Nucleosynthesis and observational evidences of magneto-rotational driven supernovae.

Abstract

About half of the heavy elements in our Universe are synthesized by one process, the rapid neutron capture process (r-process). This process requires extreme and violent environments that achieve the necessary neutron-rich conditions. Neutron star mergers and magneto rotational driven supernovae are promising candidates to host the r-process. Within this thesis, we investigate the r-process from an observational as well as a nucleosynthesis point of view. Our ultimate goal is to find out indications in favor of, or against neutron star mergers being the proposed only source of r-process elements. Therefore, we are particularly interested in the chemical evolution of heavy elements. Stars that are embedded in the environment of dwarf galaxies provide a perfect stellar laboratory, since they are usually well separated and shielded from external pollution and each galaxy has its own chemical history. In this work, we study 380 stars from 13 dwarf galaxies. In total, we derive abundances for 12 elements (i.e., magnesium, scandium, titanium, chromium, manganese, iron, nickel, zinc, strontium, yttrium, barium, and europium). To learn about the enrichment in heavy elements of dwarf galaxies, we first examine their individual chemical histories to ultimately learn about the heavy element enrichment of dwarf galaxies. Additionally, we investigate a possible delay in the enrichment of heavy elements that would be inferred from the delayed onset of neutron star mergers. Neutron star mergers have already been observed and confirmed as possible r-process site. We therefore assume that they are the only source of r-process elements and combine different observational constraints to describe the chemical features of heavy elements in the Milky Way. When combining this assumption with realistic delay times, we have not been able to explain a decreasing trend in europium versus iron abundances. We suggest that a possible explanation would be an additional source that exists in the early universe but fades later. Events that possibly can fulfill this criterion are magneto rotational driven supernovae. We use a modern state of the art hydrodynamical simulation to investigate the synthesis of elements in this type of event. In total, we calculate the nucleosynthesis of four models with different magnetic field strengths and rotation rates. We find elements up to xenon (second r-process peak) for the model with weakest magnetic field strength, which is caused by a late change of the proto-neutron star morphology. For the model with the strongest magnetic field strength, we detect a fully operating r-process. Having a detailed abundance pattern of this event calculated, we discuss possible observables such as the ejected nucleosynthetic pattern, the total amount of synthesized europium, and the possible production of gamma- and X-rays.