Effects of the equation of state on the core-crust interface of slowly rotating neutron stars

TL;DR

This paper investigates how different models of the nuclear equation of state influence the core-crust transition properties of neutron stars and their impact on various astrophysical phenomena, emphasizing the importance of accurate transition parameter determination.

Contribution

It systematically compares dynamical and thermodynamical methods for determining transition density and pressure, highlighting their effects on neutron star properties and astrophysical applications.

Findings

Transition properties are sensitive to the symmetry energy approximation.

Method choice (dynamical vs thermodynamical) affects results.

Accurate determination of transition parameters is crucial for neutron star modeling.

Abstract

We systematically study the symmetry energy effects of the transition density $n_{\rm t}$ and the transition pressure $P_{\rm t}$ around the crust-core interface of a neutron star in the framework of the dynamical and the thermodynamical method respectively. We employ both the parabolic approximation and the full expansion, for the definition of the symmetry energy. We use various theoretical nuclear models, which are suitable for reproducing the bulk properties of nuclear matter at low densities, close to saturation density as well as the maximum observational neutron star mass. Firstly we derive and present an approximation for the transition pressure $P_{\rm t}$ and crustal mass $M_{\rm crust}$. Secondly, we explore the effects of the Equation of State (EoS) on a few astrophysical applications which are sensitive to the values of $n_{\rm t}$ and $P_{\rm t}$ including neutron star oscillation frequencies, thermal relaxation of the crust, crustal fraction of the moment of inertia and the r-mode instability window of a rotating neutron star. We found that the above quantities are sensitive mainly on the applied approximation for the symmetry energy (confirming previous results). Furthermore, an additional sensitivity also exists, depending on the used method (dynamical or thermodynamical). The above findings lead us to claim that the determination of the $n_{\rm t}$ and $P_{\rm t}$ must be reliable and accurate before they are used to constrain relevant neutron star properties.