We examine the effect of spatial resolution on initial mass ejection in grid-based hydrodynamic simulations of binary neutron star mergers. The subset of the dynamical ejecta with velocities greater than ∼0.6c can generate an ultraviolet precursor to the kilonova on ∼hr timescales and contribute to a years-long non-thermal afterglow. Previous work has found differing amounts of this fast ejecta, by one- to two orders of magnitude, when using particle-based or grid-based hydrodynamic methods. Here we carry out a numerical experiment that models the merger as an axisymmetric collision in a co-rotating frame, accounting for Newtonian self-gravity, inertial forces, and gravitational wave losses. The lower computational cost allows us to reach spatial resolutions as high as 4m, or ∼3×10−4 of the stellar radius. We find that fast ejecta production converges to within 10% for a cell size of 20m. This suggests that fast ejecta quantities found in existing grid-based merger simulations are unlikely to increase to the level needed to match particle-based results upon further resolution increases. The resulting neutron-powered precursors are in principle detectable out to distances ≲200Mpc with upcoming facilities. We also find that head-on collisions at the free-fall speed, relevant for eccentric mergers, yield fast and slow ejecta quantities of order 10−2M⊙, with a kilonova signature distinct from that of quasi-circular mergers.
@article{arxiv.2108.08311,
title = {Resolving the fastest ejecta from binary Neutron Star mergers: implications for electromagnetic counterparts},
author = {Coleman Dean and Rodrigo Fernández and Brian D. Metzger},
journal= {arXiv preprint arXiv:2108.08311},
year = {2021}
}