Fragmentation of Neutron Star Matter

Background: Neutron stars are astronomical systems with nucleons submitted to extreme conditions. Due to the long range coulomb repulsion between protons, the system has structural inhomogeneities. These structural inhomogeneities arise also in expanding systems, where the fragment distribution is highly dependent on the thermodynamic conditions (temperature, proton fraction, ...) and the expansion velocity. Purpose: We aim to find the different regimes of fragment distribution, and the existence of infinite clusters. Method: We study the dynamics of the nucleons with a semiclassical molecular dynamics model. Starting with an equilibrium configuration, we expand the system homogeneously until we arrive to an asymptotic configuration (i. e. very low final densities). We study the fragment distribution throughout this expansion. Results: We found the typical regimes of the asymptotic fragment distribution of an expansion: u-shaped, power law and exponential. Another key feature in our calculations is that, since the interaction between protons is long range repulsive, we do not have always an infinite fragment. We found that, as expected, the faster the expansion velocity is, the quicker the infinite fragment disappears. Conclusions: We have developed a novel graph-based tool for the identification of infinite fragments, and found a transition from U-shaped to exponential fragment mass distribution with increasing expansion rate

The Neutrino Opacity of Neutron Star Inner Crust

The study of neutron rich matter, present in neutron star, proto-neutron stars and core-collapse supernovae, can lead to further understanding of the behavior of nuclear matter in highly asymmetric nuclei. Heterogeneous structures are expected to exist in these systems, often referred to as nuclear pasta. We have carried out a systematic study of neutrino opacity for different thermodynamic conditions in order to assess the impact that the structure has on it. We studied the dynamics of the neutrino opacity of the heterogeneous matter at different thermodynamic conditions with semiclassical molecular dynamics model already used to study nuclear multifragmentation. For different densities, proton fractions and temperature, we calculate the very long range opacity and the cluster distribution. The neutrino opacity is of crucial importance for the evolution of the core-collapse supernovae and the neutrino scattering