The determination of the mass, composition, and geometry of matter outflows
in black hole-neutron star and neutron star-neutron star binaries is crucial to
current efforts to model kilonovae, and to understand the role of neutron star
merger in r-process nucleosynthesis. In this manuscript, we review the simple
criteria currently used in merger simulations to determine whether matter is
unbound and what the asymptotic velocity of ejected material will be. We then
show that properly accounting for both heating and cooling during r-process
nucleosynthesis is important to accurately predict the mass and kinetic energy
of the outflows. These processes are also likely to be crucial to predict the
fallback timescale of any bound ejecta. We derive a model for the asymptotic
veloicity of unbound matter and binding energy of bound matter that accounts
for both of these effects and that can easily be implemented in merger
simulations. We show, however, that the detailed velocity distribution and
geometry of the outflows can currently only be captured by full 3D fluid
simulations of the outflows, as non-local effect ignored by the simple criteria
used in merger simulations cannot be safely neglected when modeling these
effects. Finally, we propose the introduction of simple source terms in the
fluid equations to approximately account for heating/cooling from r-process
nucleosynthesis in future seconds-long 3D simulations of merger remnants,
without the explicit inclusion of out-of-nuclear statistical equilibrium
reactions in the simulations.
