spectra. Four new stars exhibit a significant beryllium overabundance. Moreover, the two known Be-rich stars, HD 106038 and HD 132475, are also found consistent with Thamnos membership. Thus, all currently known Be-rich stars appear associated with the Thamnos-2 structure. The Be enhancement is accompanied by elevated Si abundances, and we detect a correlation between Be and neutron-capture elements. No comparable Be-rich population is known elsewhere in the Galaxy, pointing to a rare enrichment event. The measured A(Be)/A(Li) excess ratio bears the imprint of spallation reactions, pointing to a highly energetic event in which fast CNO nuclei fragmented upon collision with the surrounding medium. The silicon overabundance is also consistent with a hypernova origin. Such an event may have rapidly enriched the surrounding gas to [Fe/H] about -1.5 explaining the relatively high metallicities of stars formed from this material despite their old ages (about 13 Gyr).