Recent Progress in the Understanding of the r-Process

Abstract

A brief overview of the r-process is given with an emphasis on the observational implications for this process. The conditions required for the major production of the heavy r-process elements (r-elements) with mass numbers A >130 are discussed based on a generic astrophysical model where matter adiabatically expands from a hot and dense initial state. Nucleosynthesis in the neutrino-driven winds from nascent neutron stars is discussed as a specific example. Such winds readily produce the elements from Sr to Ag with A ~ 88 to 110 through charged-particle reactions in the alpha-process but appear incapable of making the heavy r-elements. Observations of elemental abundances in metal-poor stars have provided many valuable insights into the r-process. They have demonstrated that the production of the heavy r-elements must be associated with massive stars evolving on short timescales, provided evidence strongly favoring core-collapse supernovae over neutron star mergers as the major source for these elements, and shown that this source cannot produce any significant amount of the low-A elements from Na to Ge including Fe. A self-consistent astrophysical model of the r-process remains to be developed, and it appears well worthwhile to carry out more comprehensive studies on the evolution and explosion of the massive stars of ~ 8 to 11 M_sun that undergo O-Ne-Mg core collapse and produce a very insignificant amount of the low-A elements.