Free Neutron Ejection from Shock Breakout in Binary Neutron Star Mergers

Merging neutron stars generate shock waves that disintegrate heavy nuclei into nucleons, especially in the outer envelope. It is expected that some of these neutrons, having avoided capturing positrons, remain as free neutrons even after the disappearance of electron-positron pairs. To investigate how many free neutrons can be ejected from merging neutron stars, we performed special-relativistic Lagrangian hydrodynamics computations with simplified models of this phenomenon in which a spherically symmetric shock wave propagates in the hydrostatic envelope and emerges from the surface. We systematically study a wide parameter space of the size of the merging neutron stars and the energy involved in the shock waves. As a result, it is found that the mass of remaining free neutrons is 10(-7) to 10(-6). Me, which is smaller than the previously expected mass by more than two orders of magnitude. There is a preferred energy of the order of 10(48) erg that yields the maximum amount of free neutrons for large sizes of the envelope. We briefly discuss the emission from the free neutron layer and estimate the luminosity in the optical band to be about 7 x 10(41) erg s(-1) (M-n/10(-6)M(circle dot)) similar to 30 minutes after the merger.