Long-lived neutron star remnants from asymmetric binary neutron star mergers: element formation, kilonova signals and gravitational waves

ABSTRACT We present 3D general-relativistic neutrino-radiation hydrodynamics simulations of two asymmetric binary neutron star mergers producing long-lived neutron stars remnants and spanning a fraction of their cooling time-scale. The mergers are characterized by significant tidal disruption with neutron-rich material forming a massive disc around the remnant. The latter develops one-armed dynamics that is imprinted in the emitted kilo-Hertz gravitational waves. Angular momentum transport to the disc is initially driven by spiral-density waves and enhanced by turbulent viscosity and neutrino heating on longer time-scales. The mass outflows are composed by neutron-rich dynamical ejecta of mass ${\sim }10^{-3}\!-\!10^{-2}{\, {\rm M_{\odot }}}$ followed by a persistent spiral-wave/neutrino-driven wind of ${\gtrsim }10^{-2}{\, {\rm M_{\odot }}}$ with material spanning a wide range of electron fractions, ${\sim }0.1\!-\!0.55$. Dynamical ejecta (winds) have fast velocity tails up to ${\sim }0.8$ (${\sim }0.4$) c. The outflows are further evolved to days time-scale using 2D ray-by-ray radiation-hydrodynamics simulations that include an online nuclear network. We find complete r-process yields and identify the production of $^{56}$Ni and the subsequent decay chain to $^{56}$Co and $^{56}$Fe. Synthetic kilonova light curves predict an extended (near-) infrared peak a few days post-merger originating from r-process in the neutron-rich/high-opacity ejecta and UV/optical peaks at a few hours (ten minutes) post-merger originating from weak r-process (free-neutron decay) in the faster ejecta components. Additionally, the fast tail of tidal origin generates kilonova afterglows potentially detectable in radio and X band on a few to ten years time-scale. Quantitative effects originating from the tidal disruption merger dynamics are reflected in the multimessenger emissions.