Numerical Relativity Simulations of the Neutron Star Merger GW170817: Long-Term Remnant Evolutions, Winds, Remnant Disks, and Nucleosynthesis

TL;DR

This study uses numerical relativity simulations to analyze neutron star merger remnants, focusing on ejecta, winds, disks, and nucleosynthesis, revealing insights into kilonova emissions and heavy element formation.

Contribution

It provides the first detailed long-term simulations of neutron star merger remnants with microphysical models, elucidating wind properties and nucleosynthesis outcomes.

Findings

Wind driven by spiral density waves persists for 100 ms.

Disk masses around long-lived remnants are 0.1-0.2 solar masses.

Nucleosynthesis accounts for all r-process peaks, matching solar abundances.

Abstract

We present a systematic numerical-relativity study of the dynamical ejecta, winds and nucleosynthesis in neutron star merger remnants. Binaries with the chirp mass compatible with GW170817, different mass ratios, and five microphysical equations of state (EOS) are simulated with an approximate neutrino transport and a subgrid model for magnetohydrodynamics turbulence up to 100 milliseconds postmerger. Spiral density waves propagating from the neutron star remnant to the disk trigger a wind with mass flux ${\sim}0.1{-}0.5\,{\rm M_\odot/s}$ persisting for the entire simulation as long as the remnant does not collapse to black hole. This wind has average electron fraction $\gtrsim 0.3$ and average velocity ${\sim}0.1-0.17\,$c and thus is a site for the production of weak $r$-process elements (mass number $A<195$). Disks around long-lived remnants have masses ${\sim}0.1{-}0.2\,{\rm M_\odot}$, temperatures peaking at $\lesssim10\,$MeV near the inner edge, and a characteristic double-peak distribution in entropy resulting from shocks propagating through the disk. The dynamical and spiral-wave ejecta computed in our targeted simulations are not compatible with those inferred from AT2017gfo using two-components kilonova models. Rather, they indicate that multi-component kilonova models including disk winds are necessary to interpret AT2017gfo. The nucleosynthesis in the combined dynamical ejecta and spiral-wave wind in the comparable-mass long-lived mergers robustly accounts for all the $r$-process peaks, from mass number ${\sim}75$ to actinides in terms of solar abundances. Total abundandes are weakly dependent on the EOS, while the mass ratio affect the production of first peak elements.