Thamnos-2 system stars abundances

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]=~-1.5, explaining the relatively high metallicities of stars formed from this material despite their old ages (~13Gyr)