  We examine r-process nucleosynthesis in a "prompt supernova explosion" from
an 8-10 Msun progenitor star, as an alternative scenario to the "neutrino wind"
mechanism. In the present model, the progenitor star has formed an
oxygen-neon-magnesium core at its center. The core-collapse simulations are
performed with a one-dimension, 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 highly neutronized ejecta (Ye = 0.14-0.20) leads 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 Msun stars with oxygen-neon-magnesium cores can be a
promising site of r-process nuclei.
