Session LG: Mini-Symposium: Nuclear Physics and the r-Process in the Multi-messenger Era I

Nuclear Astrophysics in the New Era of Multimessenger Astronomy

One of the overarching questions animating nuclear physics today is "How does subatomic matter organize itself". Neutron stars are cosmic laboratories uniquely poised to answer this fundamental question. The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration is providing fundamental new insights into the astrophysical site for the r-process and on the nature of neutron-rich matter. In turn, the study of nuclei at new exotic-beam facilities throughout the world will help elucidate the underlying dynamics of the r-process and the structure, dynamics, and composition of neutron stars. In this presentation I will discuss how this synergy -- in combination with nuclear physics insights, modern theoretical approaches, and powerful statistical ideas -- can pave the way to understanding these fascinating objects.   

Macroscopic-microscopic fission yields for nucleosynthesis

The rapid neutron capture process (r-process) is believe to synthesize the heaviest elements found on the periodic table. This remarkable process is believed to occur in exotic environments such as compact object mergers and possibly supernovae. In the most neutron-rich components of explosive outflows, nuclear fission, or the splitting of heavy nucleus into smaller lighter fragments, may play a crucial role in directing the nuclear flow far from stability. Where exactly the fission fragments are distributed across the chart of nuclides is an open theoretical question. I will address this open issue using recent state-of-the-art fission yield calculations that employ the theoretical macroscopic-microscopic framework. I will present the impact of these calculations in simulations of the r-process.   

Sandblasting The R-process: Spallation Of R-process Nuclei Ejected From A NSNS Event

Neutron star mergers are r-process nucleosynthesis sites, which eject materials at high velocity ranging from 0.1c to 0.3c for different regions. Thus the r-process nuclei ejected from a neutron star merger event are sufficiently energetic to have spallation nuclear reactions with the interstellar medium particles. The spallation reactions tend to shift the r-process abundance patterns towards the solar data, and smooth the abundance shapes. The spallation effects depend on both the initial r-process nuclei conditions, which is determined by the astrophysical trajectories and nuclear data adopted for the r-process nucleosynthesis, and the propagation with various ejecta velocities and spallation cross-sections.   

Nuclear Masses, Neutron Capture, and the r-process

Individual neutron capture cross sections play an important role in the final isotopic abundances from a wide range of r-process scenarios. Unfortunately, the isotopes which show the greatest impact are far from stability and not within experimental reach for direct measurements in the coming years. We have discovered a previously unrecognized correlation between the neutron capture cross-section and the two-neutron separation energy. While initial studies required independent correlations for regions of different nuclear structure, recent work has shown a simple way to treat multiple regions in a consistent way, drastically improving its predictive reach. Because two-neutron separation energies can be measured with achievable rare beam intensities, the quality and quantity of S$_{2n}${} data is far more extensive than what is available for neutron capture, allowing experimentally based extrapolations. In addition to providing extended predictions, this may offer hints into where traditional reaction theories have missed underlying physics that is needed to more accurately model the capture reaction process.   

Statistical uncertainties of Skyrme-type nuclear energy density functionals and r-process nucleosynthesis

Fully understanding the impact of uncertainties in models of neutron-rich nuclei represents one of several critical steps towards understanding the formation of the heaviest elements via the rapid neutron capture (r-) process of nucleosynthesis. In this work, we consider the statistical uncertainty in the UNEDF1 nuclear energy density functional. We begin by sampling 50 points within the posterior distribution of the UNEDF1 parameter space. For each sample, we calculate nuclear binding energies, along with the nuclear capture, decay, and fission properties necessary for r-process nucleosynthesis calculations. We perform nucleosynthesis calculations for several distinct types of astrophysical conditions in which the r-process is thought to occur using each of these 50 datasets, and we report the resulting range in abundance patterns. Finally, we estimate the ability of future measurements at the Facility for Rare Isotope Beams to reduce statistical uncertainty in the UNEDF1 parameters. We repeat our analysis in order to quantify the resulting improvements to r-process nucleosynthesis simulations.   

Examining lanthanide production in merger accretion disk winds: nuclear masses and the rare-earth peak

The observations of the GW170817 electromagnetic counterpart suggested lanthanides were produced in this neutron star merger event. Lanthanide production in heavy element nucleosynthesis is subject to large uncertainties from nuclear physics and astrophysics unknowns. Specifically, the rare-earth abundance peak, a feature of enhanced lanthanide production at A$\sim $164 seen in the solar $r$-process residuals, is not robustly produced in $r$-process calculations. The proposed dynamical mechanism of peak formation requires the presence of a nuclear physics feature in the rare-earth region which may be within reach of experiments performed at, for example, the CPT at CARIBU and the upcoming FRIB. To take full advantage of such measurements, we employ Markov Chain Monte Carlo to ``reverse engineer'' the nuclear masses capable of producing a peak compatible with the observed solar $r$-process abundances and compare directly with experimental mass data. Here I will present our latest results and demonstrate how the method may be used to the learn which astrophysical conditions are consistent with both observational and experimental data. The question of where nature primarily produces the heavy elements can only be answered through such collaborative efforts between experiment, theory, and observation.   

Actinide-Rich or Actinide-Poor, Same r-Process Progenitor

The astrophysical production site of the heaviest elements in the universe remains a mystery. Incorporating heavy element signatures of metal-poor, $r$-process enhanced stars into theoretical studies of $r$-process production can offer crucial constraints on the origin of heavy elements. In this study, we introduce and apply the ``Actinide-Dilution with Matching" model to a variety of stellar groups ranging from actinide-deficient to actinide-enhanced to empirically characterize $r$-process ejecta mass as a function of electron fraction. We find that actinide-boost stars do not indicate the need for a unique and separate $r$-process progenitor. Rather, small variations of neutron richness within the same type of $r$-process event can account for all observed levels of actinide enhancements. The very low-$Y_e$, fission-cycling ejecta of an $r$-process event need only constitute 10--30\% of the total ejecta mass to accommodate most actinide abundances of metal-poor stars. We find that our empirical $Y_e$ distributions of ejecta are similar to those inferred from studies of GW170817 mass ejecta ratios, which is consistent with neutron-star mergers being a source of the heavy elements in metal-poor, $r$-process enhanced stars.