Mass ejection and nucleosynthesis in binary neutron star mergers: dependence on lifetime of remnant neutron star

By performing general relativistic hydrodynamics simulations with an approximate neutrino-radiation transfer, the properties of ejecta in dynamical and post-merger phases are investigated for the cases in which the remnant massive neutron star collapses into a black hole in <20 ms after the onset of the merger. The dynamical mass ejection is investigated in three-dimensional simulations. The post-merger mass ejection is then explored in two-dimensional axisymmetric simulations with viscosity using the three-dimensional post-merger systems as the initial conditions. We show that the typical neutron-richness of the dynamical ejecta is higher for the merger of more asymmetric binaries; hence, heavier r-process nuclei are dominantly synthesized. The post-merger ejecta has only a mild neutron-richness, producing lighter r-process nuclei, irrespective of binary mass ratios. Because of the larger disk mass, the post-merger ejecta mass is larger for more asymmetric binary mergers. Thus, the post-merger ejecta can compensate for the underproduced lighter r-process nuclei for asymmetric merger cases. As a result, by summing up both ejecta components, the solar residual r-process pattern is reproduced within the average deviation of a factor of three, irrespective of the binary mass ratio. If a massive neutron star survives for a long time, light r-process nuclei are overproduced, and the resulting abundance pattern deviates from that of the solar residuals. An implication of our results associated with the mass distribution of compact neutron star binaries is discussed.