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UBC Theses and Dissertations

Neutron star metallurgy Hoffman, Kelsey Llyn

Abstract

The crust of a neutron star plays an important role in the emission observed from it. The thermal emission generated in the core of the neutron star passes through the crust, thus it is important to know what is in the crust in order to understand how the emission is shaped and altered. The crust itself may be responsible for the observations of glitches from neutron stars and also as a source of gravitational waves. This thesis is two-fold. The first goal is to calculate the composition of the neutron star crust of a non-accreting neutron star. The second is to use the calculated crustal compositions in molecular dynamics simulations in order to determine the shear modulus and breaking strain of the crustal material. The composition of the crust is found to be dependent on how the neutron star cooled. Nuclear reactions within the crust are quenched as the star cools. The composition of the crust, envelope, and atmosphere are calculated after the nuclear reactions are quenched. With the settling timescales of the various isotopes in the crust, some of these isotopes are able to float up to the neutron star surface and form the atmosphere. Three different cooling methods were used in these calculations – modified Urca cooling, a thick crust and a thin crust – each produces different atmospheric and crustal compositions. The calculated crustal abundances are then used as initial conditions in molecular dynamics simulations. A shear force is introduced by deforming the simulation box. The shear modulus and breaking strain are calculated for the three different crustal compositions as well as for perfect pure face-centered cubic (FCC) and body-centered cubic (BCC) systems. The upper limit, from the perfect crystal lattice structure, on the breaking strain is found to ~0.11 − 0.12 and the shear modulus is found to be 6.5 × 10³º dyne/cm². These properties predict glitch amplitudes of ∆Ω/Ω∼10⁻³. The gravitational wave strain amplitudes for PSR J2124- 3358 are also predicted to be greater than the observed upper limits. This indicates that the neutron star crust is not a perfect BCC lattice which deformed to 10% of the maximum.

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