Mechanical Properties of non-accreting Neutron Star Crusts

TL;DR

This study calculates the mechanical properties of neutron star crusts with different compositions and cooling histories, finding similar shear moduli and breaking strains, which impact gravitational wave detection and SGR burst models.

Contribution

It provides new molecular dynamics simulations of neutron star crusts with various compositions, revealing their mechanical properties and implications for astrophysical phenomena.

Findings

Breaking strain ~0.1 at high density

Shear modulus ~10^{30} dyne/cm^2

Crustal properties suggest limits on gravitational wave strain amplitudes

Abstract

The mechanical properties of a neutron star crust, such as breaking strain and shear modulus, have implications for the detection of gravitational waves from a neutron star as well as bursts from Soft Gamma-ray Repeaters (SGRs). These properties are calculated here for three different crustal compositions for a non-accreting neutron star that results from three different cooling histories, as well as for a pure iron crust. A simple shear is simulated using molecular dynamics to the crustal compositions by deforming the simulation box. The breaking strain and shear modulus are found to be similar in the four cases, with a breaking strain of ~0.1 and a shear modulus of ~10^{30} dyne cm^{-2} at a density of \rho = 10^{14} g cm^{-3} for simulations with an initially perfect BCC lattice. With these crustal properties and the observed properties of {PSR J2124-3358} the predicted strain amplitude of gravitational waves for a maximally deformed crust is found to be greater than the observational upper limits from LIGO. This suggests that the neutron star crust in this case may not be maximally deformed or it may not have a perfect BCC lattice structure. The implications of the calculated crustal properties of bursts from SGRs are also explored. The mechanical properties found for a perfect BCC lattice structure find that crustal events alone can not be ruled out for triggering the energy in SGR bursts.