Europium isotope ratios in s-process element-enhanced metal-poor stars:: A new probe of the 151Sm branching

We report on the first measurement of the Eu isotope fractions (Eu-151 and Eu-153) in s-process element - enhanced metal-poor stars. We use these ratios to investigate the Sm-151 branching of s-process nucleosynthesis. The measurement was made by detailed study of Eu II lines that are significantly affected by hyperfine splitting and isotope shifts in spectra of the carbon-rich very metal poor stars LP 625 - 44 and CS 31062 - 050, observed with the Subaru Telescope High Dispersion Spectrograph. The Eu-151 fractions [fr(Eu-151) = Eu-151/(Eu-151 + Eu-153)] derived for LP 625 - 44 and CS 31062 - 050 are 0.60 and 0.55, respectively, with uncertainties of about +/- 0.05. These values are higher than found in solar system material but agree well with the predictions of recent s-process models. We derive new constraints on the temperature and neutron density during the s-process based on calculations of pulsed s-process models for the Eu-151 fraction.