Comprehensive study of mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars

Abstract

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 $\lesssim 20$ ms after the onset of the merger. The dynamical mass ejection is investigated in three-dimensional simulations. The post-merger mass ejection is investigated 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 are shown to have only a mild neutron-richness, which results in the production of 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. Our result also indicates that the (about a factor of a few) light-to-heavy abundance scatter observed in $r$-process-enhanced stars can be attributed to variation in the binary mass ratio and total mass. Implications of our results associated with the mass distribution of compact neutron star binaries and the magnetar scenario of short gamma-ray bursts are discussed.