Linelist used for the analysis of BH3

We investigate the chemical abundances of the companion in order to constrain the formation of this remarkable system. We perform a detailed analysis of high-resolution ESO-UVES spectra of the companion. 51 elements from lithium to uranium were investigated through spectral synthesis, including 15 treated in NLTE.

We compare the resulting pattern to r-process enriched stars, to nucleosynthesis models and to stars of the ED-2 stream, from which BH3 is thought to originate. The abundance pattern of the BH3 companion is consistent with that of r-I stars and is well reproduced by a combination of core-collapse supernova yields and an r-process component. The chemical patterns of four ED-2 stars closely match that of the companion, particularly after accounting for different levels of mixing of the enriched material with the ambient gas.