Fragmentation of Neutron Star Matter

TL;DR

This study investigates how neutron star matter fragments during expansion, revealing different distribution regimes and the conditions for infinite cluster formation using a novel graph-based analysis.

Contribution

We introduce a new graph-based method to identify infinite fragments and analyze the transition in fragment distributions during neutron star matter expansion.

Findings

Identified u-shaped, power law, and exponential fragment distribution regimes.

Discovered that faster expansion velocities lead to the disappearance of infinite fragments.

Developed a novel tool for detecting infinite clusters in nuclear matter simulations.

Abstract

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.