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During the past few decades, major advances have been made in modeling evolution of stars with a wide range of masses and their associated nucleosynthesis. For example, spherical and multi- dimensional simulations of core-collapse supernovae (CCSNe) with detailed neutrino transport have greatly enhanced our understanding of the explosion mechanism and elucidated several processes of heavy element production, such as the synthesis of nuclei with A=60-110 in the neutrino-heated ejecta via α-rich freeze-out (the α-process) and neutrino-aided proton capture (the νp-process). In addition, detailed simulations of neutron star mergers have led to a promising model for producing the heaviest elements via rapid neutron capture (the r-process) with fission cycling playing a crucial role. The neutrino-related processes are sensitive to neutrino oscillations and may even provide a probe of the yet-unknown neutrino mass hierarchy.

Furthermore, all the nucleosynthetic processes mentioned above depend on the understanding of properties of extremely dense matter either directly or as a medium of neutrino transport. Much progress has also been made in these fundamental aspects of nuclear theory in recent years, with new sets of nuclear equations of state and neutrino interaction cross sections calculated for astrophysical applications and having major impact on the understanding of nucleosynthesis.

The progress in the study of the r-process is particularly noteworthy. Not only have a number of astrophysical models been developed, but the masses and β-decay half-lives for a number of nuclei on or close to the r-process path have also been precisely measured. The new generation of experiments at rare-isotope beam facilities such as FRIB, RIKEN, and FAIR promise to provide even more. In addition, observations of elemental abundances in old stars of the Milky Way halo and satellite dwarf galaxies have greatly stimulated the search for the r-process sites in addition to providing a rich database for understanding the chemical evolution of the universe.

Nucleosynthesis and chemical evolution is a highly interdisciplinary problem that requires understanding the structure and reactions of exotic nuclei, the properties of matter and neutrino interactions under extreme conditions, advanced simulations of astrophysical environments, and comparison against forefront astronomical observations. Only a coordinated effort among nuclear theorists and experimentalists, astrophysical modelers, and astronomers will allow us to build a bridge between the properties of the fundamental constituents of matter and the production of nuclei in exotic astrophysical environments as reflected by their observed abundances. The goal of this five-week program is to bring together experts in these communities to identify key new developments and foster collaborations towards a deeper understanding of the origin and evolution of elements in the universe.

• Week 1: Joint workshop with the program on binary neutron star coalescence. Workshop webpage

• Week 2: Tellurium through uranium: These elements are mainly produced by the s- and r-processes. What are the main uncertainties, nuclear and astrophysical, in the understanding of these processes? Are there environments where neutron capture occurs intermediately between the s- and r-processes? How can we explain the vast observational data on these elements in stars of the Milky Way halo and satellite dwarf galaxies?

• Week 3: Elements between Strontium and Silver: These elements can be produced by the s-process (slow neutron capture) during stellar evolution and by the α- and νp- processes in neutrino-driven winds from CCSNe. Are there other mechanisms? What are the main nuclear physics uncertainties affecting the production of these elements? How do various sources contribute to these elements over the history of galaxies?

• Week 4: From big bang nucleosynthesis (BBN) products to the iron group: Observed Li abundances in old stars are lower than the BBN prediction by factor of several. How can this be explained? Are there other problems with the standard picture of BBN? What are the critical nuclear reaction rates for stellar evolution? What are the major uncertainties in current models of nucleosynthesis for the elements up to the iron group? Most of the iron in the present Milky Way is produced by Type Ia supernovae (SNe Ia) when a white dwarf exceeds a critical mass by accreting matter from a binary companion or merging with another white dwarf. What are the progenitors for SNe Ia? How does nucleosynthesis by SNe Ia depend on the progenitor? How often do SNe Ia occur over the history of different galaxies?

• Week 5: Future directions

The timing of this program is critical, as a larger number of observations on elemental abundances are now available, as well as new nuclear data and improved nuclear and astrophysical models. On the experimental side, we are about to enter a new era for nuclear astrophysics with the new generation of rare-isotope beam facilities: FAIR (GSI, Darmstadt), FRIB (NSCL, Michigan, USA), and RIBF (RIKEN, Japan). This will significantly advance the experimental frontier towards the heaviest neutron-rich isotopes close to the neutron drip-line. The impact on the understanding of the origin of the heavy elements will be huge if the efforts of all the concerned communities are integrated.