Recent chemical abundance studies for very metal-deficient stars have provided important constraints on models of individual nucleosynthesis processes. The analyses are generally made for elemental abundances, while nucleosynthesis calculations predict the abundances in isotope level. Measurements of isotope abundances give quite strong constraints on nucleosynthesis models. For measurements of isotope abundances of heavy elements from stellar spectra, the effect of hyperfine splitting is quite useful. Eu is an ideal case to measure isotope ratios, because (1) this element has only two stable isotopes with odd mass number (151 and 153), and (2) both isotopes show large hyperfine splitting with different degree. Accurate wavelengths and transition probabilities were recently reported by Lawler et al.[1] for Eu. We obtained high resolution (R=90,000), high signal-to-noise spectra for several metal-deficient stars using the Subaru Telescope High Dispersion Spectrograph. Our measurements for r-process element-enhanced stars show that the Eu isotope ratios in these stars agree very well with that in solar system material (the fraction of 151Eu is 48%). This result confirms the agreement of the abundance patterns of elements around Eu in these metal-deficient stars with that of the r-process component in solar system material in isotope level. On the other hand, the isotope ratios measured for s-process element-enhanced stars, in which the Eu isotopes are estimated to originate from s-process, have higher (the 151Eu fraction is 55-60%) than that in solar system material [2]. These results, however, agree well with the predictions of recent s-process models. We use these ratios to investigate the 151Sm branching of s-process nucleosynthesis, and 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.

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