Postmerger Mass Ejection of Low-mass Binary Neutron Stars

Abstract

Abstract We study the postmerger mass ejection of low-mass binary neutron stars (NSs) with the system mass of 2.5 M ⊙ and 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 ∼0.06–0.1 M ⊙ (depending on the EOS), being ∼30% of the initial disk masses (∼0.15–0.3 M ⊙). Postprocessing nucleosynthesis calculations indicate that the ejecta is composed mainly of light r-process nuclei with small amounts of lanthanides (mass fraction ∼0.002–0.004) and heavier species due to the modest average electron fraction (∼0.32–0.34) for a reasonable value of the viscous coefficient. Such abundance distributions are compatible with those in weak r-process stars such as HD 122563 but not with the solar r-process-like abundance patterns found in all measured r-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 solar r-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.