r-Process Nucleosynthesis: Connecting Rare-Isotope Beam Facilities with the Cosmos

The origin of heavy elements such as gold, silver, platinum, or uranium in nature remains one of the long standing, open questions in science. An answer within the next decade is in sight given a set of essential ingredients; namely, FRIB, the LIGO gravitational wave observatory, large ground and space based telescopes analyzing starlight, advanced computer models of stellar collisions and explosions, and theories explaining the properties of nuclei. This is the main conclusion of a unique review paper in Journal of Physics G. The review paper is the outcome of a interdisciplinary collaborative effort that began with a workshop "The r-process Nucleosynthesis: Connecting FRIB with the Cosmos" during the summer of 2016, initiated by Chuck Horowitz and organized within the International Nuclear Theory Collaboration program (ICNT) and the Joint Institute for Nuclear Astrophysics Center for the Evolution of the Elements (JINA-CEE).

Heavy elements such as silver, gold, platinum, and uranium are believed to be mostly produced by a so called rapid neutron capture process (r-process). Yet, where this process occurs, and what nuclear reactions are involved is not yet understood. Proposed sites include colliding neutron stars, supernova explosions, jets of gas launched into space by exploding stars, or very massive collapsing stars. The authors are however hopeful that this long standing mystery can finally be solved in the coming decade. LIGO recently detected gravitational waves from two colliding neutron stars, which enabled rapid followup observations with conventional telescopes. These observations provided the first clear evidence of heavy element synthesis in an r-process site. At the same time FRIB is now nearing completion and will provide, together with complementary facilities across the world, the nuclear data needed to determine what elements and isotopes are produced, not only when neutron stars collide, but also in other potential sites. Advanced computer models and theory are under development as well, to connect the nuclear data with the observations.

The paper is unique in the large number of international authors from various different sub-disciplines that worked together. It provides a first comprehensive overview of all aspects and developments related to the r-process problem, and a road map for the future, for both astrophysics and nuclear science. The overall message is clear - nuclear scientists, computational modelers, and astronomers must work closely together to tackle the challenging problem of the origin of the elements. Centers like JINA-CEE, a Physics Frontiers Center devoted to create interdisciplinary connections between astrophysics and nuclear physics, will therefore play an essential role in this endeavor.

r process graph

The image shows nuclei colored dark as the ones that are the most important to be measured to understand heavy element synthesis. Everything on the left of the thick grey line is expected to be measureable at FRIB.