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We report on the first measurement of the Eu isotope fractions (151Eu and153Eu) in s-process-element-enhanced, metal-poor stars. We use these ratios toinvestigate the 151Sm branching of s-process nucleosynthesis. The measurementwas made by detailed study of Eu II lines that are significantly affected byhyperfine splitting and isotope shifts in spectra of the carbon-rich, verymetal-poor stars LP625-44 and CS31062-050, observed with the Subaru TelescopeHigh Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) =151Eu/(151Eu+153Eu)] derived for LP625-44 and CS31062-050 are 0.60 and 0.55,respectively, with uncertainties of about +/- 0.05. These values are higherthan found in solar-system material, but agree well with the predictions ofrecent s-process models. We derive new constraints on the temperature andneutron density during the s-process based on calculations of pulsed s-processmodels for the 151Eu fraction.Comment: 10 pages, 2 figures, ApJL in pres

Europium Isotope Ratios in s -Process Element-enhanced Metal-poor Stars: A New Probe of the 151 Sm Branching

Europium Isotope Ratios in s -Process Element-enhanced Metal-poor Stars: A New Probe of the ¹⁵¹ Sm Branching