Nucleosynthesis in explosive environments : neutron star mergers and core-collapse supernovae

Abstract

Several nucleosynthesis processes are responsible for the production of the chemical elements in the universe. Explosive ejecta in core-collapse supernovae typically produce intermediate-mass elements up to the iron-group nuclei, although the exact compositions depend on the parameters of the supernova, such as the structure of the pre-supernova progenitor, the explosion energy and neutrino luminosities. Understanding the connection between these parameters and the nuclear composition of the ejecta is an ongoing field of research in nuclear astrophysics. Open questions also remain surrounding the late-time ejecta (the neutrino-driven wind), which could host either a weak r-process or the νp-process. Research on the rapid neutron-capture process requires the knowledge of the properties of exotic nuclei far from stability. Since these nuclei cannot be produced under laboratory conditions, we have to rely on theoretical predictions (e.g., mass models), introducing large uncertainties. In addition, the astrophysical environment of the r-process is still unknown, although recent observational data support mergers of two neutron stars as a promising site. Furthermore, observations of metal-poor stars enriched with r-process material suggest a robust abundance pattern for the strong r-process, which provides a solid benchmark to test our models against. In this thesis, several aspects of explosive nucleosynthesis are studied. In the first part, the theoretical framework of nucleosynthesis calculations is discussed, with a focus on the r-process and fission reactions. The second part highlights the role of fission on r-process calculations. Finally, we report on nucleosynthesis calculations for core-collapse supernova models, on the one hand in spherical symmetry in order to contrain the so-called PUSH method, as well as in axisymmetric models.