Pasta Nucleosynthesis: Molecular dynamics simulations of nuclear statistical equilibrium

TL;DR

This study uses large-scale molecular dynamics simulations to investigate nuclear pasta shapes in neutron star mergers, revealing their impact on nucleosynthesis and validating models at higher densities.

Contribution

It introduces advanced GPU-based molecular dynamics simulations to analyze nuclear pasta effects on nucleosynthesis during neutron star mergers, extending validity beyond traditional models.

Findings

Final compositions agree with nuclear statistical equilibrium at higher temperatures.

Non-equilibrium structures like rod-like nuclei appear at low temperatures and higher proton fractions.

The MD model extends understanding of matter behavior at densities above traditional equilibrium models.

Abstract

Background: Exotic non-spherical nuclear pasta shapes are expected in nuclear matter at just below saturation density because of competition between short range nuclear attraction and long range Coulomb repulsion. Purpose: We explore the impact of nuclear pasta on nucleosynthesis, during neutron star mergers, as cold dense nuclear matter is ejected and decompressed. Methods: We perform classical molecular dynamics simulations with 51200 and 409600 nucleons, that are run on GPUs. We expand our simulation region to decompress systems from an initial density of 0.080 fm^{-3} down to 0.00125 fm^{-3}. We study proton fractions of Y_P=0.05, 0.10, 0.20, 0.30, and 0.40 at T =0.5, 0.75, and 1.0 MeV. We calculate the composition of the resulting systems using a cluster algorithm. Results: We find final compositions that are in good agreement with nuclear statistical equilibrium models for temperatures of 0.75 and 1 MeV. However, for proton fractions greater than Y_P=0.2 at a temperature of T = 0.5 MeV, the MD simulations produce non-equilibrium results with large rod-like nuclei. Conclusions: Our MD model is valid at higher densities than simple nuclear statistical equilibrium models and may help determine the initial temperatures and proton fractions of matter ejected in mergers.