The influence of neutrinos on r-process nucleosynthesis in the ejecta of black hole–neutron star mergers

Abstract

During the merger of a black hole and a neutron star, baryonic mass can\nbecome unbound from the system. Because the ejected material is extremely\nneutron-rich, the r-process rapidly synthesizes heavy nuclides as the material\nexpands and cools. In this work, we map general relativistic models of black\nhole-neutron star (BHNS) mergers into a Newtonian smoothed particle\nhydrodynamics (SPH) code and follow the evolution of the thermodynamics and\nmorphology of the ejecta until the outflows become homologous. We investigate\nhow the subsequent evolution depends on our mapping procedure and find that the\nresults are robust. Using thermodynamic histories from the SPH particles, we\nthen calculate the expected nucleosynthesis in these outflows while varying the\nlevel of neutrino irradiation coming from the postmerger accretion disk. We\nfind that the ejected material robustly produces r-process nucleosynthesis even\nfor unrealistically high neutrino luminosities, due to the rapid velocities of\nthe outflow. Nonetheless, we find that neutrinos can have an impact on the\ndetailed pat- tern of the r-process nucleosynthesis. Electron neutrinos are\ncaptured by neutrons to produce protons while neutron capture is occurring. The\nproduced protons rapidly form low mass seed nuclei for the r-process. These low\nmass seeds are eventually incorporated into the first r-process peak at A~78,\nproducing mainly Ge and Se. We consider the mechanism of this process in detail\nand discuss if it can impact galactic chemical evolution of the first peak\nr-process nuclei.\n