Astromaterial Science and Nuclear Pasta (1606.03646v2)

Abstract: We define `astromaterial science' as the study of materials in astronomical objects that are qualitatively denser than materials on earth. Astromaterials can have unique properties related to their large density, though they may be organized in ways similar to more conventional materials. By analogy to terrestrial materials, we divide our study of astromaterials into hard and soft and discuss one example of each. The hard astromaterial discussed here is a crystalline lattice, such as the Coulomb crystals in the interior of cold white dwarfs and in the crust of neutron stars, while the soft astromaterial is nuclear pasta found in the inner crusts of neutron stars. In particular, we discuss how molecular dynamics simulations have been used to calculate the properties of astromaterials to interpret observations of white dwarfs and neutron stars. Coulomb crystals are studied to understand how compact stars freeze. Their incredible strength may make crust "mountains" on rotating neutron stars a source for gravitational waves that the Laser Interferometer Gravitational-Wave Observatory (LIGO) may detect. Nuclear pasta is expected near the base of the neutron star crust at densities of $10^{14}$ g/cm$^3$. Competition between nuclear attraction and Coulomb repulsion rearranges neutrons and protons into complex non-spherical shapes such as sheets (lasagna) or tubes (spaghetti). Semi-classical molecular dynamics simulations of nuclear pasta have been used to study these phases and calculate their transport properties such as neutrino opacity, thermal conductivity, and electrical conductivity. Observations of neutron stars may be sensitive to these properties, and can be be used to interpret observations of supernova neutrinos, magnetic field decay, and crust cooling of accreting neutron stars. We end by comparing nuclear pasta shapes with some similar shapes seen in biological systems.

• The paper demonstrates that astromaterial science classifies dense celestial matter into hard Coulomb crystals and soft nuclear pasta found in neutron star crusts.
• The paper employs molecular and semi-classical dynamics simulations to uncover nuclear pasta's complex non-spherical geometries and its effects on thermal and electrical properties.
• The paper highlights that the extreme strength of neutron star crusts—up to ten billion times stronger than steel—could produce detectable gravitational waves through 'mountain' formation.

An Overview of Astromaterial Science and Nuclear Pasta

The paper titled "Astromaterial Science and Nuclear Pasta," authored by M.E. Caplan and C.J. Horowitz, provides an advanced examination of materials characterized by extraordinarily high densities, markedly distinct from terrestrial counterparts. Termed 'astromaterials,' these materials are inherent to celestial bodies such as white dwarfs and neutron stars, prompting the definition of a new field of paper: astromaterial science.

Astromaterial science classifies astromaterials into two principal categories akin to terrestrial material classifications—'hard' and 'soft.' Hard astromaterials, exemplified by Coulomb crystals, exist in the cores of cold white dwarfs and the crusts of neutron stars. Soft astromaterials, represented by nuclear pasta, are found within neutron stars' crusts where extreme densities facilitate distinctive non-spherical nuclear configurations.

Hard Astromaterials: Coulomb Crystals

The paper of Coulomb crystals involves understanding the crystallization process of ionized matter under extreme densities and pressures, epitomized in white dwarfs and neutron stars. At the core of these objects, the intense gravitational forces result in materials crystallizing beyond densities possible on Earth. The investigation utilizes molecular dynamics to simulate the material properties of these dense Coulomb crystals, particularly in the context of neutron stars.

Through simulations, the research identifies an extraordinary strength of neutron star crust material, posited to be up to ten billion times stronger than steel, an observation with significant implications for the potential detection of gravitational waves. The robustness of these crystal formations implies they can sustain substantial deformations, allowing for the emergence of 'mountains' on rotating neutron stars that can generate continuous gravitational waves detectable by observatories such as LIGO.

Soft Astromaterials: The Phenomenon of Nuclear Pasta

Nuclear pasta represents a unique phase of nuclear matter expected at the densely packed base of neutron star crusts. Here, nucleons arrange into complex structures due to the competing forces of nuclear attraction and Coulomb repulsion, forming non-standard shapes like tubes (spaghetti), sheets (lasagna), and various other geometries. These configurations are analyzed through semi-classical molecular dynamics simulations, offering insights into their potential influence on neutron star phenomena.

The properties of nuclear pasta, such as neutrino opacity, electrical and thermal conductivities, and shear viscosity, are critical to interpreting several astrophysical observations related to neutron stars. For example, the crustal cooling rates in accreting neutron stars and the decay dynamics of magnetic fields might provide observable evidence of nuclear pasta's influence.

Implications and Future Prospects

Theoretical and practical implications of this research are extensive. For theoretical astrophysics, understanding these unique materials contributes to models of neutron star structure, thermal dynamics, and gravitational wave emission. Practically, the potential observation of gravitational waves emanating from neutron star "mountains" could enhance gravitational wave astronomy, offering a new method to explore the universe's most extreme physicochemical environments.

Moreover, the analogy between nuclear pasta and biological soft materials suggests potential interdisciplinary applications, bridging nuclear physics and materials science disciplines. As observational capabilities advance, they might confirm these theoretical models, significantly impacting how astrophysical phenomena are understood.

This paper highlights the intricacies of astromaterial science, emphasizing the need for continued computational and observational research to elucidate the properties and implications of these extraordinary materials. With ongoing advancements in astronomical detection and simulation technologies, the frontier of astromaterial science and its understanding of the universe’s most extreme matter continues to expand.