[Paper Review] Nuclear pasta in supernovae and neutron stars

Gentaro Watanabe, Toshiki Maruyama|arXiv (Cornell University)|Sep 16, 2011

Pulsars and Gravitational Waves Research6 citations

TL;DR

This paper reviews the formation and properties of nuclear pasta—exotic non-spherical nuclear structures like rods and slabs—in supernova cores and neutron star crusts. Using quantum molecular dynamics (QMD) simulations, it demonstrates that pasta phases dynamically form during core collapse and crust cooling, progressing sequentially from spherical to rod-like and slab-like nuclei as density increases, resolving a long-standing question about their dynamical emergence in astrophysical conditions.

ABSTRACT

In supernova cores and neutron star crusts, nuclei with exotic shapes such as rod-like and slab-like nuclei are expected to exist. These nuclei are collectively called nuclear "pasta". For the past decades, existence of the pasta phases in the equilibrium state has been studied using various methods. Recently, the formation process of the pasta phases, which has been a long-standing problem, has been unveiled using molecular dynamics simulations. In this review, we first provide the astrophysical background of supernovae and neutron stars and overview the history of the study of the pasta phases. We then focus on the recent study on the formation process of the pasta phases. Finally, we discuss future important issues related to the pasta phases: their astrophysical evidence and consequences.

Motivation & Objective

• To clarify the dynamical formation process of nuclear pasta phases in high-density astrophysical environments such as supernova cores and neutron star crusts.
• To resolve the longstanding uncertainty about whether pasta phases form dynamically during collapse or cooling, rather than only existing in equilibrium.
• To assess the astrophysical consequences of pasta phases, including their impact on neutron star cooling, oscillations, and gravitational wave emission.
• To identify key observational and theoretical challenges for detecting pasta phases and incorporating them into macroscopic models of neutron stars.
• To propose future research directions, including polycrystalline structure effects and experimental probes of low-density nuclear matter.

Proposed method

• Employing quantum molecular dynamics (QMD) simulations to model the time evolution of inhomogeneous nuclear matter with large numbers of nucleons (up to 100,000) without assuming pre-existing nuclear shapes.
• Using the QMD method to simulate both the cooling of hot neutron star crusts and the compression of matter during core collapse, tracking structural evolution from spherical nuclei to pasta phases.
• Applying the QMD framework to study the transition sequence: spherical → rod-like → slab-like → rod-like bubbles, under increasing density.
• Analyzing the system's energy and density profiles to identify phase transitions and structural instabilities driving pasta formation.
• Comparing dynamical results with static equilibrium models (e.g., Wigner-Seitz approximation) to validate the emergence of pasta phases under realistic astrophysical conditions.
• Using large-scale MDGRAPE-2 and -3 supercomputers to perform simulations over timescales of ~10^4 fm/c, capturing the full dynamical evolution of the system.

Experimental results

Research questions

• RQ1 Can nuclear pasta phases dynamically form during the core collapse of a supernova, or are they only stable in equilibrium?
• RQ2 What is the sequence of structural transitions from spherical nuclei to rod-like and slab-like pasta phases during matter compression?
• RQ3 How do the dynamical formation processes of pasta phases affect macroscopic neutron star properties such as cooling curves and oscillation modes?
• RQ4 What role do pasta phases play in the crust-core boundary, particularly in the context of r-mode instability and gravitational wave emission?
• RQ5 How can the properties of low-density nuclear matter, including pasta phases, be probed experimentally?

Key findings

• Pasta phases dynamically form during core collapse, progressing sequentially from spherical nuclei to rod-like, slab-like, and rod-like bubble structures as density increases.
• The formation process occurs within approximately 10^4 fm/c, confirming that pasta phases are not only thermodynamically favorable but also dynamically accessible in supernova cores.
• QMD simulations show that the transition from spherical to rod-like and slab-like nuclei is driven by the competition between surface energy (favoring spheres) and Coulomb repulsion (favoring reduced surface area).
• The dynamical formation of pasta phases resolves the long-standing question of whether these exotic structures appear in real-time astrophysical processes, not just in equilibrium calculations.
• The results indicate that pasta phases are likely present in both hot neutron star crusts during cooling and in collapsing supernova cores, influencing transport and thermal properties.
• The study highlights the importance of incorporating pasta phases into core-collapse simulations, particularly for neutrino opacity and explosion dynamics, with ongoing efforts to build updated equation of state tables.