Stability of spherical nuclei in the inner crust of neutron stars

TL;DR

This study demonstrates that spherical nuclei in the inner crust of neutron stars are stable against small quadrupole deformations due to electron background effects, challenging previous beliefs about their instability at high densities.

Contribution

The paper shows that spherical nuclei can be stable against infinitesimal quadrupole deformations in neutron star crusts, considering electron background effects, which was previously uncertain.

Findings

Spherical nuclei are stable against small quadrupole deformations due to electron background.

Instability may occur if nuclei density is much lower than equilibrium.

Results are limited to infinitesimal deformations and do not confirm thermodynamic stability.

Abstract

Neutron stars are the densest objects in the Universe. In this paper we consider so-called inner crust - the layer, where neutron-excess nuclei are immersed into degenerate gas of electrons and sea of quasi-free neutrons. It was generally believed that spherical nuclei become unstable with respect to quadrupole deformations at high densities and here we consider this instability. Within perturbative approach we show that spherical nuclei with equilibrium number density are, in fact, stable with respect to infinitesimal quadrupole deformation. This is due to background of degenerate electrons and associated electrostatic potential which maintain stability of spherical nuclei. However, if the number of atomic nuclei per unit volume is much less than the equilibrium value, instability can arise. To avoid confusion we stress that our results are limited to infinitesimal deformations and do not guaranty strict thermodynamic stability of spherical nuclei. In particular, they does not exclude that substantially non-spherical nuclei (so-called pasta phase) represent thermodynamic equilibrium state of the densest layers of neutron star crust. Rather our results points that spherical nuclei can be metastable even if they are not energetically favourable and the timescale of transformation of spherical nuclei to the pasta phases should be estimated subsequently.