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 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.

We have conducted 19 state-of-the-art 3D core-collapse supernova simulations spanning a broad range of progenitor masses. This is the largest collection of sophisticated 3D supernova simulations ever performed. We have found that while the majority of these models explode, not all do, and that even models in the middle of the available progenitor mass range may be less explodable. This does not mean that those models for which we did not witness explosion would not explode in Nature, but that they are less prone to explosion than others. One consequence is that the ‘compactness’ measure is not a metric for explodability. We find that lower-mass massive star progenitors likely experience lower-energy explosions, while the higher-mass massive stars likely experience higher-energy explosions. Moreover, most 3D explosions have a dominant dipole morphology, have a pinched, wasp-waist structure, and experience simultaneous accretion and explosion. We reproduce the general range of residual neutron-star masses inferred for the galactic neutron-star population. The most massive progenitor models, however, in particular vis à vis explosion energy, need to be continued for longer physical times to asymptote to their final states. We find that while the majority of the inner ejecta have Y_e = 0.5, there is a substantial proton-rich tail. This result has important implications for the nucleosynthetic yields as a function of progenitor. Finally, we find that the non-exploding models eventually evolve into compact inner configurations that experience a quasi-periodic spiral SASI mode. We otherwise see little evidence of the SASI in the exploding models.

The detection of a kilo/macronova electromagnetic counterpart (AT 2017gfo) 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 event. The blue and red components of AT 2017gfo have been interpreted as the signature of multi-component ejecta in the merger dynamics. However, the explanation of AT 2017gfo in terms of the properties of the ejecta and of the ejection mechanisms is still incomplete. In this work, we analyze AT 2017gfo 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 AT 2017gfo 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 also constrains the total ejected mass associated to AT 2017gfo to be between 0.042 and 0.077 , the observation angle of the source to be between and , and the mass of the disk to be .

We study the gravitational wave (GW) signal from eight new 3D core-collapse supernova simulations. We show that the signal is dominated by f- and g-mode oscillations of the protoneutron star (PNS) and its frequency evolution encodes the contraction rate of the latter, which, in turn, is known to depend on the star’s mass, on the equation of state, and on transport properties in warm nuclear matter. A lower-frequency component of the signal, associated with the standing accretion shock instability, is found in only one of our models. Finally, we show that the energy radiated in GWs is proportional to the amount of turbulent energy accreted by the PNS.