COOLING NEUTRON-STARS WITH ACCRETED ENVELOPES

The relationships between the effective surface temperature T-eff and the internal temperature T-b of nonmagnetized neutron stars with and w ithout accreted envelopes are calculated for T-eff > 5 x 10(4) K. We u se updated equations of state and radiative opacities, and we improve considerably the electron conductive opacity. We examine various model s of accreted layers (H, He, C, and O subshells produced by nuclear bu rning of accreted matter). The resulting T-eff-T-b relationship is rem arkably insensitive to the details of the models and depends mainly on the accreted mass Delta M. For T-eff > 10(5) K, the accreted matter i s generally more heat transparent. Even a small accreted mass (Delta M greater than or similar to 10(-13) M.) affects appreciably the coolin g of a neutron star, leading to higher T-eff at the neutrino cooling s tage and to lower T-eff at the subsequent photon stage. We illustrate this by simulating the standard cooling of neutron stars. The presence of accreted matter yields better agreement of our model cooling curve s with the blackbody fits to the ROSAT spectral observations of coolin g neutron stars, without invoking quark matter or superfluidity in the neutron star cores.