![]() Sophisticated models are required to quantitatively study all the features related to the merger and postmerger phase. Simulations in numerical relativity (NR) are the most appropriate tool to study the dynamical phases of BNS mergers: the late inspiral (the last ∼20 orbits), the merger, and its aftermath. As the two NSs approach each other, finite size (tides) and hydrodynamic effects become progressively more relevant, and the inspiral terminates when the binary reaches the mass-shedding limit (Roche lobe overflow) ( 13). In particular, the inspiral can be well described by a sequence of circular orbits until shortly before merger. ![]() Nevertheless, the evolution of close circularized binaries can still be considered as an adiabatic process as long as the radiation-reaction timescale is much longer than the orbital period. BNS systems lose orbital angular momentum because of the emission of GWs, so BNS space-times are dynamical. Their self-gravity (or compactness C A = GM A/ c 2 R A ∼ 0.15, with A labeling one of the NSs) cannot be neglected. Isolated NSs are characterized by strong but stationary gravitational fields. However, it is not clear whether NS mergers produce all the r-process nuclei or whether other astrophysical phenomena are required to explain the observed chemical abundances in our galaxy and satellites. The fact that NS mergers produce some r-process nuclei is now firmly established by the multimessenger observations of GW170817 ( 12). NS mergers are also thought to be an important, if not a dominant, astrophysical site of production of r-process elements, such as gold ( 12). With third-generation detectors, or for rare very nearby events, it will be possible to observe GWs emitted by the merger product of two NSs possibly constraining the presence of phase transitions at several times nuclear densities and temperatures of tens of MeV ( 8– 11). GW observation of inspiraling NSs can be used to measure the tidal deformability of the stars, probe the interior structure of NSs, and constrain the nature of matter at supernuclear densities ( 5– 7). Many more multimessenger observations of NS mergers are expected in the next years as the ground-based laser-interferometer detectors LIGO and Virgo reach their design sensitivity and as KAGRA and LIGO India join the network ( 4). The detection of a BNS merger (GW170817) by LIGO/Virgo and electromagnetic (EM) observer partners had a profound impact on our understanding of gravity, the physics of dense matter, the origin of short γ-ray bursts (SGRBs), and the site of production of r-process elements ( 1– 3). Binary systems composed of two NSs have provided the first evidence for the existence of gravitational waves (GWs). Neutron star (NS) mergers are at the heart of some of the most pressing problems in nuclear astrophysics.
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