Current Status of r-Process Nucleosynthesis

Abstract

The rapid neutron capture process (r-process) is believed to be responsible for about half of the production of the elements heavier than iron and contributes to abundances of some lighter nuclides as well. A universal pattern of r-process element abundances is observed in some metal-poor stars of the Galactic halo. This suggests that a well-regulated combination of astrophysical conditions and nuclear physics conspires to produce such a universal abundance pattern. The search for the astrophysical site for r-process nucleosynthesis has stimulated interdisciplinary research for more than six decades. There is currently much enthusiasm surrounding evidence for r-process nucleosynthesis in binary neutron star mergers in the multiwavelength follow-up observations of kilonova/gravitational-wave GRB170807A/GW170817. Nevertheless, there remain questions as to the contribution over the history of the Galaxy to the current solar-system r-process abundances from other sites such as neutrino-driven winds or magnetohydrodynamical ejection of material from core-collapse supernovae. In this review we highlight some current issues surrounding the nuclear physics input, astronomical observations, galactic chemical evolution, and theoretical simulations of r process astrophysical environments with goal of outlining a path toward resolving the remaining mysteries of the r-process.

Can Neutron-star Mergers Explain the r-process Enrichment in Globular Clusters?

“…Various other astrophysical scenarios have been proposed for contributing to the observed abundances of r-process elements (see Kajino et al 2019, for a review). One such scenario is via the formation of a dense accretion disk during the collapse of a massive, rapidly-rotating star -the collapsar scenario (Woosley 1993;Siegel et al 2019;Siegel 2019).…”

“…Astrophysical mechanisms for synthesizing the heaviest elements in the Universe are poorly understood, yet essential in explaining nucleosynthetic abundances observable today. Roughly half the elements heavier than iron are formed through the rapid capture of neutrons in a dense, neutronrich environment, known as r-process nucleosynthesis (e.g., Cowan et al 2019;Kajino et al 2019). In these environments, the rate of neutron capture overcomes the rate of βdecay of radioactive nuclei, which converts the heavy nuclei into more stable isotopes with higher atomic numbers.…”