In the cases of the r-process (and the p-process), we do not have well-measured nuclear data for the highly radioactive isotopes involved, mature models of the potential astrophysical sites, or conclusive direct observations. In part to address the nuclear uncertainties, the 2007 and 2015 Long Range Plans of the Nuclear Science Advisory Committee recommended continued support for current radioactive ion beam facilities and the construction of the Facility for Rare Isotope Beams. However, the nuclear data these accelerators, and their peers being operated and constructed around the world, will provide is alone not enough to correct our knowledge concerning the formation of these elements. We propose an effort to mature the modeling of many of the suggested astrophysical sites of the r-process, which will also reveal much about the nucleosynthesis of elements from oxygen to germanium. This effort is not independent of the experimental efforts, as calculations of the r-process and p-process rely on the nuclear data. However, the details of the nuclear data that are needed vary between the different potential sites. Winnowing the list of possible r-process or p-process sites reduces the nuclear data needs, allowing the simulations we propose herein to guide the experimental efforts in nuclear astrophysics.

The observed pattern of r-process species requires a combination of high-entropy and neutron-rich material, acting on a timescale of seconds, with the exact balance varying between the different proposed sites. Arguments based on the chemical evolution history of our galaxy call for as many as three sites to explain the development of the r-process abundance pattern over time; for instance, a weak (low atomic mass) r-process, possibly associated with ordinary core-collapse supernovae, a main r-process, sometimes associated with peculiar supernovae driven by magnetohydrodynamic jets and a fission-recycling (high atomic mass) r-process, thought to be associated with neutron star mergers. Simulations of each of these scenarios rely on a similar toolkit: magnetohydrodynamics, thermonuclear kinetics, the equation of state of nuclear matter, and radiation transport of neutrinos.

Under this SciDAC proposal, we will improve existing astrophysical application codes to take full advantage of the coming generation of exascale computers and develop new modules with greater physical fidelity that is only achievable on these machines. In parallel, we will improve the microscopic physics in the simulations, implementing improved nuclear equations of states and neutrino opacities and, where appropriate, approximating the impact of neutrino oscillations. The simulations we will run using these new/improved codes will begin with the final stages of stellar evolution and push outward the state-of-the-art in the modeling of many of the proposed sites of the r-process. Among the products of our simulations will be the finest estimates to date of the photonic, neutrino, gravitational wave and nucleosynthesis signatures of these cosmic explosions. We will also seek to quantify the uncertainty in these predictions. In particular, we will examine both the nuclear physics and astrophysics uncertainties in the production of r-process isotopes and other nucleosynthesis. The development of codes under this proposal will leverage developments supported by the Exascale Models of Stellar Explosions team of the DOE Exascale Computing Project. We also expect to leverage development funded at LANL by the Advanced Technology Development and Mitigation (ATDM) subprogram of the DoE Advanced Simulation and Computing (ASC) Program.