We report the first measurement of the odd-isotope fractions for barium, f odd,Ba in two extremely metal-poor stars: a CEMP-r/s star HE 0338-3945 ([Fe/H] = −2.42 ± 0.11) and an r-II star CS 31082-001 ([Fe/H] = −2.90 ± 0.13). The measured f odd,Ba values are 0.23 ± 0.12 corresponding to 34.3 ± 34.3% of the r-process contributions for HE 0338-3945 and 0.43 ± 0.09 corresponding to 91.4 ± 25.7% of the r-process contribution to Ba production for CS 31082-001. The high r-process signature of barium in CS 31082-001 (91.4 ± 25.7%) suggests that the majority of the heavy elements in this star were synthesised via an r-process path, while the lower r-process value (34.3 ± 34.3%) found in HE 0338-3945 indicates that the heavy elements in this star formed through a mix of sprocess and r-process synthesis. These conclusions are consistent with studies based on AGB model calculations to fit their abundance distributions.

No abstract