Core-collapse supernovae and the equation of state

Abstract

Core-collapse supernovae (CCSN) mark the end of the life of massive stars and are cosmic laboratories for physics at the extremes. Numerical simulations of these explosions are essential to understanding the complex mechanisms that are involved. All four fundamental interactions have to be taken into account, which requires the combined knowledge of astrophysics, nuclear physics, particle physics, and observations. A key ingredient in simulations is the equation of state (EOS), which determines the contraction behavior of the proto-neutron star (PNS), and thus impacts neutrino energies and explosion dynamics. However, the EOS for hot and dense matter is still not fully understood and CCSN simulations rely on phenomenological EOS models that differ in their underlying theory as well as nuclear physics input. In this thesis, we investigate the impact of uncertainties in the EOS in CCSN simulations. Further, we present an extension of the high-density EOS models to lower densities and temperatures, which enables us to perform long-time simulations of CCSN, following the shock evolution up to several seconds after bounce. In the first part of this thesis, we present the first systematic study on the effect of different nuclear matter properties of the EOS in CCSN simulations. We investigate the impact of varying the nucleon effective mass, incompressibility, symmetry energy, and nuclear saturation point on the PNS contraction and its implication on the shock evolution. This allows us to examine possible reasons for differences in simulations with commonly used EOS models. We find that the contraction behavior of the PNS is mainly governed by the effective mass, which determines the thermal nucleonic contributions to the EOS. Larger effective masses result in smaller pressures at nuclear densities and a lower thermal index. This modifies the density, and thus the PNS contraction behavior, and consequently the shock propagation. We observe that variations in the symmetry energy impact the electron fraction, entropy, and temperature in the PNS interior. Our results suggest that differences among CCSN EOS mainly originate from their different nuclear matter properties. We verify that our models give reasonable modifications to the mass-radius relation of cold neutron stars and further investigate details of the explosion dynamics. Moreover, our EOS models are tested in different CCSN simulation codes, which yield similar results. Finally, we show that the choice of neutrino treatment impacts the PNS interior. In the second part, we perform long-time CCSN simulations that follow the shock evolution several seconds after bounce, which requires a large simulation domain. To this end, we present a formalism for a high-density EOS transition to lower densities and temperatures. This formalism is tested for various EOS models and different progenitors in spherical symmetry. Additionally, we verify its functionality in cylindrical symmetry and for several neutrino transport schemes. With the transition, we perform the first long-time CCSN simulations in FLASH for exploding models, following the shock expansion up to five seconds after bounce. Different CCSN scenarios are investigated, varying the rotational profile and the explosion energetics by enhancing the neutrino energy deposition in the neutrino leakage scheme. We find that additional rotation and heating favors neutrino-driven winds, which impacts the diagnostic energy. Our results indicate that rotation decreases the mass accretion and reduces neutrino luminosities, as suggested in previous studies. Moreover, the results are compared to simulations performed with an M1 neutrino transport scheme. This allows us to analyze differences in the electron fraction, which need to be considered for future nucleosynthesis studies.