Neutrino radiation hydrodynamics in hot and dense nuclear matter and the role of microphysics in simulations of massive stars

Abstract

The main results of my doctoral studies were obtained from core collapse simulations of massive stars using a numerical model based on radiation hydrodynamics and three-flavour Boltzmann neutrino transport in spherical symmetry. It was continuously further developed with respect to the involved microphysics, such as neutrino-matter interactions, a nuclear reaction network for low temperatures and densities and the equation of state (EoS) for hot and dense nuclear matter. These improvements made it possible to extend the simulation times from about $1$ second to more than 20 seconds of physical time and allowed a detailed and for the first time consistent radiation hydrodynamics investigation of the neutrino driven wind, which develops during the early post-explosion phase of massive stars due to the continued neutrino energy deposition. The neutrinos that diffuse out of the central object, a protoneutron star (PNS), heat the material on top of the PNS surface. This heat is partly converted into kinetic energy which drives a matter outflow, known as the neutrino driven wind. Neutrino driven explosions of massive Fe-core progenitors of 10 and 18 solar mass were modelled using enhanced neutrino opacities. This was necessary because the explosion mechanism of such stars is a subject of active research and by present standard knowledge only working in multi-dimensional models. In the case of a special progenitor star, the less massive 8.8 solar mass O-Ne-Mg-core, the explosion in spherical symmetry was found even without enhanced opacities. The obtained post-explosion evolution is in qualitative agreement with previous static steady-state and parametrized dynamic wind models. On the other hand, we find generally smaller neutrino luminosities and mean energies, the neutrino driven wind is proton-rich for more then 10 seconds and the PNS properties and the contraction behaviour differ from the assumptions made in previous wind studies. Despite the moderately large entropies per baryon of about 100 and the fast expansion timescale, the conditions found are unlikely to favour r-process nucleosynthesis. In addition, we discuss the formation of stellar mass black holes via PNS collapse. The simulations are launched from several massive progenitors of 40 and 50 solar mass. In the absence of an earlier explosion, the PNS collapses to a black hole due to the continued mass accretion. We analyse the electron-neutrino luminosity dependencies and construct a simple approximation for the electron-neutrino luminosity. Furthermore, we analyse different (mu,tau)-neutrino pair-reactions separately and compare the differences during the post-bounce phase. We also investigate the connection between the increasing (mu,tau)-neutrino luminosity and the PNS contraction during the accretion phase before black hole formation. Comparing the different post-bounce phases of the progenitor models under investigation, we find large differences in the emitted neutrino spectra. These differences and the analysis of the electron-neutrino luminosity indicate a strong progenitor model dependency of the emitted neutrino signal. Including an EoS for strange quark matter based on the simple and widely used MIT bag model, we are able to study the appearance of quark matter in core collapse simulations. The transition from hadronic matter to quark matter is modelled via a Gibbs construction which results in an extended mixed phase. Assuming small bag constants, the phase transition occurs during the early post-bounce phase of massive progenitor stars at densities near nuclear saturation which are found at the PNS centre. The simulations are launched from 10, $13 and 15 solar mass stars, where in the absence of an earlier explosion the PNSs contract due to the continued mass accretion on a timescale of 100 ms. A direct consequence of the phase transition is the formation of a strong second accretion shock at the phase boundary between the mixed and the pure hadronic phases. It even turns into a dynamic shock and overtakes the first shock, which remained unaffected from the happenings inside the PNS. In other words, a new explosion mechanism is discovered, where moderate explosion energies of 1 Bethe are obtained. As soon as this second shock propagates over the sphere of last scattering where neutrinos decouple from matter, a second neutrino burst is released which may possibly be detectable for a future Galactic event, if a quark phase transition has taken place.