Bound Debris Expulsion from Neutron Star Merger Remnants

Abstract

Many studies have found that neutron star mergers leave a fraction of the stars’ mass in bound orbits surrounding the resulting massive neutron star or black hole. This mass is a site of r-process nucleosynthesis and can generate a wind that contributes to a kilonova. However, comparatively little is known about the dynamics determining its mass or initial structure. Here we begin to investigate these questions, starting with the origin of the disk mass. Using tracer particle as well as discretized fluid data from numerical simulations, we identify where in the neutron stars the debris came from, the paths it takes in order to escape from the neutron stars’ interiors, and the times and locations at which its orbital properties diverge from those of neighboring fluid elements that end up remaining in the merged neutron star.

The Dynamics of Debris Disk Creation in Neutron Star Mergers

“…In Zenati et al (2023) we showed how the material escaping from neutron star merger remnants is selected, demonstrating, among other things, that it originates from a wide range of radii within the original neutron stars. This effort was significantly aided by the use of simulation data describing both the motion of tracer particles and snapshots of fluid properties.…”

“…Once the stars are in contact, pressure forces along the contact surface squeeze matter outward, roughly parallel to the orbital axis. However, only a small fraction of the matter expelled from the merged star's interior escapes the remnant, whether it results in a long-lived neutron star or a black hole (Most et al 2021;Hajela et al 2022;Combi & Siegel 2023;Zenati et al 2023). In fact, in the simulation we analyze here, most of the matter that evades the quick collapse to a black hole (only ∼4 ms after the neutron stars touch, at simulation time 16.7 ms) is nonetheless captured within a few more milliseconds; only ≈0.016M e remains in orbit for a longer duration.…”

“…In very coarse terms, the entire population of particles follows a similar path. In the first 1-2 ms after the two stars touch (≈13.3 ms; Zenati et al 2023), the particles having low angular momentum (u f  1.5) become more deeply bound, particularly for those with |u f | = 1. The sense of evolution reverses sharply ≈1.5 ms before the event horizon forms; from then until the creation of the black hole, most of the particles increase in angular momentum and decrease in binding energy.…”

“…Although calculations of neutron star merger dynamics all agree that the great majority of the neutron stars' mass is retained within the black hole or long-lived neutron star remnant, and a small fraction of the mass outside the remnant is propelled outward at mildly relativistic speeds (Davies et al 1994;Rosswog & Davies 2002, Bauswein et al 2013Hotokezaka et al 2013;Kiuchi et al 2014;Fernández et al 2015;Foucart et al 2016;Lehner et al 2016;Radice et al 2016Radice et al , 2018Ruiz et al 2016Ruiz et al , 2021Sekiguchi et al 2016;Dietrich et al 2017;Fujibayashi et al 2018;Ciolfi & Kalinani 2020;Vincent et al 2020;Murguia-Berthier et al 2021;Nedora et al 2021;Sarin & Lasky 2021;Combi & Siegel 2023;Most & Quataert 2023;Zenati et al 2023;Camilletti et al 2024), a large fraction of all the observable properties-the gamma-ray burst, the nucleosynthesis, and at least part of the Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.…”