SciDAC4 TEAMS Initiative Summary:
We are the products of nearly 14 billion years of cosmic chemical evolution, stretching back to the
fusion of newly formed neutrons and protons into helium and other light elements in the Big Bang.
In the ninety six years since Eddington suggested that the billion-year brightness of stars could
be powered by the transmutation of the elements, our understanding of cosmic chemical evolution has matured greatly.
However, questions still remain, chief among these the source of the heaviest elements.
As a result, the Committee on the Physics of the Universe convened by the National Research Council
in 2000 included the question "How Were the Elements from Iron to Uranium Made?" in their list
of Eleven Science Questions for the New Century. Burbidge, Burbidge, Fowler & Hoyle (1957; Rev. Mod.
Phys., 29 547) and Cameron (1957; PASP, 69 201) divided the observed abundance of the isotopes heavier
than iron between three distinct processes. On the basis of the correlation between nuclear properties
and cosmic abundances, we know that roughly half of these isotopes are the result of a slow neutron
capture process, termed the s-process. Based on well-measured nuclear data from stable nuclei and mature
models of stellar evolution, we have considerable confidence that the s-process occurs over thousands
of years in the hydrogen and helium-rich nuclear burning shells that sit atop the inert carbon-oxygen
core in an Asymptotic Giant Branch star. This confidence is buoyed by observations of red-giant stars
with large over abundances of s-process elements like barium and radioactive technetium.
Most of the isotopes that the s-process can not explain result from a rapid neutron capture process,
the r-process, while a scattering of rarer proton-rich isotopes are ascribed to a p-process, originally
thought to be driven by proton capture.
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.