CRUSTAL IMPURITIES AND THE INTERNAL TEMPERATURE OF A NEUTRON-STAR CRUST

When the upper part of a neutron star crystallizes to form the crust, the constituting ions are trapped in the lattice as a result of the lo w diffusion rates in the solid phase. As a consequence, the local comp osition of the crust corresponds to the equilibrium condition at the m elting paint and not at the present internal temperature. The inclusio n of the small entropic contribution to the free energy does not lead to marked changes in our view of the microscopic structure of a neutro n star crust, because the melting temperature is much smaller than the typical energies at play in the crystal cell. However, mixing between layers characterized by different nuclear species is found to be an i mportant effect for the production of impurities. We show that one sho uld expect an increase of the thermal diffusion time through the crust at small temperatures, because of the decrease of thermal conductivit y in relatively thin impurity-rich layers acting as isolating shields.