We present the first comprehensive study of r-process element nucleosynthesis in the ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus systems. The evolution of the BH-accretion tori is simulated for seconds with a Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment closure scheme for the transport of electron neutrinos and antineutrinos. The investigated cases are guided by relativistic double neutron star (NS-NS) and NS-BH merger models, producing ∼3-6 M BHs with rotation parameters of A BH ∼ 0.8 and tori of 0.03-0.3 M . Our nucleosynthesis analysis includes the dynamical (prompt) ejecta expelled during the CBM phase and the neutrino and viscously driven outflows of the relic BH-torus systems. While typically ∼20-25% of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ∼1%, but neutrino heating enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide distribution of electron fractions (0.1-0.6) and entropies, they produce heavy elements from A ∼ 80 up to the actinides, with relative contributions of A > ∼ 130 nuclei being subdominant and sensitively dependent on BH and torus masses and the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases can reproduce the solar abundances amazingly well for A > ∼ 90. Varying contributions of the torus ejecta might account for observed variations of lighter elements with 40 Z 56 relative to heavier ones, and a considerable reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly asymmetric NS-BH mergers, might explain the composition of heavy-element deficient stars.

We investigate systematically the dynamical mass ejection, r-process nucleosynthesis, and properties of electromagnetic counterparts of neutron-star (NS) mergers in dependence on the uncertain properties of the nuclear equation of state (EoS) by employing 40 representative, microphysical high-density EoSs in relativistic, hydrodynamical simulations. The crucial parameter determining the ejecta mass is the radius R 1.35 of a 1.35 M ⊙ NS. NSs with smaller R 1.35 ("soft" EoS) eject systematically higher masses. These range from ∼10 −3 M ⊙ to ∼10 −2 M ⊙ for 1.35-1.35 M ⊙ binaries and from ∼5 × 10 −3 M ⊙ to ∼2 × 10 −2 M ⊙ for 1.2-1.5 M ⊙ systems (with kinetic energies between ∼5 × 10 49 erg and 10 51 erg). Correspondingly, the bolometric peak luminosities of the optical transients of symmetric (asymmetric) mergers vary between 3 × 10 41 erg s −1 and 14 × 10 41 erg s −1 (9 × 10 41 erg s −1 and 14.5 × 10 41 erg s −1 ) on timescales between ∼2 h and ∼12 h. If these signals with absolute bolometric magnitudes from −15.0 to −16.7 are measured, the tight correlation of their properties with those of the merging NSs might provide valuable constraints on the high-density EoS. The r-process nucleosynthesis exhibits a remarkable robustness independent of the EoS, producing a nearly solar abundance pattern above mass number 130. By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4 × 10 −5 yr −1 (4 × 10 −4 yr −1 ) for a soft (stiff) NS EoS, if NS mergers are the main source of heavy r-nuclei. The production ratio of radioactive 232 Th to 238 U attains a stable value of 1.64-1.67, which does not exclude NS mergers as potential sources of heavy r-material in the most metal-poor stars.

We introduce a new, powerful method to constrain properties of neutron stars (NSs). We show that the total mass of GW170817 provides a reliable constraint on the stellar radius if the merger did not result in a prompt collapse as suggested by the interpretation of associated electromagnetic emission. The radius R 1.6 of nonrotating NSs with a mass of 1.6 M can be constrained to be larger than 10.68−0.04 km, and the radius R max of the nonrotating maximum mass configuration must be larger than 9.60 +0.14 −0.03 km. We point out that detections of future events will further improve these constraints. Moreover, we show that a future event with a signature of a prompt collapse of the merger remnant will establish even stronger constraints on the NS radius from above and the maximum mass M max of NSs from above. These constraints are particularly robust because they only require a measurement of the chirp mass and a distinction between prompt and delayed collapse of the merger remnant, which may be inferred from the electromagnetic signal or even from the presence/absence of a ringdown gravitational-wave (GW) signal. This prospect strengthens the case of our novel method of constraining NS properties, which is directly applicable to future GW events with accompanying electromagnetic counterpart observations. We emphasize that this procedure is a new way of constraining NS radii from GW detections independent of existing efforts to infer radius information from the late inspiral phase or postmerger oscillations, and it does not require particularly loud GW events.

Although the rapid neutron-capture process, or r-process, is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60, the astrophysical site of this process has not been identified yet. Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars (NSs) and within milliseconds afterwards. For the first time we make use of relativistic hydrodynamical simulations of such events, defining consistently the conditions that determine the nucleosynthesis, i.e., neutron enrichment, entropy, early density evolution and thus expansion timescale, and ejecta mass. We find that 10 −3 -10 −2 M are ejected, which is enough for mergers to be the main source of heavy (A 140) galactic r-nuclei for merger rates of some 10 −5 yr −1 . While asymmetric mergers eject 2-3 times more mass than symmetric ones, the exact amount depends weakly on whether the NSs have radii of ∼15 km for a "stiff" nuclear equation of state (EOS) or ∼12 km for a "soft" EOS. R-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model, we expect high emission in the B-V-R bands for 1-2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass.