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 (151Euand 153Eu) for the extremely metal-poor, r-process-enhanced star CS31082-001,based on high-resolution spectra obtained with the Subaru Telescope HighDispersion Spectrograph. We have also obtained new measurements of this ratiofor two similar stars with previous europium isotope measurements, CS22892-052and HD115444. The measurements were made using observations of the Eu lines inthese spectra that are most significantly affected by isotope shifts andhyperfine splitting. The fractions of 151Eu derived for CS31082-001,CS22892-052, and HD115444 are 0.44, 0.51, and 0.46, respectively, withuncertainties of about +/-0.1. CS31082-001, the first star with a meaningfulmeasurement of U outside of the solar system, is known to exhibit peculiarabundance ratios between the actinide and rare-earth elements (e.g., Th/Eu),ratios that are significantly different from those for other stars with largeexcesses of r-process elements, such as our two comparison objects.Nevertheless, our analysis indicates that the Eu isotope ratio of CS31082-001agrees, within the errors, with those of other r-process-enhanced objects, andwith that of solar-system material.Comment: 16 pages, 4 figures, Apj, in pres