Nuclear pasta structures at high temperatures

TL;DR

This paper models high-temperature nuclear pasta structures using relativistic mean field theory, revealing their possible configurations, stability conditions, and phase transitions relevant to neutron star and supernova phenomena.

Contribution

It provides a detailed theoretical analysis of nuclear pasta structures at high temperatures, including phase diagrams and stability criteria, using three-dimensional relativistic mean field calculations.

Findings

Different pasta structures can transform into each other via volume-preserving deformations.

Free energy differences among structures are on the order of tens of keV, indicating possible coexistence.

Critical conditions for disordering and transition to uniform matter depend on density, temperature, and composition.

Abstract

We investigate nuclear pasta structures at high temperatures in the framework of relativistic mean field model with Thomas-Fermi approximation. Typical pasta structures (droplet, rod, slab, tube, and bubble) are obtained, which form various crystalline configurations. The properties of those nuclear pastas are examined in a three-dimensional geometry with reflection symmetry, where the optimum lattice constants are fixed by reproducing the droplet/bubble density that minimizes the free energy adopting spherical or cylindrical approximations for Wigner-Seitz cells. It is found that different crystalline structures can evolve into each other via volume conserving deformations. For fixed densities and temperatures, the differences of the free energies per baryon of nuclear pasta in various shapes and lattice structures are typically on the order of tens of keV, suggesting the possible coexistence of those structures. As temperature increases, the thermodynamic fluctuations are expected to disrupt the long-range ordering in nuclear pasta structures. We then estimate the critical conditions for nuclear pasta to become disordered and behave like liquid, which are found to be sensitive to the densities, temperatures, proton fractions, and nuclear shapes. If we further increase temperature, eventually the nonuniform structures of nuclear pasta become unstable and are converted into uniform nuclear matter. The phase diagrams of nuclear matter are then estimated, which should be useful for understanding the evolutions of neutron stars, supernova dynamics, and binary neutron star mergers.