While the origin of r-process nuclei remains a long-standing mystery, recent spectroscopic studies of extremely metal poor stars in the Galactic halo strongly suggest that it is associated with core-collapse supernovae. In this study we examine r-process nucleosynthesis in a "prompt supernova explosion" from an 8-10 M☉ progenitor star as an alternative scenario to the "neutrino wind" mechanism, which has also been considered a promising site of the r-process. In the present model, the progenitor star has formed an oxygen-neon-magnesium (O-Ne-Mg) core (of mass 1.38 M☉) at its center. Its smaller gravitational potential, as well as the smaller core that is in nuclear statistical equilibrium at the time of core bounce, as compared with the iron cores in more massive stars, may allow the star to explode hydrodynamically rather than by delayed neutrino heating. The core-collapse simulations are performed with a one-dimensional, Newtonian hydrodynamic code. We obtain a very weak prompt explosion in which no r-processing occurs. We further simulate energetic prompt explosions by enhancement of the shock-heating energy in order to investigate conditions necessary for the production of r-process nuclei in such events. The r-process nucleosynthesis is calculated using a nuclear reaction network code including relevant neutron-rich isotopes with reactions among them. The highly neutronized ejecta (Ye ≈ 0.14-0.20) lead to robust production of r-process nuclei; their relative abundances are in excellent agreement with the solar r-process pattern. Our results suggest that prompt explosions of 8-10 M☉ stars with O-Ne-Mg cores can be a promising site of r-process nuclei. The mass of the r-process material per event is about 2 orders of magnitude larger than that expected from Galactic chemical evolution studies. We propose, therefore, that only a small fraction of r-process material is ejected via "mixing-fallback" mechanism of the core matter, wherein most of the r-process material falls back onto the proto-neutron star. A lower limit on the age of the universe is derived by application of the uranium-thorium (U-Th) chronometer pair by comparison with the observed ratio of these species in the highly r-process-enhanced, extremely metal poor star CS 31082-001. The inferred age is 14.1 ± 2.4 Gyr—the same as that obtained previously based on the neutrino wind scenario with the same nuclear mass formula. This suggests that chronometric estimates obtained using the U-Th pair are independent of the astrophysical conditions considered.
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