Improving nuclear physics models for neutron star crusts

The composition of the inner and outer crusts of neutron stars is explored, using nuclear models with considerable improvements. We first directly compare the Hartree-Fock-Bogoliubov method and the extended Thomas-Fermi + Strutinsky integral (ETFSI) method in the inner crust, and find a large energy discrepancy, caused by the lack of neutron paring correlations in the ETFSI method. We implement neutron pairing using the local-density approximation, and find that the energy discrepancy is essentially eliminated. We systematically investigate the inner crust using a variety of Skyrme energy density functionals, and find that there is a strong correlation between the pure neutron matter (PNM) equation of state (EoS) of a functional, and its predictions for the proton fraction and pressure in the inner crust. This calls for the use of Skyrme functionals with more stringent constraints on the low-density part of the PNM EoS. Finally, we present a new mass model for the outer crust, where the existing DZ10 model is supplemented with a Gaussian process term. This new model provides reliable extrapolation beyond the limit of measured nuclei, and crucially comes with error bars, so that predictions for the outer crust composition can be given with confidence intervals.