Nuclear heating and melted layers in the inner crust of an accreting neutron star

A neutron star in a long-lived, low-mass binary can easily accrete enough m atter to replace its entire crust. Previous authors noted that an accreted crust, being formed from the burning of accreted hydrogen and helium, allow s a series of nonequilibrium reactions, at densities greater than or simila r to 6 x 10(11) g cm(-3), which release a substantial amount of heat (simil ar to 1 MeV per accreted nucleon). Recent calculations by Schatz et al. sho wed that the crystalline lattice of an accreted crust is also likely to be quite impure. This paper discusses the thermal structure of such a neutron star and surveys how the crust reactions and impurities affect the crust te mperature. During accretion rapid enough to make the accreted hydrogen and helium burn stably (M similar to 10(-8) M . yr(-1); typical of the brightes t low-mass neutron star binaries), most of the heat released in the crust i s conducted into the core, where neutrino emission regulates the temperatur e. As a result there is an inversion of the thermal gradient: the temperatu re decreases with depth in the inner crust. The thermal structure in the cr ust at these high accretion rates is insensitive to the temperature in the hydrogen/helium burning shell. When the crust is very impure, the temperatu re can reach approximate to 8 x 10(8) K at densities greater than or simila r to 6 x 10(11) g cm(-3). This peak temperature depends mostly on the amoun t of heat released and the thermal conductivity and in particular is roughl y independent of the core temperature. The high crust temperatures are suff icient to melt the crystalline lattice in thin layers where electron captur es have substantially reduced the nuclear charge.