INTERNAL TEMPERATURES AND COOLING OF NEUTRON-STARS WITH ACCRETED ENVELOPES

The relationships between the effective surface (T-eff) and internal t emperatures of neutron stars (NSs) with and without accreted envelopes are calculated for T-eff > 5 x 10(4) K using new data on the equation of state and opacities in the outer NS layers. We examine various mod els of accreted layers (H, He, C, O shells produced by nuclear transfo rmations in accreted matter). We employ new Opacity Library (OPAL) rad iative opacities for H, He, and Fe. In the outermost NS layers, we imp lement the modern OPAL equation of state for Fe, and the Saumon-Chabri er equation of state for H and He. The updated thermal conductivities of degenerate electrons include the Debye-Waller factor for the electr on-phonon scattering in solidified matter, while in liquid matter they include the contributions from electron-ion collisions (evaluated wit h non-Born corrections and with the ion structure factors in responsiv e electron background) and from the electron-electron collisions. For T-eff < 10(5.5) K, the electron conduction in non-degenerate layers of the envelope becomes important, reducing noticeably the temperature g radient. The accreted matter further decreases this gradient at T-eff > 10(5) K. Even a small amount of accreted matter (with mass greater t han or similar to 10(-16)M(circle dot)) affects appreciably the NS coo ling, leading to higher T-eff at the neutrino cooling stage and to low er T-eff at the subsequent photon stage.