Thermal conductivity and phase separation of the crust of accreting neutron stars

TL;DR

This study uses molecular dynamics simulations to investigate how impurities and phase separation affect the thermal conductivity of accreting neutron star crusts, with implications for gravitational wave emission.

Contribution

It introduces detailed simulations of neutron star crust composition, revealing impurity scattering reduces thermal conductivity and phase separation occurs, which was not previously modeled.

Findings

Impurity scattering significantly reduces thermal conductivity.

Phase separation leads to compositional heterogeneity in the crust.

Implications for gravitational wave emission from neutron stars.

Abstract

Recently, crust cooling times have been measured for neutron stars after extended outbursts. These observations are very sensitive to the thermal conductivity $\kappa$ of the crust and strongly suggest that $\kappa$ is large. We perform molecular dynamics simulations of the structure of the crust of an accreting neutron star using a complex composition that includes many impurities. The composition comes from simulations of rapid proton capture nucleosynthesys followed by electron captures. We find that the thermal conductivity is reduced by impurity scattering. In addition, we find phase separation. Some impurities with low atomic number $Z$ are concentrated in a subregion of the simulation volume. For our composition, the solid crust must separate into regions of different compositions. This could lead to an asymmetric star with a quadrupole deformation that radiates gravitational waves. Observations of crust cooling can constrain impurity concentrations.