In this study we explore nucleosynthesis in the dynamic ejecta of compact binary mergers. We are particularly interested in the question how sensitive the resulting abundance patterns are to the parameters of the merging system. Therefore, we systematically investigate combinations of neutron star masses in the range from 1.0 to 2.0 M⊙ and, for completeness, we compare the results with those from two simulations of a neutron star black hole merger. The ejecta masses vary by a factor of 5 for the studied systems, but all amounts are (within the uncertainties of the merger rates) compatible with being a major source of the cosmic r‐process. The ejecta undergo robust r‐process nucleosynthesis which produces all the elements from the second to the third peak in close‐to‐solar ratios. Most strikingly, this r‐process is extremely robust, and all 23 investigated binary systems yield practically identical abundance patterns. This is mainly the result of the ejecta being extremely neutron rich (Ye ≈0.04) and the r‐process path meandering along the neutron drip line so that the abundances are determined entirely by nuclear rather than astrophysical properties. While further questions related to galactic chemical evolution need to be explored in future studies, we consider this robustness together with the ease with which both the second and third peak are reproduced as strong indications that compact binary mergers are prime candidates for the sources of the observed unique heavy r‐process component.

We examine magnetorotationally driven supernovae as sources of r-process elements in the early Galaxy. On the basis of thermodynamic histories of tracer particles from a three-dimensional magnetohydrodynamical core-collapse supernova model with approximated neutrino transport, we perform nucleosynthesis calculations with and without considering the effects of neutrino absorption reactions on the electron fraction (Y e ) during post-processing. We find that the peak distribution of Y e in the ejecta is shifted from ∼ 0.15 to ∼ 0.17 and broadened toward higher Y e due to neutrino absorption. Nevertheless, in both cases the second and third peaks of the solar r-process element distribution can be well reproduced. The rare progenitor configuration that was used here, characterized by a high rotation rate and a large magnetic field necessary for the formation of bipolar jets, could naturally provide a site for the strong r-process in agreement with observations of the early galactic chemical evolution.

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.

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Context. Light elements and nitrogen surface abundances together can constrain the mixing efficiencies in massive stars on the main sequence, because moderate mixing in the surface layers leads to depletion of light elements but only later to enrichment in nitrogen. Aims. We want to test the rotational mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars. Methods. The GENEC is a 1D code containing sophisticated prescriptions for rotational mixing. We implemented an extended reaction network into this code including the light elements Li, Be, and B, which allowed us to perform calculations testing the rotation-induced mixing. Results. We followed 9, 12, and 15 M models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of light isotopes for faster rotating stars and more massive stars. Conclusions. We find that the mixing prescriptions used in the present rotating models for massive single stars can account for most of the observations; however, the uncertainties are quite large, making it hard to draw a firm conclusion on the mixing scenario.