Diffusive heat blanketing envelopes of neutron stars

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 rho(b) similar to 10(8) - 10(10) g cm(-3)) for diffusively equilibrated and non-equilibrated distributions of ion species at different masses Delta M 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 Delta M. The obtained relations are approximated by analytic expressions which are convenient for modelling the evolution of neutron stars.