New Nuclear Reaction Flow during r-Process Nucleosynthesis in Supernovae: Critical Role of Light, Neutron-rich Nuclei

We study the role of light, neutron-rich nuclei during r-process nucleosynthesis in supernovae. Most previous studies of the r-process have concentrated on the reaction flow of heavy, unstable nuclei. Although the nuclear reaction network includes a few thousand heavy nuclei, only limited reaction flow through light nuclei near the stability line has been used in those studies. However, in a viable scenario of the r-process in neutrino-driven winds, the initial condition is a high-entropy hot plasma consisting of neutrons, protons, and electron-positron pairs experiencing an intense flux of neutrinos. In such environments, light nuclei, as well as heavy nuclei, are expected to play important roles in the production of seed nuclei and r-process elements. Thus, we have extended our fully implicit nuclear reaction network so that it includes all nuclei up to the neutron-drip line for Z ≤ 10, in addition to a larger network for Z ≥ 10. In the present nucleosynthesis study, we utilize a wind model of massive Type II supernova explosions to study the effects of this extended network. We find that a new nuclear reaction flow path opens in the very light, neutron-rich region. This new nuclear reaction flow can change the final heavy-element abundances by as much as an order of magnitude.