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 hydrodynamical simulations 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 solar-like distribution of r-process elements with mass numbers $A \gtrsim 90$ 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 dynamically 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, at least if the dynamical component dominates the overall ejecta. Part II of this work will study the light curve produced by the dynamical ejecta of our four NS merger models.

Comprehensive study of mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars

“…Pioneering simulations for binary neutron star mergers with an approximate neutrino treatment in Newtonian gravity (Rosswog et al 1999;Korobkin et al 2012) as well as those in an approximate framework of general-relativistic gravity but without weak interaction (Goriely et al 2011;Bauswein et al 2013) have shown the ejection of very neutron-rich matter (in terms of the electron fraction, Y e < 0.1). However, in this decade, both including the general relativistic gravity, which results in a more violent merger process and hence in higher matter temperature, and taking weak interaction processes into account have significantly altered our understanding on the neutron-richness of the ejecta (Sekiguchi et al 2015;Palenzuela et al 2015;Sekiguchi et al 2016;Foucart et al 2016;Radice et al 2018;Kullmann et al 2022). These works have shown that the dynamical ejecta have a broad distribution of neutron richness (Y e ≈ 0.05-0.4), and as a result, a wide variety of heavy nuclei are now considered to be synthesized via the rapid neutron capture (r-) process (Wanajo et al 2014;Goriely et al 2015;Radice et al 2018;Kullmann et al 2022).…”

Opacity of the highly ionized lanthanides and the effect on the early kilonova

“…In neutron star merger, the heaviest elements are produced mainly in the tidal ejecta component, i.e., in the ejecta that is mostly distributed towards the equatorial plane (e.g., Bauswein et al 2013;Just et al 2015;Sekiguchi et al 2015;Kullmann et al 2022;Just et al 2022). Hence, the kilonova observed in the equatorial direction is likely to show the effect of the presence of lanthanides.…”

The Production of Actinides in Neutron Star Mergers

“…Earlier r-process nucleosynthesis studies that adopted either the parametrized BNSM ejecta properties or the outflow trajectories extracted from numerical simulations modeling the post-merger evolution of BNSMs, revealed that a significant amount of actinides with mass fraction X act > ∼ 10 −3 is only produced in ejecta with Y e < ∼ 0.2 [32][33][34][35][36][37][38][39][40][41][42][43][44][45]. This can be qualitatively understood from relating the averaged nuclear mass number at the end of the rprocess ( A f ) to the averaged initial seed nuclear mass number ( A s ) and the abundance ratio of free neutrons to seed nuclei (R n/s ) before the onset of r-process by…”