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

摘要

A neutron star in a long-lived, low-mass binary can easily accrete enoughmatter to replace its entire crust. Previous authors noted that an accretedcrust, being formed from the burning of accreted hydrogen and helium, allows aseries of non-equilibrium reactions, at densities >6e11 g/cc, which release asubstantial amount of heat (1 MeV per accreted nucleon). Recent calculations bySchatz et al. showed that the crystalline lattice of an accreted crust is alsolikely to be quite impure. This paper discusses the thermal structure of such aneutron star and surveys how the crust reactions and impurities affect thecrust temperature. During accretion rapid enough to make the accreted hydrogenand helium burn stably (near the Eddington accretion rate; typical of thebrightest low-mass neutron star binaries), most of the heat released in thecrust is conducted into the core, where neutrino emission regulates thetemperature. As a result there is an inversion of the thermal gradient: thetemperature decreases with depth in the inner crust. The thermal structure inthe crust at these high accretion rates is insensitive to the temperature inthe hydrogen/helium burning shell. When the crust is very impure, thetemperature can reach approximately 8e8 K at densities > 6e11 g/cc. This peaktemperature depends mostly on the amount of heat released and the thermalconductivity and in particular is roughly independent of the core temperature.The high crust temperatures are sufficient to melt the crystalline lattice inthin layers where electron captures have substantially reduced the nuclearcharge.