Europium Isotope Ratios in s-Process Element-enhanced Metal-poor Stars: A new probe of the 151 Sm branching

Abstract

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 + 151Eu)] 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 sprocess 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. Show more