2003ApJ...592L..67A

We report on the first measurement of the Eu isotope fractions (¹⁵¹Eu and ¹⁵³Eu) in s-process element-enhanced metal-poor stars. We use these ratios to investigate the ¹⁵¹Sm 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 ¹⁵¹Eu fractions [fr(¹⁵¹Eu)=¹⁵¹Eu/(¹⁵¹Eu+¹⁵³Eu)] 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 ¹⁵¹Eu fraction.