Session J07: R-Process Nucleosynthesis

Sites of the r-process: supernovae and mergers: Recent Successes and Current Issues

Many of the heavy elements in the universe are produced through the rapid capture of neutrons onto iron peak elements, the so-called r-process. Sites for this crucial heavy element production are necessarily extreme and leading proposals invoke conditions at the heart of the engines behind supernovae and gamma-ray bursts. These models include neutrino-driven winds, magnetically-contained winds, magnetically-driven outflows, and disk winds around a collapsed star as well as the winds and dynamical ejecta from the merger of two neutron stars or a neutron star and a black hole. Here we review these sites, their successes and problems in an effort to gain a more complete picture of the production of the heavy elements made in the r-process.   

R-process production and transients from neutron star mergers

The mergers of neutron stars with neutron star or black hole binary companions have long been suspected to be a major site of astrophysical r-process production. The first direct detection of a binary neutron star merger in August of 2017 (GW1701817) provided a critical test of this theory. I will explain the physics governing the nature of r-process transients, and discuss how we can decode mergers’ radioactively-powered emission to learn about merger-driven nucleosynthesis. I will apply these insights to GW170817, and outline what we learned about the r-process from this event, and what questions remain.   

Nuclear and neutrino physics of the r-process

The production of the elements heavier than iron in the universe has long been associated with neutron-capture processes. The most neutron-rich isotopes are created by rapid (r ) neutron-capture nucleosynthesis in extreme astrophysical environments. Specifics of these environments and the location of the astrophysical sites in which the r process occurs have remained open problems. It has been reported that observations of the gravitational wave event GW170817 and its electromagnetic counterpart suggest that neutron star mergers are a site of r-process nucleosynthesis. Still many questions remain, such as the nature of the astrophysical conditions within the merger responsible for element synthesis and whether mergers can account for all galactic r-process production. If we hope to fully understand the connection between this discovery and the origin of r-process elements, uncertainties in neutrino and nuclear astrophysics must be reduced. I will highlight the role played by neutrinos and nuclear masses.