Dynamical ejecta of neutron star mergers with nucleonic weak processes I: Nucleosynthesis

Abstract

We present a coherent study of the impact of neutrino interactions on the r-process element nucleosynthesis and the heating rate produced by the radioactive elements synthesised in the dynamical ejecta of neutron star-neutron star (NS-NS) mergers. We have studied the material ejected from four NS-NS merger systems based on the hydrodynamical simulations of Ardevol-Pulpillo et al. ( 2019) which handle neutrino effects in an elaborate way by including neutrino equilibration with matter in optically thick regions and re-absorption in optically thin regions. We find that the neutron richness of the dynamical ejecta is significantly affected by the neutrinos emitted by the post-merger remnant, in particular when compared to a case neglecting all neutrino interactions. Our nucleosynthesis results show that a solarlike distribution of r-process elements with mass numbers A > ∼ 90 is produced, including a significant enrichment in Sr and a reduced production of actinides compared to simulations without inclusion of the nucleonic weak processes. The composition of the ejected matter as well as the corresponding rate of radioactive decay heating are found to be rather independent of the system mass asymmetry and the adopted equation of state. This approximate degeneracy in abundance pattern and heating rates can be favourable for extracting the ejecta properties from kilonova observations. Part II of this work will study the light curve produced by the dynamical ejecta of our four NS merger models.

Dynamical ejecta of neutron star mergers with nucleonic weak processes II: Kilonova emission

“…Another, more general argument may be connected to the tendency 16 that dynamical ejecta are expelled in all directions, albeit with a greater mass loading in equatorial than polar directions (Bauswein et al 2013;Sekiguchi et al 2015;Radice et al 2018b;Kullmann et al 2021). The dynamical ejecta should therefore overcast all non-relativistic postmerger ejecta to a substantial extent, such that a reprocessing of the emission from interior ejecta by the enveloping dynamical ejecta can be naturally expected, in particular at early times (t < ∼ 1−1.5 d) when most of the dynamical ejecta are still optically thick.…”

“…In this study, we take a step towards resolving this question by investigating the kilonova that results from dynamical ejecta modeled by using a state-of-the-art neutrino scheme, namely the Improved Leakage-Equilibration-Absorption Scheme (ILEAS; Ardevol-Pulpillo et al 2019), which in particular includes neutrino-absorption effects. The present study follows up on the nucleosynthesis analysis presented in Kullmann et al (2021) (called "Part I" hereafter) and discusses the kilonova emission for exactly the same models.…”

The Neutrino Fast Flavor Instability in Three Dimensions

“…In both cases, the matter ejected pollutes the surrounding environment with metals that later form more metal-rich stars, planets, and life. Furthermore, although mergers are the prime candidate source of some of the heaviest elements in the universe, neutrinos play a deciding role in determining whether the ejecta is sufficiently neutronrich to efficiently form these elements (e.g., [4,5]).…”