Crust-core interface and bulk neutron star properties

TL;DR

This paper investigates how higher-order terms in the symmetry energy expansion affect the crust-core transition and neutron star properties, highlighting limitations of the parabolic approximation and proposing more accurate modeling approaches.

Contribution

It introduces a comprehensive analysis of higher-order effects on the crust-core transition and neutron star characteristics, improving upon the traditional parabolic approximation.

Findings

Larger slope parameter L leads to slower convergence of the energy expansion.

Universal relation between transition density n_t and L is established.

Parabolic approximation overestimates transition density and pressure.

Abstract

The nuclear symmetry energy plays an important role in the description of the properties of finite nuclei as well as neutron stars. Especially, for low values of baryon density, the accurate description of the crust-core interface strongly depends on the symmetry energy. Usually, the well known parabolic approximation is employed for the definition of the symmetry energy without avoiding some drawbacks. In the present paper, a class of nuclear models, suitable for the description of the inner and outer core of neutron stars, is applied in studying the effect of higher orders of the expansion of the energy on the location of the crust-core transition. The thermodynamical and dynamical methods are used for the determination of the transition density $n_{\rm t}$ and pressure $P_{\rm t}$. The corresponding energy density functional is applied for the study of some relevant properties of both nonrotating and slowly rotating neutron stars. We found that the larger the value of the slope parameter $L$, the slower the convergence of the expansion. In addition, a universal relation is presented between $n_{\rm t}$ and $L$, by employing the full expression and dynamical approach. The crustal moment of inertia is very sensitive to the location of the transition while the effects are moderated concerning the critical angular velocity of the $r$-mode instability and minimum mass configuration. The effect on the tidal deformability is less but not negligible. In any case, the use of the parabolic approximation leads to the overestimation of $n_{\rm t}$ and $P_{\rm t}$ and consequently, on inaccurate predictions. Moreover, in some cases, even the matching process at the interface may affect considerably the predictions, introducing errors of the same order with the one due to the employed method.