Search

Merging neutron stars produce "kilonovae"—electromagnetic transients powered by the decay of unstable nuclei synthesized via rapid neutron capture (the r-process) in material that is gravitationally unbound during inspiral and coalescence. Kilonova emission, if accurately interpreted, can be used to characterize the masses and compositions of merger-driven outflows, helping to resolve a long-standing debate about the origins of r-process material in the Universe. We explore how the uncertain properties of nuclei involved in the r-process complicate the inference of outflow properties from kilonova observations. Using r-process simulations, we show how nuclear physics uncertainties impact predictions of radioactive heating and element synthesis. For a set of models that span a large range in both predicted heating and final abundances, we carry out detailed numerical calculations of decay product thermalization and radiation transport in a kilonova ejecta with a fixed mass and density profile. The light curves associated with our models exhibit great diversity in their luminosities, with peak brightness varying by more than an order of magnitude. We also find variability in the shape of the kilonova light curves and their color, which in some cases runs counter to the expectation that increasing levels of lanthanide and/or actinide enrichment will be correlated with longer, dimmer, redder emission. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The pattern of isotopic abundances produced in rapid neutron capture, or r-process, nucleosynthesis is sensitive to the nuclear physics properties of thousands of unstable neutron-rich nuclear species that participate in the process. It has long been recognized that the some of the most influential pieces of nuclear data for r-process simulations are ß-decay lifetimes. In light of experimental advances that have pushed measurement capabilities closer to the classic r-process path, we revisit the role of individual ß-decay rates in the r process. We perform ß-decay rate sensitivity studies for a main (A > 120) r process in a range of potential astrophysical scenarios. We study the influence of individual rates during (n)-(n) equilibrium and during the post-equilibrium phase where material moves back toward stability. We confirm the widely accepted view that the most important lifetimes are those of nuclei along the r-process path for each astrophysical scenario considered. However, we find in addition that individual ß-decay rates continue to shape the final abundance pattern through the post-equilibrium phase, for as long as neutron capture competes with ß decay. Many of the lifetimes important for this phase of the r process are within current or near future experimental reach. [ABSTRACT FROM AUTHOR]

Rapid neutron capture nucleosynthesis involves thousands of nuclear species far from stability, whose nuclear properties need to be understood in order to accurately predict nucleosynthetic outcomes. Recently sensitivity studies have provided a deeper understanding of how the r process proceeds and have identified pieces of nuclear data of interest for further experimental or theoretical study. A key result of these studies has been to point out the importance of individual neutron capture rates in setting the final r-process abundance pattern for a 'main' (A ~ 130 peak and above) r process. Here we examine neutron capture in the context of a 'weak' r process that forms primarily the A ~ 80 r-process abundance peak. We identify the astrophysical conditions required to produce this peak region through weak rprocessing and point out the neutron capture rates that most strongly influence the final abundance pattern. [ABSTRACT FROM AUTHOR]

The site of the rapid neutron capture process (r process) is one of the open challenges in all of physics today. The r process is thought to be responsible for the creation of more than half of all elements beyond iron. The scientific challenges to understanding the origin of the heavy elements beyond iron lie in both the uncertainties associated with astrophysical conditions that are needed to allow an r process to occur and a vast lack of knowledge about the properties of nuclei far from stability. One way is to disentangle the nuclear and astrophysical components of the question. On the nuclear physics side, there is great global competition to access and measure the most exotic nuclei that existing facilities can reach, while simultaneously building new, more powerful accelerators to make even more exotic nuclei. On the astrophysics side, various astrophysical scenarios for the production of the heaviest elements have been proposed but open questions remain. This paper reports on a sensitivity study of the r process to determine the most crucial nuclear masses to measure using an r-process simulation code, several mass models (FRDM, Duflo-Zuker, and HFB-21), and three potential astrophysical scenarios. [ABSTRACT FROM AUTHOR]

Introduction: The birth environment of the Solar System can be understood through a combination of astronomical observations, astrophysical modeling, and meteoritic measurements of short- and long-lived radionuclides. Understanding the origins of radionuclides produced via the rapid neutron capture process (r-process) of nucleosynthesis is important because many isotopes born from this process serve as chronometers that constrain the origin and evolution of our galaxy. The astrophysical setting for the r-process had often been thought to be core-collapse supernovae [1]. However, the recent observation of the gravitational wave event GW170817 [2] and the inference of lanthanide elements in the ejecta demonstrate that kilanova events (e.g., neutron star mergers) are potential production sites of rprocess elements [3,4]. Two general models have been put forth by cosmochemists to explain the abundances of Solar System radionuclides [5]. The first is a one-phase model of the interstellar medium (ISM) in which single, discrete events are responsible for the solar abundances deduced from studies of meteoritical materials [6,7]. The ISM is populated by longlived nuclides as a result of these discrete nucleosynthetic events and it is enriched in short-lived radionuclides (SLRs) from the most recent event. Recent studies invoking kilonovae sources of r-process nuclides have focused on this type of model (e.g.,[8]). The alternative model, involving a two-phase ISM, suggests that the Solar System sampled averages accrued over time in the star-forming region in which the Sun formed [9]. Actinides are not the best arbiters for these two models, but they are used for cosmochronology, the dating of the Galaxy. Here we use modern nucleosynthesis simulations of potential r-process sites to obtain the production ratios of specific actinides in these events and determine if these new production ratios are consistent with meteorite data and established cosmochronology constraints (i.e., the 10 to 13 Gyr age of the Galaxy). Methods: Nucleosynthesis simulations were run using the reaction framework Portable Routines for Integrated nucleoSynthesis Modeling (PRISM). PRISM can simulate the production of individual isotopes given a set of trajectories that describe the astrophysical parameters of a given nucleosyhthesis event and their associated initial conditions. This allows for the extraction of production ratios of the radionuclides of interest. It is important to use ratios normalized to a stable, reference nuclide to account for the differences in the chemistry between the nuclides. In the case of the actinides, there is no stable reference, so the long-lived isotope 232Th is used. After the production ratios are determined, the solar abundance ratios of the radionuclides normalized to 232Th are divided by the production ratios. This accounts for variations in the production ratios for the different nuclides. The resulting value is a normalized relative abundance (a). Here we ran four different r-process events through PRISM: simple dynamical ejecta trajectory sets of two neutron star merger environments, a neutron star-black hole merger event [10], a simulation of the accretion disk of a neutron star merger [11], and a collapsar simulation [12]. The outputs from these events were used to extract the production ratios of interest (i.e., 235U/232Th, 238U/232Th, 244Pu/232Th, and 247Cm/232Th). Results and Discussion: For the neutron star merger events and the neutron star-black hole event, the a values for 238U/232Th and 244Pu/232Th are in good agreement with those predicted by models for a Galactic age of 13 Gyr [5]. However, there is significant disagreement between the values for 235U/232Th and 247Cm/232Th predicted by our models and predictions based on the age of the Galaxy [5]. The collapsar simulation does not produce significant amounts of the actinides. The results obtained from the neutron star merger events and the neutron star-black hole event in this work demonstrate that three actinides out of five (232Th, 238U, 244Pu) are predicted to be produced at a level that is consistent with previous production ratios estimated from supernova environments [5]. In these cases, the kilonova origin is consistent with constraints from cosmochronology. Conversely, the predictions for 235U and 247Cm are inconsistent with cosmochronology constraints; their relative abundances cannot be reconciled with reasonable ages for the Galaxy. Either the models are incomplete, or these nuclides were not produced in kilonova events. Nonetheless, this work demonstrates that in some cases the actinide production ratios of r-process nuclides are indistinguishable from previous supernova estimates, but in other cases large discrepancies exist. Examining the production ratios obtained for other environments, such as magnetorotational supernovae, will be useful to further assess the possible site(s) of the r-process. [ABSTRACT FROM AUTHOR]

The JYFLTRAP double Penning trap at the Ion Guide Isotope Separator On-Line facility has been used to measure the atomic masses of 13 neutron-rich rare-earth isotopes. Eight of the nuclides, 161Pm, 163Sm, 164,165Eu, 167Gd, and 165,167,168Tb, were measured for the first time. The systematics of the mass surface has been studied via one- and two-neutron separation energies as well as neutron pairing-gap and shell-gap energies. The proton-neutron pairing strength has also been investigated. The impact of the new mass values on the astrophysical rapid neutron capture process has been studied. The calculated abundance distribution results in a better agreement with the solar abundance pattern near the top of the rare-earth abundance peak at around A≈165. [ABSTRACT FROM AUTHOR]

Background: The rapid neutron capture process is one of the main nucleosynthesis processes of elements heavier than Fe. Uncertainties in nuclear properties, such as masses, half-lives, and β-delayed neutron probabilities can cause orders of magnitude of variation within astrophysical r-process simulations. Presently, theoretical models are used to make global predictions of various nuclear properties for the thousands of nuclei required for these simulations, and measurements are required to benchmark these models, especially far from stability. Purpose: β-decay strength distributions can be used to not only inform astrophysical r-process simulations, but also to provide a stringent test for theoretical calculations. The aim of this work is to provide accurate strength distributions for 69,71Coβ decay. Method: The technique of total absorption spectroscopy was used to measure the β decay of 69,71Co for the first time at the National Superconducting Cyclotron Laboratory. The ions were implanted in a double-sided silicon strip detector at the center of the Summing NaI(Tl) detector and identified using standard particle identification methods. The response of the detection system to the β-decay electron and subsequent γ-ray radiation was fit to the observed experimental data using a χ²-minimization technique. Results: β-feeding intensities and Gamow-Teller strength distributions were extracted from the fits of the experimental data. The β-decay intensities show that there is a large percentage of feeding to levels above 2 MeV, which have not been observed in previous studies. The resultant β-feeding intensities and Gamow-Teller strength distributions were compared to shell model and quasiparticle random phase approximation (QRPA) calculations. Conclusions: Comparing experimentally determined β-decay strength distributions provides a test of models, which are commonly used for global β-decay properties for astrophysical calculations. This work highlights the importance of performing detailed comparisons of models to experimental data, particularly far from stability and as close to the r-process path as possible. [ABSTRACT FROM AUTHOR]

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on 69Ni, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented. [ABSTRACT FROM AUTHOR]