Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H] > −1 in the disk of the Milky Way cannot be reproduced while accounting for the delaytime distribution (DTD) of coalescence times (∝ t −1 ) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions (∝ t −1.5 ) or power laws combined with a strong burst of mergers before the onset of Type Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50 % of the Eu production in the early universe, before the onset of Type Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

Neutron star mergers have been predicted since the 1970's, supported by the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered as nucleosynthesis sites of the rapid neutron-capture process (r-process), being responsible for making about half of all heavy elements beyond Fe and being the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron-star matter are consistent with solar r-process abundances of heavy nuclei. More recently neutron star mergers have also been identified with short duration Gamma-Ray Bursts via their IR afterglow, only explainable by the opacities of heavy (rather than only Fe-group) nuclei. Two other observations support rare events like neutron star mergers as a dominant scenario for the production of the heaviest r-process nuclei: (a)The discrepancy between the latest admixtures of two long-lived radioactivities ( 60 Fe and 244 Pu) found on earth seems to exclude the origin of the latter from core collapse supernovae. (b)The ratio of [Eu/Fe], with Eu being dominated by r-process contributions, shows a strong scatter in low metallicity stars up to [Fe/H]<-2, arguing for a strongly reduced occurrence rate in comparison to core-collapse supernovae. The high neutron densities in ejected matter permit a violent r-process, encountering fission cycling of the heaviest nuclei in regions far from (nuclear) stability. Uncertainties in nuclear properties, like nuclear masses, betadecay half-lives, fission barriers and fission fragment distributions affect the detailed abundance distributions. The modeling of the astrophysical events depends also on the hydrodynamic treatment, i.e. SPH vs. grid calculations, Newtonian vs. GR approaches, the occurrence of a neutrino wind after the merger and before the emergence of a black hole, and finally the properties of black hole accretion disks. We will discuss the effect of both (nuclear and modelling) uncertainties and conclude that binary compact mergers are probably a or the dominant site of the production of r-process nuclei in our Galaxy. A small caveat exists with respect to explaining the behavior of [Eu/Fe] at lowest metallicities and the question whether neutron star mergers can already contribute at such early times in galactic evolution.2 Thielemann et al.

Comparing observational abundance features with nucleosynthesis predictions of stellar evolution or explosion simulations, we can scrutinize two aspects: (a) the conditions in the astrophysical production site and (b) the quality of the nuclear physics input utilized. We test the abundance features of r-process nucleosynthesis calculations for the dynamical ejecta of neutron star merger simulations based on three different nuclear mass models: The Finite Range Droplet Model, the (quenched version of the) Extended Thomas Fermi Model with Strutinsky Integral, and the Hartree-Fock-Bogoliubov mass model. We make use of corresponding fission barrier heights and compare the impact of four different fission fragment distribution models on the final r-process abundance distribution. In particular, we explore the abundance distribution in the second r-process peak and the rare-earth sub-peak as a function of mass models and fission fragment distributions, as well as the origin of a shift in the third r-process peak position. The latter has been noticed in a number of merger nucleosynthesis predictions. We show that the shift occurs during the r-process freeze-out when neutron captures and β-decays compete and an (n,γ)-(γ,n) equilibrium is no longer maintained. During this phase neutrons originate mainly from fission of material above A = 240. We also investigate the role of β-decay half-lives from recent theoretical advances, which lead either to a smaller amount of fissioning nuclei during freeze-out or a faster (and thus earlier) release of fission neutrons, which can (partially) prevent this shift and has an impact on the second and rare-earth peak as well.

We report on a method, PUSH, for artificially triggering core-collapse supernova explosions of massive stars in spherical symmetry. We explore basic explosion properties and calibrate PUSH to reproduce SN 1987A observables. Our simulations are based on the GR hydrodynamics code AGILE combined with the neutrino transport scheme IDSA for electron neutrinos and ASL for the heavy flavor neutrinos. To trigger explosions in the otherwise non-exploding simulations, the PUSH method increases the energy deposition in the gain region proportionally to the heavy flavor neutrino fluxes. We explore the progenitor range 18 -21 M ⊙ . Our studies reveal a distinction between high compactness (HC) (compactness parameter ξ 1.75 > 0.45) and low compactness (LC) (ξ 1.75 < 0.45) progenitor models, where LC models tend to explode earlier, with a lower explosion energy, and with a lower remnant mass. HC models are needed to obtain explosion energies around 1 Bethe, as observed for SN 1987A. However, all the models with sufficiently high explosion energy overproduce 56 Ni and fallback is needed to reproduce the observed nucleosynthesis yields. 57−58 Ni yields depend sensitively on the electron fraction and on the location of the mass cut with respect to the shell structure of the progenitor. We identify a progenitor and a suitable set of parameters that fit the explosion properties of SN 1987A assuming 0.1 M ⊙ of fallback. We predict a neutron star with a gravitational mass of 1.50 M ⊙ . We find correlations between explosion properties and the compactness of the progenitor model in the explored mass range. However, a more complete analysis will require exploring of a larger set of progenitors.

We present the nucleosynthesis of magneto-rotational supernovae (MR-SNe) including neutrino-driven and magneto-rotational-driven ejecta based, for the first time, on two-dimensional simulations with accurate neutrino transport. The models analysed here have different rotation and magnetic fields, allowing us to explore the impact of these two key ingredients. The accurate neutrino transport of the simulations is critical to analyse the slightly neutron rich and proton rich ejecta that are similar to the, also neutrino-driven, ejecta in standard supernovae. In the model with strong magnetic field, the r-process produces heavy elements up to the third r-process peak (A ∼ 195), in agreement with previous works. This model presents a jet-like explosion with proton-rich jets surrounded by neutron rich material where the r-process occurs. We have estimated a lower limit for 56Ni of 2.5 × 10−2M⊙, which is still well below the expected hypernova value. Longer simulations including the accretion disk evolution are required to get a final prediction. In addition, we have found that the late evolution is critical in a model with weak magnetic field in which lately ejected neutron rich matter produces elements up to the second r-process peak. Even if we cannot yet provide conclusions for hypernova nucleosynthesis, our results agree with observations of old stars and radioactive isotopes in supernova remnants. This makes MR-SNe a good additional scenario to neutron star mergers for the synthesis of heavy elements and brings us closer to understand their origin and the role of MR-SNe in the early galaxy nucleosynthesis.

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron capture process (r-process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives (≃15.6 million years) of the radioactive r-process nuclei iodine-129 and curium-247 preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic ratio 129I/247Cm = 438 ± 184 with nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.

Abstract. Core-collapse supernovae are the first polluters of heavy elements in the galactic history. As such, it is important to study the nuclear compositions of their ejecta, and understand their dependence on the progenitor structure (e.g., mass, compactness, metallicity). Here, we present a detailed nucleosynthesis study based on two long-term, two-dimensional core-collapse supernova simulations of a 11.2 M and a 17.0 M star. We find that in both models nuclei well beyond the iron group (up to Z ≈ 44) can be produced, and discuss in detail also the nucleosynthesis of the p-nuclei 92,94 Mo and 96,98 Ru. While we observe the production of 92 Mo and 94 Mo in slightly neutron-rich conditions in both simulations, 96,98 Ru can only be produced efficiently via the νp-process. Furthermore, the production of Ru in the νp-process heavily depends on the presence of very proton-rich material in the ejecta. This disentanglement of production mechanisms has interesting consequences when comparing to the abundance ratios between these isotopes in the solar system and in presolar grains.

Several extremely metal-poor stars are known to have an enhanced thorium abundance. These actinide-boost stars have likely inherited material from an r-process that operated under different conditions than the r-process that is reflected in most other metal-poor stars with no actinide enhancement.In this article, we explore the sensitivity of actinide production in r-process calculations on the hydrodynamical conditions as well as on the nuclear physics. We find that the initial electron fraction Y e is the most important factor determining the actinide yields and that the abundance ratios between long-lived actinides and lanthanides like europium can vary for different conditions in our calculations. In our setup, conditions with high entropies systematically lead to lower actinide abundances relative to other r-process elements. Furthermore, actinide-enhanced ejecta can be distinguished from the "regular" composition also in other ways, most notably in the second r-process peak abundances.