New Views of the $r$-Process

Nucleosynthesis via rapid neutron capture, the $r$-process, is responsible for approximately half of the solar abundances of the nuclei with mass numbers $A > 100$. Five decades after this process was proposed, two outstanding issues remain: (1) which astrophysical environments can provide the physical conditions required for the $r$-process? and (2) what is the detailed nuclear physics input that governs the yield pattern of nuclei from an $r$-process? Both issues are crucial for a full understanding of the $r$-process. This talk will mainly address the issue of the astrophysical sites. While there are no self-consistent models that can produce a robust $r$-process, observations of elemental abundances in old stars of the Galactic halo over the past decade have provided important guidance to the overall nucleosynthetic characteristics of astrophysical $r$-process sources. For example, these observations strongly suggest that the source for the heaviest $r$-process nuclei produces none or very little of the Fe group and lighter nuclei. On the theoretical front, several new mechanisms other than rapid ($r$) or slow ($s$) neutron capture were found to produce the nuclei with $60 < A < 100$ that were thought to be made dominantly by the $r$ and $s$-processes. Major results from the stellar observations will be highlighted. Their implications for astrophysical models of the $r$-process will be discussed. Existing models and possible improvements will be reviewed based on the observational implications.