Domains and defects in nuclear "pasta"

TL;DR

This study uses large-scale molecular dynamics simulations to analyze the structure and defects of nuclear pasta in neutron star crusts, highlighting the importance of simulation size for accurate property predictions.

Contribution

It presents the largest nuclear pasta simulations to date and introduces an algorithm for identifying sub-domains, advancing understanding of pasta topology and defects.

Findings

Qualitative agreement between small and large simulations in structure

Finite size effects influence defect formation and structure factors

Large simulations are necessary for accurate transport property predictions

Abstract

Nuclear pasta topology is an essential ingredient to determine transport properties in the inner crust of neutron stars. We perform semi-classical molecular dynamics simulations of nuclear pasta for proton fractions $Y_p=0.30$ and $Y_p=0.40$ near one third of nuclear saturation density, $n=0.05\,\mathrm{fm}^{-3}$, at a temperature $T=1.0\,\mathrm{MeV}$. Our simulations are, to our knowledge, the largest nuclear pasta simulations to date and contain up to $3\,276\,800$ nucleons in the $Y_p=0.30$ and $819\,200$ nucleons in the $Y_p=0.40$ case. An algorithm to determine which nucleons are part of a given sub-domain in the system is presented. By comparing runs of different sizes we study finite size effects, equilibration time, the formation of multiple domains and defects in the pasta structures, as well as the structure factor dependence on simulation size. Although we find qualitative agreement between the topological structure and the structure factors of runs with $51\,200$ nucleons and those with $819\,200$ nucleons or more, we show that simulations with hundreds of thousands of nucleons may be necessary to accurately predict pasta transport properties.