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Hydromagnetic equilibrium in non-barotropic multifluid neutron stars

Traditionally, the subject of hydromagnetic equilibrium in neutron stars has been addressed in the context of standard magnetohydrodynamics, with matter obeying a barotropic equation of state. In this paper we take a step towards a more realistic treatment of the problem by considering neutron stars with interior superfluid components. In this multifluid model stratification associated with a varying matter composition (the relative proton to neutron density fraction) enters as a natural ingredient, leading to a non-barotropic system. After formulating the hydromagnetic equilibrium of superfluid/superconducting neutron stars as a perturbation problem, we focus on the particular case of a three-fluid system consisting of superfluid neutrons and normal protons and electrons. We determine the equilibrium structure of dipolar magnetic fields with a mixed poloidal-toroidal composition. We find that, with respect to barotropic models, stratification has the generic effect of leading to equilibria with a higher fraction of magnetic energy stored in the toroidal component. However, even in models with strong stratification the poloidal and toroidal components are comparable, with the former contributing the bulk of the magnetic energy.