The Elasticity of Nuclear Pasta

Abstract: The elastic properties of neutron star crusts are relevant for a variety of currently observable or near-future electromagnetic and gravitational wave phenomena. These phenomena may depend on the elastic properties of nuclear pasta found in the inner crust. We present large scale classical molecular dynamics simulations where we deform nuclear pasta. We simulate idealized samples of nuclear pasta and describe their breaking mechanism. We also deform nuclear pasta that is arranged into many domains, similar to what is known for the ions in neutron star crusts. Our results show that nuclear pasta may be the strongest known material, perhaps with a shear modulus of $10^{30}\,\text{ergs/cm}^3$ and breaking strain greater than 0.1.

• The paper reveals that nuclear pasta exhibits extraordinary strength with shear moduli up to 10^31 erg/cm^3 and breaking strains exceeding 0.1.
• It utilizes large-scale classical molecular dynamics with two-body potentials to simulate deformation and failure in both idealized and polycrystalline configurations.
• The findings imply that the anisotropic elasticity of nuclear pasta could influence neutron star events such as crust breaking and gravitational wave emissions.

On the Elasticity of Nuclear Pasta

The paper "The Elasticity of Nuclear Pasta" by Caplan, Schneider, and Horowitz presents a thorough examination of the elastic properties of nuclear pasta—complex structures within the inner crust of neutron stars. These properties potentially influence observable electromagnetic and gravitational wave phenomena.

Simulation and Methodology

The research utilizes large-scale classical molecular dynamics simulations to explore the deformation and breaking mechanisms of nuclear pasta. The study simulates both idealized nuclear pasta configurations and more complex arrangements that mimic the polycrystalline nature of neutron star crusts. The model employed is grounded in a two-body potential that accurately reflects the binding behaviors observed in both symmetric nuclear and pure neutron matter. This simulation framework permits an exploration of the elastic attributes of nuclear pasta by subjecting it to controlled strains.

Results

The simulations reveal that nuclear pasta may exhibit extraordinary mechanical strength, with estimates of shear modulus reaching up to $10^{31}\,\text{erg/cm}^3$ and breaking strains exceeding 0.1. Notably, these values might reflect nuclear pasta as the strongest known material. The results highlight the anisotropic and robust nature of particular pasta configurations such as the "lasagna" phase. When such configurations are strained, they undergo a buckling before fracturing, a process characterized in detail by the authors.

Moreover, the study details the role of helicoidal defects that act as transient connectors between pasta layers during deformation. These defects contribute to a shear stress that is pivotal in how nuclear pasta responses are interpreted. Importantly, simulations of domains with varying orientations suggest that nuclear pasta's shear modulus is contingent on these complexities, which classical analytic techniques struggle to predict accurately.

Theoretical Implications and Astrophysical Significance

Nuclear pasta's strong shear modulus could profoundly affect neutron star phenomena. The paper speculates on implications for crust breaking events, such as magnetar outbursts and neutron star mergers, suggesting that nuclear pasta might delay these phenomena until even higher strains are realized, compared to the outer crust. This insight has significant ramifications for understanding gravitational wave sources, notably in the context of continuous wave signals potentially detectable by instruments like LIGO and Virgo.

Future Directions

The paper encourages further investigations into the elasticity of nuclear pasta, emphasizing the challenge of analytical modeling due to the microdomain structure and complex geometries of pasta phases. Further research could explore how superfluid characteristics might influence these elastic properties. Additionally, there is an impetus to examine the nuclear pasta's role in neutron star "mountains," which could be responsible for gravitational wave emissions.

In conclusion, this study provides critical insight into the mechanical properties of nuclear pasta, underscoring its potential significance in both theoretical and observational astrophysics. The approach and findings detailed herein set the stage for deeper explorations into the material science within neutron star crusts.