SUPERNOVAE THEORY: STUDY OF ELECTRO-WEAK PROCESSES DURING GRAVITATIONAL COLLAPSE OF MASSIVE STARS

Abstract : The physics of supernova requires the understanding of both the complex hydrodynamical phenomena (such as transfer of energy, neutrino transport, shock) as well as the microphysics related to the dense and hot matter. In the framework of type II Supernovae theory, currently most of numerical simulations that simulate the supernova core collapse up to the formation and propagation of the shock wave fail to reproduce the observed explosion of the outer layers of massive stars. The reason for that could be due both to hydrodynamical phenomena such as rotation, convection, and general relativity, and to some microphysical processes involved in the picture and not yet completely understood. The aim of this work is to investigate some of these microphysical inputs, namely the electro-weak processes, that play a crucial role during the gravitational collapse and to analyse their effects by means of hydrodynamical simulations. Among nuclear processes which occur in core-collapse supernova, the most important electro-weak process taking place during the collapse is the electron capture; it occurs both on free protons and on protons bound in nuclei. This capture is essential to determine the evolution of the lepton fraction of the core during the neutronization phase. It affects the efficiency of the bounce and, as a consequence, the strength of the shock wave. Moreover, both the equation of state of supernova matter and electron capture rates in nuclei are modified by the effective mass of nucleons in nuclei, induced by many-body correlations in the dense medium, and its temperature dependence. In the first part of the thesis, a nuclear model aimed at studying the nuclear effective mass is presented. We show how we have included in a energy density functional (EDF) approach a surface-peaked nucleon effective mass to mimic some effects beyond Hartree-Fock. We have added a term to the Skyrme functional, in order to reproduce the enhancement of the effective mass at the nuclear surface, increasing the level density around the Fermi surface. We apply this framework to analyse the mean field properties in 40Ca and 208Pb nuclei, and the pairing properties at zero and finite temperature in the nucleus 120Sn. New calculations to evaluate the temperature dependence of the nucleon effective mass in the microscopic RPA framework are underway. The second part of the thesis is devoted to the supernova models I have worked on. The results obtained within a one-zone approximation, such as the ones achieved in spherically symmetric one-dimensional Newtonian and General Relativistic codes are presented. Even if there are many facet of a supernova event that cannot be consistently captured by a spherically symmetric model, and observations of neutron star kicks, or inhomogeneous ejecta, invoke multi-dimensional effects, one dimensional simulations allow a first and detailed study of different specific inputs and can focus on the uncertainties in nuclear physics. In particular, we show that, introducing a temperature dependent nuclear symmetry en- ergy (via a temperature dependent nucleon effective mass) into the supernova simulations, in the case of a one-zone and a spherically symmetric one dimensional Newtonian code with neutrino transport, the deleptonization is systematically reduced, and the effect on the shock wave energetics is non-negligible. Furthermore, the results obtained with a General Relativistic code, without neutrino transport but with the evolution equation for neutrinos already implemented in a multi-group fashion, are analysed. We study the impact of the equation of state and the electron capture on the collapse phase. A Newtonian version of the code has been implemented, thus comparisons are carried out. The obtained results are in global agreement with the literature. The development of numerical codes done in this thesis in order to simulate the corecollapse is suitable to test microscopical properties of the matter and can provide a useful tool for future research projects.
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Submitted on : Wednesday, February 16, 2011 - 2:03:34 PM
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A. F. Fantina. SUPERNOVAE THEORY: STUDY OF ELECTRO-WEAK PROCESSES DURING GRAVITATIONAL COLLAPSE OF MASSIVE STARS. Astrophysique [astro-ph]. Université Paris Sud - Paris XI, 2010. Français. ⟨tel-00566480⟩

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