Session L04: Sensitive Reaction Studies for Nuclear Astrophysics

Live

The study of the rapid neutron capture or "r process" of nucleosynthesis offers the opportunity to glean insight into where the heavy elements (those above iron) on the periodic table are created in nature. Astrophysical sites, namely, supernovae and compact object mergers, e.g. two neutron stars, have long been touted as possible candidates. However, simulations of these environments require the input of thousands of pieces of nuclear data for which no experimental information is available. These unknown nuclear inputs may greatly impact the nucleosynthesis, leading to large uncertainties in the resultant abundances. It is therefore imperative to identify the most important nuclear inputs which leverage the final patterns. We report on recent studies that probe the variation in key nuclear quantities like masses, neutron capture rates, half-lives, branching ratios and fission. We show how these quantities influence the nucleosynthesis and discuss possible insights that can be gathered by current and future observations. We conclude that a targeted reduction of nuclear physics uncertainties either by new measurements or by improved nuclear models will allow for more robust r-process predictions.   

Live

In very neutron-rich conditions, as can be found in neutron star merger environments, the $r$ process can synthesize up to the heaviest, most unstable nuclei, the fissioning actinides. The aim to pinpoint whether fission is indeed occurring in $r$-process scenarios is of particular interest following the observations of the electromagnetic counterpart for the GW170817 neutron star merger event which strongly implied the presence of lanthanide species. However, presently there is no evidence that this event produced elements heavier than the lanthanides such as gold, platinum, and beyond, which includes the actinides. Understanding the effects of fission in the $r$ process requires knowledge of fission properties for hundreds of nuclei on the neutron-rich side of stability, about which little is experimentally known. The $r$-process nucleosynthesis studies I will present explicitly connect the nuclear data to $r$-process observational signatures. For instance, a feature of enhanced lanthanide production, the $r$-process rare-earth abundance peak, could be intimately linked to the nuclear structure and deformation of neutron-rich lanthanide species or produced via late-time fission deposition. We will examine these possibilities in the context of a numerical approach utilizing Markov Chain Monte Carlo which seeks to find conditions capable of producing a rare-earth peak consistent with both observational and experimental data. Since nuclear fission is an especially exotic and energetic process, we will discuss ways in which the presence of fission may lead to observable signatures in astrophysical scenarios, thereby confirming the production of the heaviest $r$-process elements. We will also explore the potential for future experimental and theoretical efforts to refine our knowledge of fission in the $r$ process. The question of where nature primarily produces the heavy elements can only be answered through such collaborative efforts between experiment, observation, and theory.   

Not Participating

The origin of the r-process elements remains the biggest unsolved question in our understanding of chemical evolution in the Milky Way, but joint electromagnetic and gravitational wave observations are starting to put significant constraints on their origin. The most likely astrophysical sites for the formation of these nuclei involve dynamic events in the lives of neutron stars: the merger of a neutron star and another compact object or the neutron stars birth. In these environments, nuclear physics plays a paramount role in determining both the evolution of the dense object itself and what nuclei are synthesized in material that is ejected from the system. In this talk, I will discuss nucleosynthesis and matter ejection during neutron star mergers. I will also discuss electromagnetic observables associated with neutron star mergers that give us a direct window into the formation of the r-process elements.