PRODUCTION OF ALL THE r -PROCESS NUCLIDES IN THE DYNAMICAL EJECTA OF NEUTRON STAR MERGERS

Recent studies suggest that binary neutron star (NS–NS) mergers robustly produce heavy r -process nuclei above the atomic mass number A ∼ 130 because their ejecta consist of almost pure neutrons (electron fraction of Y e < 0.1). However, the production of a small amount of the lighter r -process nuclei ( A ≈ 90–120) conflicts with the spectroscopic results of r -process-enhanced Galactic halo stars. We present, for the first time, the result of nucleosynthesis calculations based on the fully general relativistic simulation of a NS–NS merger with approximate neutrino transport. It is found that the bulk of the dynamical ejecta are appreciably shock-heated and neutrino processed, resulting in a wide range of Y e (≈0.09–0.45). The mass-averaged abundance distribution of calculated nucleosynthesis yields is in reasonable agreement with the full-mass range ( A ≈ 90–240) of the solar r -process curve. This implies, if our model is representative of such events, that the dynamical ejecta of NS–NS mergers could be the origin of the Galactic r -process nuclei. Our result also shows that radioactive heating after ∼1 day from the merging, which gives rise to r -process-powered transient emission, is dominated by the β-decays of several species close to stability with precisely measured half-lives. This implies that the total radioactive heating rate for such an event can be well constrained within about a factor of two if the ejected material has a solar-like r -process pattern.