On r-process nucleosynthesis from matter ejected in binary neutron starmergers

TL;DR

This study uses general-relativistic simulations to analyze neutron star merger ejecta and nucleosynthesis, revealing consistent heavy element production, smaller ejecta masses than previously assumed, and modest kilonova brightness, impacting detection prospects.

Contribution

It provides the first systematic analysis of ejecta properties and nucleosynthesis yields across different EOSs and masses, highlighting their robustness and implications for kilonova observability.

Findings

Ejecta properties and nucleosynthesis yields are robust across EOSs and masses.

Ejected mass is less than 10^{-3} solar masses, lower than typical assumptions.

Kilonova brightness peaks around magnitude -13 in the H-band, with low detection prospects.

Abstract

When binary systems of neutron stars merge, a very small fraction of their rest mass is ejected, either dynamically or secularly. This material is neutron-rich and its nucleosynthesis could provide the astrophysical site for the production of heavy elements in the universe, together with a kilonova signal confirming neutron-star mergers as the origin of short gamma-ray bursts. We perform full general-relativistic simulations of binary neutron-star mergers employing three different nuclear-physics EOSs, considering both equal- and unequal-mass configurations, and adopting a leakage scheme to account for neutrino radiative losses. Using a combination of techniques, we carry out an extensive and systematic study of the hydrodynamical, thermodynamical, and geometrical properties of the matter ejected dynamically, employing the WinNet nuclear-reaction network to recover the relative abundances of heavy elements produced by each configurations. Among the results obtained, three are particularly important. First, we find that both the properties of the dynamical ejecta and the nucleosynthesis yields are robust against variations of the EOS and masses, and match very well the observed chemical abundances. Second, using a conservative but robust criterion for unbound matter, we find that the amount of ejected mass is $\lesssim 10^{-3}\,M_{\odot}$, hence at least one order of magnitude smaller than what normally assumed in modelling kilonova signals. Finally, using a simplified and gray-opacity model we assess the observability of the infrared kilonova emission finding, that for all binaries the luminosity peaks around $\sim1/2$ day in the $H$-band, reaching a maximum magnitude of $-13$, and decreasing rapidly after one day. These rather low luminosities make the prospects for detecting kilonovae less promising than what assumed so far.