Cooling Neutron Stars with Accreted Envelopes

Gilles Chabrier1, Alexander Y. Potekhin2;1;3, and Dmitry G. Yakovlev2 ABSTRACT The relationships between the e ective surface temperature Te and internal temperature Tb of nonmagnetized neutron stars with and without accreted envelopes are calculated for Te > 5 104 K. We use updated equations of state and radiative opacities, and we improve considerably the electron conductive opacity. We examine various models of accreted layers (H, He, C, O subshells produced by nuclear burning of accreted matter). The resulting Te Tb relationship is remarkably insensitive to the details of the models and depends mainly on the accreted mass M . For Te > 105 K, the accreted matter is generally more heat-transparent. Even a small accreted mass ( M > 10 13M ) a ects appreciably the cooling of a neutron star, leading to higher Te at the neutrino cooling stage and to lower Te 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 curves with the blackbody ts to the ROSAT spectral observations of cooling neutron stars, without invoking quark matter or super uidity in the neutron star cores. Subject headings: stars: neutron { pulsars: general { dense matter Accepted for publication in the Astrophysical Journal Letters