The coalescence of a binary containing a neutron star (NS) and either another NS or a black hole (BH) leads to the formation of a central object that takes most of the mass of the binary. The remaining material (typically a few to tens of percent of a solar mass) is ejected through various channels: (1) dynamically through tidal forces or shock compression, (2) on longer timescales in a wind driven by either neutrino- or viscous heating.
These nonrelativistic ejecta components can lead to the production of r-process elements and to the generation of an electromagnetic counterpart (kilonova), which can aid in the localization of the gravitational wave signal and to provide additional information about the system. Because the timescales over which these ejecta are generated, the two components have different nucleosynthetic and kinematic properties, with observational implications.
The image shows snapshots of a 2D simulation in which the different components (obtained from the output of a 3D merger simulation) are tagged by tracer fluids shown with different colors. For this particular parameter choice, fallback accretion from gravitationally bound material is suppressed by the onset of the viscously-driven wind on a timescale of ~1s.
More information here.