Heavy Element Nucleosynthesis in Hyperaccreting Neutron Stars

We investigate thermonuclear feedback in hyperaccreting neutron star environments at accretion rates typical of short-period compact object binaries in common envelopes. Our general relativistic hydrodynamic models account for weak reactions, neutrino energy loss, nuclear energy release, pair production, and relativistic degenerate equations of state, with an option for azimuthal (angular momentum carrying) accretion. At the stellar surface, accretion drives an outgoing shock which develops convective currents in its wake where nucleons cycle through multiple heating (photo-disintegration) and cooling (r- and p-process element synthesis) phases, ejecting material with abundances that depend on the accretion rate among other factors. The flows are proton-rich, high entropy, and experience short millisecond expansion-timescales. We find that heavy element nuclei form through a disequilibrium effect between neutrons and alpha-particles, without requiring an excess of neutrons.