The Rapid Proton Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

We calculate the nucleosynthesis during stable nuclear burning on anaccreting neutron star. This is appropriate for weakly magnetic neutron starsaccreting at near-Eddington rates in low mass X-ray binaries, and for mostaccreting X-ray pulsars. We show that the nuclear burning proceeds via therapid proton capture process (rp process), and makes nuclei far beyond the irongroup. The final mixture of nuclei consists of elements with a range of massesbetween approximately A=60 and A=100. The average nuclear mass of the ashes isset by the extent of helium burning via (alpha,p) reactions, and depends on thelocal accretion rate. Our results imply that the crust of these accreting neutron stars is madefrom a complex mixture of heavy nuclei, with important implications for itsthermal, electrical and structural properties. A crustal lattice as impure asour results suggest will have a conductivity set mostly by impurity scattering,allowing more rapid Ohmic diffusion of magnetic fields than previouslyestimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11 postscript figures