Crystallization of the inner crust of a neutron star and the influence of shell effects

TL;DR

This paper investigates the crystallization process of the neutron star's inner crust, emphasizing the influence of shell effects and nuclear surface tension on its composition and properties at finite temperatures.

Contribution

It extends previous work on the outer crust to the inner crust, analyzing the effects of nuclear ingredients and shell effects using a variational approach with multiple nuclear models.

Findings

Shell effects are significant at low densities near the outer crust.

Surface tension at extreme isospin values is crucial for accurate inner crust predictions.

Model dependence mainly arises from the smooth part of the nuclear functional.

Abstract

Context. In the cooling process of a non-accreting neutron star, the composition and properties of the crust are thought to be fixed at the finite temperature where nuclear reactions fall out of equilibrium. A lower estimation for this temperature is given by the crystallization temperature, which can be as high as $\approx 7\times 10^9$ K in the inner crust, potentially leading to sizeable differences with respect to the simplifying cold-catalyzed matter hypothesis. Aims. We extend the recent work by Fantina et al. (2019) on the outer crust, to the study of the crystallization of the inner crust and the associated composition in the one-component plasma approximation. Methods. The finite temperature variational equations for non-uniform matter in both the liquid and the solid phases are solved using a compressible liquid-drop approach with parameters optimized on four different microscopic models which cover the present uncertainties in nuclear modeling. Results. We consider separately the effect of the different nuclear ingredients with their associated uncertainties, namely the nuclear equation of state, the surface properties in the presence of a uniform gas of dripped neutrons, and the proton shell effects arising from the ion single-particle structure. Our results suggest that the highest source of model dependence comes from the smooth part of the nuclear functional. Conclusions. We show that shell effects play an important role at the lowest densities close to the outer crust, but the most important physical ingredient to be settled for a quantitative prediction of the inner crust properties is the surface tension at extreme isospin values.