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.
