Revisiting the Rheology of Neutron Star Crusts with Molecular Dynamics

TL;DR

This paper reevaluates the rheology of neutron star crusts using molecular dynamics, emphasizing the importance of convergence at slow strain rates for accurate crust-breaking predictions.

Contribution

It highlights the non-convergence of previous results and proposes criteria for quasi-static flow in neutron star crust simulations, advocating for slower strain rates.

Findings

Convergence should be observed at strain rates slower than 10^{-5} ω_p.

Simulations with ~10^5 particles across 10 grains are computationally feasible.

Proper convergence affects the understanding of crust failure in magnetars.

Abstract

Explosive events from magnetars are likely due to the catastrophic release of stress in their crusts, but the behavior of crustal matter beyond linear elasticity is poorly understood. We argue here that seminal results from molecular dynamics informing crust breaking calculations are non-converged, and must be revisited. We estimate the criteria for quasi-static, rate-independent flow by comparing imposed deformation timescales to grain boundary diffusion in polycrystals. We argue that convergence in this regime should be observed at strain rates slower than $10^{-5}\,\omega_p$ (plasma frequency $\omega_p$) in simulations of $N\approx10^5$ particles across order 10 grains at a quarter of the melting temperature. Though computationally expensive, this is tractable with modern methods and GPU supercomputers.