Stability of interlinked neutron vortex and proton flux tube arrays in a neutron star: equilibrium configurations

TL;DR

This study uses 3D simulations to explore the complex interactions and equilibrium configurations of neutron vortices and proton flux tubes in a neutron star's outer core, revealing tangled vortex states due to competing forces.

Contribution

First detailed 3D GPE simulations of neutron vortex and proton flux tube interactions in neutron star conditions, highlighting frustration and metastability.

Findings

Vortex tangles form due to vortex-vortex and vortex-flux-tube interactions.

Multiple metastable states exist with close energies.

Tangled vorticity may be common in neutron star cores.

Abstract

Three-dimensional, Gross-Pitaevskii equation (GPE) simulations are presented of the interaction between neutron superfluid vortices and proton superconductor flux tubes in a rotating, harmonic trap, representing an idealised model of the outer core of a neutron star. Low-energy states of the neutron condensate are calculated by evolving the GPE in imaginary time in the presence of a prescribed, static, rectilinear flux tube array. The calculations are carried out as a function of the angle between the global magnetic and rotation axes, and the amplitude and sign of the current-current and density couplings between the neutron and proton condensates. It is found that the system is frustrated by the competition between vortex-vortex repulsion and vortex-flux-tube attraction (pinning), leading to the formation of vortex tangles and "glassy" behaviour characterized by multiple metastable states spaced closely in energy. The dimensionless parameters in the simulations are ordered as one expects in a neutron star, but the dynamic range is many orders of magnitude smaller than in reality, so caution must be exercised when assessing the astrophysical implications. Nevertheless the results suggest that tangled vorticity may be endemic in neutron star outer cores.