Simulating Binary Neutron Star Mergers with Finite-temperature Equations of State: The influences of the slope of the symmetry energy and artificial heating

TL;DR

This study uses advanced numerical simulations of binary neutron star mergers to explore how the slope of the symmetry energy affects ejecta composition and r-process nucleosynthesis, with limited impact on gravitational wave signals.

Contribution

It introduces a new set of simulations varying the symmetry energy slope and analyzes its effects on ejecta and gravitational waves, highlighting observable electromagnetic signatures.

Findings

Ejecta composition varies significantly with symmetry energy slope.

Gravitational wave signals are not sensitive to the symmetry energy slope.

R-process yields differ notably at lower mass numbers depending on the symmetry energy slope.

Abstract

We present a new set of numerical-relativity simulations of merging binary neutron stars, aiming to identify possible observable signatures of the slope of the symmetry energy $L_{\rm sym}$. To achieve this goal, we employ a set of equations of state based on a parameterization of the covariant density functional theory of nuclear matter that allows controlled variations of $L_{\rm sym}$ and the skewness $Q_{\rm sat}$, holding the latter fixed. For a set of our simulations, we identify a steep energy gradient in the equation of state at subsaturation densities, which acts as a source of heating with subsequent stiffening produced by thermal support. Accounting for related structural modifications in the tidal deformability reconciles our results with theoretical expectations. On the other hand, we show that gravitational waves are unlikely to distinguish the role of $L_{\rm sym}$. In contrast to this, we find that the ejecta composition is significantly altered in our simulations, which employ an M1 moment scheme, when $L_{\rm sym}$ is varied. Based on our extracted dynamical ejecta properties, we compute r-process yields and find that they are distinct for the different $L_{\rm sym}$, especially at lower mass numbers $A \lesssim 120$. This suggests that electromagnetic counterparts are more likely to exhibit signatures; however, a direct connection to $L_{\rm sym}$ remains a challenge, given the complex interplay between details of the ejecta properties and the kilonova signal.