  When binary systems of neutron stars merge, a very small fraction of their
rest mass is ejected, either dynamically or secularly. This material is
neutron-rich and its nucleosynthesis could provide the astrophysical site for
the production of heavy elements in the universe, together with a kilonova
signal confirming neutron-star mergers as the origin of short gamma-ray bursts.
We perform full general-relativistic simulations of binary neutron-star mergers
employing three different nuclear-physics EOSs, considering both equal- and
unequal-mass configurations, and adopting a leakage scheme to account for
neutrino radiative losses. Using a combination of techniques, we carry out an
extensive and systematic study of the hydrodynamical, thermodynamical, and
geometrical properties of the matter ejected dynamically, employing the WinNet
nuclear-reaction network to recover the relative abundances of heavy elements
produced by each configurations. Among the results obtained, three are
particularly important. First, we find that both the properties of the
dynamical ejecta and the nucleosynthesis yields are robust against variations
of the EOS and masses, and match very well the observed chemical abundances.
Second, using a conservative but robust criterion for unbound matter, we find
that the amount of ejected mass is $\lesssim 10^{-3}\,M_{\odot}$, hence at
least one order of magnitude smaller than what normally assumed in modelling
kilonova signals. Finally, using a simplified and gray-opacity model we assess
the observability of the infrared kilonova emission finding, that for all
binaries the luminosity peaks around $\sim1/2$ day in the $H$-band, reaching a
maximum magnitude of $-13$, and decreasing rapidly after one day. These rather
low luminosities make the prospects for detecting kilonovae less promising than
what assumed so far.
