Some of the heavy elements, such as gold and europium (Eu), are almost exclusively formed by the rapid neutron capture process (r-process). However, it is still unclear which astrophysical site between core-collapse supernovae and neutron star -neutron star (NS-NS) mergers produced most of the r-process elements in the universe. Galactic chemical evolution (GCE) models can test these scenarios by quantifying the frequency and yields required to reproduce the amount of europium (Eu) observed in galaxies. Although NS-NS mergers have become popular candidates, their required frequency (or rate) needs to be consistent with that obtained from gravitational wave measurements. Here we address the first NS-NS merger detected by LIGO/Virgo (GW170817) and its associated Gamma-ray burst and analyze their implication on the origin of r-process elements. The range of NS-NS merger rate densities of 320−4740 Gpc −3 yr −1 provided by LIGO/Virgo is remarkably consistent with the range required by GCE to explain the Eu abundances in the Milky Way with NS-NS mergers, assuming the solar r-process abundance pattern for the ejecta. Under the same assumption, this event has produced about 1 − 5 Earth masses of Eu, and 3 − 13 Earth masses of gold. When using theoretical calculations to derive Eu yields, constraining the role of NS-NS mergers becomes more challenging because of nuclear astrophysics uncertainties. This is the first study that directly combines nuclear physics uncertainties with GCE calculations. If GW170817 is a representative event, NS-NS mergers can produce Eu in sufficient amounts and are likely to be the main r-process site.

The possibility that nucleosynthesis in neutron star mergers may reach fissioning nuclei introduces significant uncertainties in predicting the relative abundances of r-process material from such events. We evaluate the impact of using sets of fission yields given by the 2016 GEF code for spontaneous (sf), neutron-induced ((n,f)), and β-delayed (βdf) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energydependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF 2016 can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of 254 Cf spontaneous fission to the heating at ∼ 100 days, such that a light curve observation consistent with such late-time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the 254 Cf β-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2-1.4 M neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z,N )(n,f) and (Z,N )βdf that control the relative abundances near the second peak. We find the "hot spots" for β-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N = 184 shell closure and the location of currently available experimental decay data. arXiv:1810.08133v2 [nucl-th]

Neutron star mergers offer unique conditions for the creation of the heavy elements and additionally provide a testbed for our understanding of this synthesis known as the r-process. We have performed dynamical nucleosynthesis calculations and identified a single isotope, 254 Cf, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which 254 Cf has the greatest impact to the light curve. Future observations in the middle-IR which are bright during this regime could indicate the production of actinide nucleosynthesis.

The rapid-neutron-capture ("r -") process is responsible for synthesizing many of the heavy elements observed in both the Solar System and Galactic metal-poor halo stars. Simulations of r -process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r -process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger and explore if varying levels of neutron richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β-decay, and mass model. For most cases, the actinides are over-produced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinideboost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of a neutron star merger are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r -process-enhanced, metal-poor stars.

The mergers of binary neutron stars, as well as black hole–neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining 32 different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics uncertainties typically generate at least one order of magnitude uncertainty in key quantities such as the nuclear heating (one and a half orders of magnitude at 1 day post-merger), the bolometric luminosity (one order of magnitude at 5 days post-merger), and the inferred mass of material from the bolometric luminosity (factor of 8 when considering the 8–10 day region). Since particular nuclear processes are critical for determining the electromagnetic signal, we provide tables of key nuclei undergoing β-decay, α-decay, and spontaneous fission important for heating at different times, identifying decays that are common among the many nuclear input combinations.