Impact of nuclear matter properties on the nucleosynthesis and the kilonova from binary neutron star merger ejecta

TL;DR

This study investigates how different nuclear matter properties influence the ejecta composition and kilonova light curves from binary neutron star mergers, revealing complex dependencies and degeneracies that limit EOS constraints.

Contribution

It systematically analyzes the impact of varied finite-temperature nuclear EOSs on nucleosynthesis and kilonova signals using simulations and post-processing models.

Findings

EOS significantly affects ejecta yields and kilonova brightness.

Dynamical ejecta are more directly linked to EOS parameters.

Strong degeneracy limits the use of kilonova observations to constrain nuclear matter.

Abstract

Material expelled from binary neutron star (BNS) mergers can harbor r-process nucleosynthesis and power a Kilonova (KN), both intimately related to the astrophysical conditions of the ejection. In turn such conditions indirectly depend on the equation of state (EOS) describing matter inside the neutron star. Therefore, in principle the above observables can hold valuable information on nuclear matter, as the merger gravitational wave signal already does. In this work, we consider the outcome of a set of BNS merger simulations employing different finite-temperature nuclear EOSs. The latter are obtained from a Skyrme-type interaction model where nuclear properties, such as the incompressibility and the nucleon effective mass at saturation density, are systematically varied. We post-process the ejecta using a reaction network coupled with a semi-analytic KN model, to asses the sensitivity on the input EOS of the final yields and the KN light curves. Both of them are found to be non-trivially influenced by the EOS, with the overall outcome being dominated by the heterogeneous outflows from the remnant disk, hosting a variable degree of neutron-rich material. The dynamical ejecta can be more directly related to the EOS parameters considered, however, we find its role in the yields production and the KN emission too entangled with the other ejecta components, in order to infer solid correlations. This result highlights the strong degeneracy that intervenes between the merger outcome and the behaviour of the intrinsic nuclear matter, and places itself as a limit to the employment of EOS-constraining approaches of such kind.