Dynamical ejecta from binary neutron star mergers: Impact of residual eccentricity and equation of state implementation

TL;DR

This study investigates how small residual eccentricity and different equation of state implementations affect the matter ejected during binary neutron star mergers, highlighting their impact on ejecta predictions crucial for astrophysical observations.

Contribution

The paper quantifies the effects of residual eccentricity and EOS implementation on ejecta mass, emphasizing their importance in accurate merger simulations.

Findings

Residual eccentricity of ~0.01 causes 25-30% variation in ejecta mass.

Eccentricity influences matter ejection through core bounces during merger.

Commonly used 1% eccentricity in simulations may introduce significant errors.

Abstract

Predicting the properties of the matter ejected during and after a neutron star merger is crucial to our ability to use electromagnetic observations of these mergers to constrain the masses of the neutron stars, the equation of state of dense matter, and the role of neutron star mergers in the enrichment of the Universe in heavy elements. Our ability to reliably provide such predictions is however limited by a broad range of factors, including the finite resolution of numerical simulations, their treatment of magnetic fields, neutrinos, and neutrino-matter interactions, and the approximate modeling of the equation of state of dense matter. In this manuscript, we study specifically the role that a small residual eccentricity and different implementations of the same equation of state have on the matter ejected during the merger of a $1.3M_\odot-1.4M_\odot$ binary neutron star system. We find that a residual eccentricity $e\sim 0.01$, as measured $\sim 4-6$ orbits before merger, causes $O(25\%-30\%)$ changes in the amount of ejected mass, mainly due to changes in the amount of matter ejected as a result of core bounces during merger. We note that $O(1\%)$ residual eccentricities have regularly been used in binary neutron star merger simulations as proxy for circular binaries, potentially creating an additional source of error in predictions for the mass of the dynamical ejecta.