Dynamical ejecta and nucleosynthetic yields from eccentric binary neutron-star mergers

TL;DR

This study investigates eccentric binary neutron-star mergers, analyzing their gravitational-wave signals and nucleosynthetic yields, revealing larger ejecta and velocity compared to circular mergers, with implications for kilonova observations.

Contribution

It provides the first detailed analysis of dynamical ejecta and nucleosynthesis in eccentric neutron-star mergers across various equations of state and orbital parameters.

Findings

Ejected mass can reach up to 0.1 solar masses.

Ejecta velocity can be as high as 0.75c.

Nucleosynthetic yields are insensitive to orbital details.

Abstract

With the recent advent of multi-messenger gravitational-wave astronomy and in anticipation of more sensitive, next-generation gravitational-wave detectors, we investigate the dynamics, gravitational-wave emission, and nucleosynthetic yields of numerous eccentric binary neutron-star mergers having different equations of state. For each equation of state we vary the orbital properties around the threshold of immediate merger, as well as the binary mass ratio. In addition to a study of the gravitational-wave emission including $f$-mode oscillations before and after merger, we couple the dynamical ejecta output from the simulations to the nuclear-reaction network code \texttt{SkyNet} to compute nucleosynthetic yields and compare to the corresponding results in the case of a quasi-circular merger. We find that the amount and velocity of dynamically ejected material is always much larger than in the quasi-circular case, reaching maximal values of $M_{\rm ej, max} \sim 0.1 \, M_{\odot}$ and $v_{\rm max}/c \sim 0.75$. At the same time, the properties of this material are rather insensitive to the details of the orbit, such as pericenter distance or post-encounter apoastron distance. Furthermore, while the composition of the ejected matter depends on the orbital parameters and on the equation of state, the relative nucleosynthetic yields do not, thus indicating that kilonova signatures could provide information on the orbital properties of dynamically captured neutron-star binaries.