PRODUCTION OF ALL THE <i>r</i> -PROCESS NUCLIDES IN THE DYNAMICAL EJECTA OF NEUTRON STAR MERGERS

Abstract

Recent studies suggest that binary neutron star (NS-NS) mergers robustly\nproduce the heavy r-process nuclei above the atomic mass number A ~ 130 because\nof their ejecta consisting of almost pure neutrons (electron fraction of Y_e &lt;\n0.1). However, little production of the lighter r-process nuclei (A = 90-120)\nconflicts with the spectroscopic results of r-process-enhanced Galactic halo\nstars. We present, for the first time, the result of nucleosynthesis\ncalculations based on the fully general-relativistic simulation of a NS-NS\nmerger with approximate neutrino transport. It is found that the bulk of the\ndynamical ejecta are appreciably shock-heated and neutrino-processed, resulting\nin a wide range of Y_e (= 0.09-0.45). The mass-averaged abundance distribution\nof calculated nucleosynthesis yields is in reasonable agreement with the\nfull-mass range (A = 90-240) of the solar r-process curve. This implies, if our\nmodel is representative of such events, that the dynamical ejecta of NS-NS\nmergers can be the origin of the Galactic r-process nuclei. Our result also\nshows that the radioactive heating after ~ 1 day from the merging, giving rise\nto r-process-powered transient emission, is dominated by the beta-decays of\nseveral species close to stability with precisely measured half-lives. This\nimplies that the total radioactive heating rate for such an event can be well\nconstrained within about a factor of two if the ejected material has a\nsolar-like r-process pattern.\n