Isotopic fractions of europium, barium, and samarium in metal-poor stars

We present our analysis of the isotopic fractions of europium (Eu), barium (Ba), and samarium (Sm) in three metal-poor stars with contrasting nucleosynthetic enrichment histories. We have obtained new high-resolution (R ~ 120,000) and high-S/N (S/N ~ 160-1000) spectra using the 2dCoudé Spectrograph at the 2.7m Harlan J. Smith Telescope at McDonald Observatory. The isotopic fractions are measured by fitting the observed spectra with synthetic spectra, generated using the most recent laboratory measurements of the hyperfine structure constants and isotope shifts. In the metal-poor giant HD 175305, we measure a Sm isotopic fraction that suggests an r-process origin and a Eu isotopic fraction that is compatible with this interpretation. In the very metal-poor giant HD 196944, we measure a Sm isotopic fraction that suggests an s-process origin. We are unable to measure any isotopic fractions for these species in the very metal-poor and n-capture-poor giant HD 122563 due to the weakness of these lines. We suggest that the Sm isotopic fraction may be a better indicator of s- or r-process nucleosynthesis than the Eu isotopic fraction in some cases. We also propose that the combination of several isotopic fraction measurements in the same metal-poor star can allow direct quantification of the relative s- and r-process nucleosynthesis contributions to n-capture material across a range of nuclear mass numbers, and provide a test of s-process nucleosynthesis predictions at the isotopic level.