We construct new models of outer heat blanketing envelopes of neutron stars composed of binary ion mixtures (H — He, He — C, C — Fe) in and out of diffusive equilibrium. To this aim, we generalize our previous work on diffusion of ions in isothermal gaseous or Coulomb liquid plasmas to handle non-isothermal systems. We calculate the relations between the effective surface temperature T_s and the temperature T_b at the bottom of heat blanketing envelopes (at a density ρ_b ∼ 10⁸—10¹⁰ g cm⁻³) for diffusively equilibrated and non-equilibrated distributions of ion species at different masses ΔM of lighter ions in the envelope. Our principal result is that the T_s — T_b relations are fairly insensitive to detailed distribution of ion fractions over the envelope (diffusively equilibrated or not) and depend almost solely on ΔM. The obtained relations are approximated by analytic expressions which are convenient for modeling the evolution of neutron stars.

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