Nucleosynthesis in massive rotating stars

Abstract

Most stars we see in the sky produce the energy they radiate away by central fusion. Most of them are fusing hydrogen to helium. After a star has exhausted hydrogen in its centre it contracts and can eventually start the fusion of helium to carbon. Massive stars are dened as stars with at least eight times the mass of the Sun, which is the critical mass for a star needed to start the carbon fusion after central helium has been exhausted. After three further fusion phases an iron core is formed and no further energy can be gained. When this core reaches a critical mass, the Chandrasekhar mass, it collapses and many of them explode in a Supernova, a stellar explosion, which is one of the most energetic events known in the universe. During such an explosion parts of the newly synthesised chemical elements are ejected and leads to an enrichment of heavy elements in the interstellar gas from which later generations of stars are formed. Massive stars are important for the formation and structure of the observed universe as well as for its chemical enrichment. They are therefore also fundamental physical constituents of our solar system and of life on earth. Massive stars have surface temperatures higher than 100000 K and are over ten-thousand times more luminous than the Sun, but their life is much shorter. The way how massive stars evolve, depends mainly on three dierent parameters, namely their initial mass, composition and rotation rate. It was shown by the research in the past 50 years of modelling massive stars, that rotation can strongly aect the way how massive stars evolve. Not only their fate can be changed by rotation eects, but also the chemical signature in the Supernova and wind ejecta. Still, the transport of matter and angular momentum, an essential part of physics inside rotating stars, is not yet fully understood. In this project, I worked, on the one hand, on constraining the rotation induced mixing by looking at the surface evolution of the light element boron. On the other hand, I focussed in the main part of my work on the nucleosynthesis of heavy elements beyond iron by neutron captures during the helium and carbon burning phases in massive stars, the so-called slow neutron capture process or s process. An interesting and not yet fully studied question is, how stellar rotation may aect the s process. In this work, the Geneva stellar evolution code (GenEC) and the Basel nuclear reaction network (BasNet) have been combined. It was found that the combination of meridional circulations toghether with shear mixing can well explain the depletion of boron at the surface of massive stars in the vicinity of the Sun. With a grid of massive star models including the eects of rotation, it was found that rotation induced mixing can enhance the production of nuclei by the s process strongly. This might be a solution for some yet unexplained features in the chemical pattern of very old stars in the Milky Way.