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 timescales. The mass outflows are composed by neutron-rich dynamical ejecta of mass $\sim 10^{-3}-10^{-2}{M}_{\odot }$ followed by a persistent spiral-wave/neutrino-driven wind of $\gtrsim 10^{-2}{M}_{\odot }$ 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 timescale 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 postmerger originating from $r$-process in the neutron-rich/high-opacity ejecta and UV/optical peaks at a few hours (ten minutes) postmerger 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.

17 pages, 16 figures, 2 appendices. Updated with corrected results for free-neutron decay