The r-Process: History, Required Conditions, Astrophysical Sites, and Observations
Friedrich-Karl Thielemann, John J. Cowan
TL;DR
This review traces the history and current understanding of the r-process, combining solar-system residual analyses with site-independent nuclear-physics predictions and a comprehensive survey of proposed astrophysical sites. It emphasizes that a single r-process site cannot explain the entire solar pattern and that multiple environments—most notably neutron-star mergers for the main, and magneto-rotational supernovae or collapsars for additional components—shape the observed abundances. The authors compare predictions to observations, including kilonovae and metal-poor stars, and discuss galactic chemical evolution and cosmochronology to assess the timing and frequency of contributing events. The work highlights the need for improved nuclear data and continued multi-messenger observations to identify the dominant sites and their roles across cosmic time.
Abstract
This review of the rapid-neutron-capture (i.e. r-) process starts with determining the Solar System r-abundance pattern via first obtaining (and subtracting) the contribution from the slow-neutron capture (s-) process. We emphasize the extensive work in this area by our late colleague Roberto Gallino and continue in an overview, concentrating on attempts to reproduce the solar r-process pattern with historical site-independent approaches, based on nuclear physics far from stability. In a second step we address the existing proposals for astrophysical sites. Among stellar observations we start with available observations of individual events before analyzing low-metallicity stars, which witness r-process contributions in the early Galaxy. We conclude with a comparison of observations and model predictions, focusing on our present ability to identify the responsible individual astrophysical sites by their imprint in Galactic evolution.