2026ApJ...999L..16J

We investigate the nucleosynthesis and kilonova emission based on numerical-relativity binary neutron star merger simulations that incorporate a two-moment neutrino-transport scheme. Unlike in previous works with simpler neutrino treatments, a massive, fast (up to v = 0.3c), proton-rich neutrino-driven wind develops in the postmerger phase of the simulations as long as the merger remnant does not collapse to a black hole. We evolve the ejecta for 100 days after the merger using 2D ray-by-ray radiation-hydrodynamics simulations coupled in situ to a complete nuclear network. The most abundant nucleosynthesis products are He, ⁵⁶Ni, and ⁵⁶Co. We find a total yield of ∼10^–3M_⊙ of ⁵⁶Ni for all mergers that produce massive neutron star remnants, independently of the mass ratio and equations of state. After a few days, the decay of ⁵⁶Ni and later ⁵⁶Co becomes the primary source of heating in the matter expanding above the remnant. As a result, the kilonova light curve flattens on timescales of days for polar observation angles. This is an important effect that should be included in future models of kilonova light curves. Furthermore, the observation of a ⁵⁶Ni-⁵⁶Co feature could serve as a clear indicator for the presence of a long-lived neutron star remnant in future kilonova observations.