We present a systematic numerical relativity study of the mass ejection and the associated electromagnetic transients and nucleosynthesis from binary neutron star (NS) mergers. We find that a few 10−3 M ⊙ of material is ejected dynamically during the mergers. The amount and the properties of these outflows depend on binary parameters and on the NS equation of state (EOS). A small fraction of these ejecta, typically ∼10−6 M ⊙, is accelerated by shocks formed shortly after merger to velocities larger than 0.6c and produces bright radio flares on timescales of weeks, months, or years after merger. Their observation could constrain the strength with which the NSs bounce after merger and, consequently, the EOS of matter at extreme densities. The dynamical ejecta robustly produce second and third r-process peak nuclei with relative isotopic abundances close to solar. The production of light r-process elements is instead sensitive to the binary mass ratio and the neutrino radiation treatment. Accretion disks of up to ∼0.2 M ⊙ are formed after merger, depending on the lifetime of the remnant. In most cases, neutrino- and viscously driven winds from these disks dominate the overall outflow. Finally, we generate synthetic kilonova light curves and find that kilonovae depend on the merger outcome and could be used to constrain the NS EOS.

Gravitational waves detected from the binary neutron star (NS) merger GW170817 constrained the NS equation of state by placing an upper bound on certain parameters describing the binary's tidal interactions. We show that the interpretation of the UV/optical/infrared counterpart of GW170817 with kilonova models, combined with new numerical relativity results, imply a complementary lower bound on the tidal deformability parameter. The joint constraints tentatively rule out both extremely stiff and soft NS equations of state.

We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally-and shock-driven ejecta, mostly distributed over a broad ∼ 60 • angle from the orbital plane, and, to a lesser extent, by thermally driven winds at high latitudes. Ejecta from eccentric mergers are typically more neutron rich than those of quasi-circular mergers. We find neutrino cooling and heating to affect, quantitatively and qualitatively, composition, morphology, and total mass of the outflows. This is also reflected in the infrared and radio signatures of the binary. The final nucleosynthetic yields of the ejecta are robust and insensitive to input physics or merger type in the regions of the second and third r-process peaks. The yields for elements on the first peak vary between our simulations, but none of our models is able to explain the Solar abundances of first-peak elements without invoking additional first-peak contributions from either neutrino and viscously-driven winds operating on longer timescales after the mergers, or from core-collapse supernovae.

The detection of a kilo/macronova electromagnetic counterpart (AT2017gfo) of the first gravitational wave signal compatible with the merger of two neutron stars (GW170817) has confirmed the occurrence of r-process nucleosynthesis in this kind of events. The blue and red components of AT2017gfo have been interpreted as the signature of multicomponent ejecta in the merger dynamics. However, the explanation of AT2017gfo in terms of the properties of the ejecta and of the ejection mechanisms is still incomplete. In this work, we analyse AT2017gfo with a new semi-analytic model of kilo/macronova inferred from general relativistic simulations of the merger and long-term numerical models of the merger aftermath. The model accounts for the anisotropic emission from the three known mass ejecta components: dynamic, winds and secular outflows from the disk. The early multi-band light-curves of AT2017gfo can only be explained by the presence of a relatively low opacity component of the ejecta at high latitudes. This points to the key role of weak interactions in setting the ejecta properties and determining the nucleosynthetic yields. Our model constrains also the total ejected mass associated to AT2017gfo to be between 0.042 and 0.077 M ; the observation angle of the source to be between π/12 and 7π/36; and the mass of the disk to be 0.08M .