Vortex Dynamics in the Neutron Star Inner Crust

TL;DR

This study uses 3D simulations to analyze vortex motion and pinning in the neutron star inner crust, revealing how various factors influence vortex unpinning and potential starquake-related phenomena.

Contribution

It provides new insights into vortex-lattice interactions, unpinning mechanisms, and the effects of lattice heterogeneity in neutron star crusts.

Findings

Pinning efficiency is higher for attractive nucleus-vortex interactions.

Unpinning is influenced by lattice orientation, composition, temperature, and pinning energy.

Unpinning can propagate through the lattice, potentially causing vortex avalanches.

Abstract

We study the superfluid vortex motion in the neutron star inner crust through direct three-dimensional simulations of the coupled dynamics of the vortex and the nuclear lattice. We demonstrate the pinning of an initially moving vortex to the lattice through excitation of lattice vibrations, and show that the efficiency of this process is higher for attractive than for repulsive nucleus-vortex interactions. We explore the unpinning of a vortex under the action of the applied Magnus force, and find that it is influenced by multiple parameters, including the sign of the pinning force, the lattice orientation, composition, temperature, and the energy of pinning to individual nucleus. In lattices with multiple grains, the unpinning transition is triggered inside the grains with weaker pinning, propagates along the vortex (mediated by the excited Kelvin waves) and crosses into grains with stronger pinning. This is likely to effectively decrease the critical force at which vortices unpin and to produce extended regions of unpinned vorticity. Shearing of the crust lattice (e.g., by a starquake) initiates the unpinning of the vortices that are crossing the slip plane. A close encounter of an unpinned vortex with a pinned vortex would cause the latter to unpin, perhaps initiating an unpinning avalanche of many vortices.