Nuclear pasta in cold non-accreting neutron stars: symmetry energy effects

The densest part of neutron-star crusts may contain very exotic nuclear configurations, so-called nuclear pasta. We investigate the effect of nuclear symmetry energy on the existence of such phases in cold non-accreting neutron stars. For this purpose, we apply four Brussel-Montreal functionals based on extended Skyrme effective interactions, which parameters were accurately calibrated to reproduce both experimental data on nuclei and realistic neutron-matter equations of state. These functionals differ in their predictions for the density dependence of the symmetry energy. Within the 4th-order extended Thomas-Fermi method, we find pasta phases occupy a larger region of the crust for models with higher symmetry energies at relevant densities in agreement with previous studies based on pure Thomas-Fermi approximation and liquid-drop models. However, the incorporation of microscopic corrections consistently calculated with the Strutinsky integral method leads to a significant shift of the onset of the pasta phases to higher densities due to the enhanced stability of spherical clusters. As a result, the pasta region shrinks significantly, and the role of symmetry energy weakens. This study sheds light on the importance of quantum effects for reliably describing pasta phases in neutron stars.