The role of weak interactions in dynamic ejecta from binary neutron star mergers

Abstract

Abstract. Weak reactions are critical for the neutron richness of the matter dynamically ejected after the merger of two neutron stars. The neutron richness, defined by the electron fraction (Y e ), determines which heavy elements are produced by the r-process and thus directly impacts the kilonova light curve. In this work, we have performed a systematic and detailed post-processing study of the impact of weak reactions on the distribution of the electron fraction and of the entropy on the dynamic ejecta obtained from an equal mass neutron star binary merger simulated in full general relativity and with microscopic equation of state. Previous investigations indicated that shocks increase Y e , however our results show that shocks can also decrease Y e , depending on their thermodynamical conditions. Moreover, we have found that neutrino absorption are key and need to be considered in future simulations. We also demonstrated that the angular dependence of the neutrino luminosity and the spatial distribution of the ejecta can lead to significant difference in the electron fraction distribution. In addition to the detailed study of the Y e evolution and its dependences, we have performed nucleosynthesis calculations. They clearly point to the necessity of improving the neutrino treatment in current simulations to be able to predict the contribution of neutron star mergers to the chemical history of the universe and to reliable calculate their kilonova light curves.

Neutron star merger remnants

“…Because free neutrons are abundant, the absorption opacities for ν e are larger than those forν e , while pair processes, responsible for keeping ν μ,τ and their antiparticles in equilibrium, decouple at much larger densities and temperatures inside the remnant. Electron neutrino and positron absorption on neu-trons increases substantially the electron fraction in the material, with a larger effect in hotter remnants and along the polar regions, where neutrino fluxes are more intense due to the lower optical depths [49,[125][126][127][128][129].…”

“…Neutrinos emitted from the hottest and densest part of the remnant irradiate the low density part of the disk (and the expanding wind) thus increasing substantially the electron fraction in the material [183]. The larger effects are for hotter remnants and along the polar regions, where neutrino fluxes are more intense due to the lower optical depths [49,[123][124][125][126][127][128][129]183]. The combined effect of magnetohydrodynamics and neutrino processes is likely to play an important role in the dynamics and should be further explored by future simulations [105,188,190].…”

Effects of chiral effective field theory equation of state on binary neutron star mergers

“…Oscillations of the remnant can cause waves in the surrounding debris material which can also steepen into shocks. Since the r-process nucleosynthesis and the resulting kilonova signal depend on the thermal history, the electron fraction, and the velocity of the ejecta, it is very important to identify the different ejecta components and the corresponding ejection mechanisms (see [96] for a discussion).…”

Full transport model of GW170817-like disk produces a blue kilonova

“…Several mechanisms can produce these outflows [25,26]. Tidal ejecta typically produce a red component, while shock-driven, near-polar dynamical ejecta can potentially be blue [27][28][29][30][31]. Wind off of a remnant hypermassive, supramassive, or stable neutron star can also be blue [24,32,33].…”