Measurement of the Europium Isotope Ratio for the Extremely Metal poor, r-Process-enhanced Star CS 31082-001^*

We report the first measurement of the isotope fraction of europium (¹⁵¹Eu and ¹⁵³Eu) for the extremely metal poor, r-process-enhanced star CS 31082-001, based on high-resolution spectra obtained with the Subaru Telescope High Dispersion Spectrograph. We have also obtained new measurements of this ratio for two similar stars with previous europium isotope measurements, CS 22892-052 and HD 115444. The measurements were made using observations of the Eu lines in these spectra that are most significantly affected by isotope shifts and hyperfine splitting. The fractions of ¹⁵¹Eu derived for CS 31082-001, CS 22892-052, and HD 115444 are 0.44, 0.51, and 0.46, respectively, with uncertainties of about ±0.1. CS 31082-001, the first star with a meaningful measurement of U outside of the solar system, is known to exhibit peculiar abundance ratios between the actinide and rare earth elements (e.g., Th/Eu), ratios that are significantly different from those for other stars with large excesses of r-process elements, such as our two comparison objects. Nevertheless, our analysis indicates that the Eu isotope ratio of CS 31082-001 agrees, within the errors, with those of other r-process-enhanced objects and with that of solar system material.