Postmerger Mass Ejection of Low-mass Binary Neutron Stars

We study the postmerger mass ejection of low-mass binary neutron stars (NSs) with the system mass of 2.5Mand subsequent nucleosynthesis by performing general-relativistic, neutrino-radiation viscous-hydrodynamics simulations in axial symmetry. We find that the merger remnants are long-lived massive NSs surviving more than several seconds, irrespective of the nuclear equations of state (EOSs) adopted. The ejecta masses of our fiducial models are similar to 0.06-0.1M(depending on the EOS), being similar to 30% of the initial disk masses (similar to 0.15-0.3M). Postprocessing nucleosynthesis calculations indicate that the ejecta is composed mainly of lightr-process nuclei with small amounts of lanthanides (mass fraction similar to 0.002-0.004) and heavier species due to the modest average electron fraction (similar to 0.32-0.34) for a reasonable value of the viscous coefficient. Such abundance distributions are compatible with those in weakr-process stars such as HD 122563 but not with the solarr-process-like abundance patterns found in all measuredr-process-enhanced metal-poor stars. Therefore, low-mass binary NS mergers should be rare. If such low-mass NS mergers occur, their electromagnetic counterparts, kilonovae, will be characterized by an early bright blue emission because of the large ejecta mass as well as the small lanthanide fraction. We also show, however, that if the effective turbulent viscosity is very high, the electron fraction of the ejecta could be low enough that the solarr-process-like abundance pattern is reproduced and the lanthanide fraction becomes so high that the kilonova would be characterized by early bright blue and late bright red emissions.