Three-Dimensional General-Relativistic Magnetohydrodynamic Simulations of Remnant Accretion Disks from Neutron Star Mergers: Outflows and $r$-Process Nucleosynthesis

The merger of binary neutron stars, or of a neutron star and a stellar-mass black hole, can result in the formation of a massive rotating torus around a spinning black hole. In addition to providing collimating media for $\gamma$-ray burst jets, unbound outflows from these disks are an important source of mass ejection and rapid neutron capture ($r$-process) nucleosynthesis. We present the first three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of neutrino-cooled accretion disks in neutron star mergers, including a realistic equation of state valid at low densities and temperatures, self-consistent evolution of the electron fraction, and neutrino cooling through an approximate leakage scheme. After initial magnetic field amplification by magnetic winding, we witness the vigorous onset of turbulence driven by the magnetorotational instability (MRI). The disk quickly reaches a balance between heating from MRI-driven turbulence and neutrino cooling, which regulates the midplane electron fraction to a low equilibrium value $Y_e\approx 0.1$. Over the 380-ms duration of the simulation, we find that a fraction $\approx 20\%$ of the initial torus mass is unbound in powerful outflows with asymptotic velocities $v\approx 0.1c$ and electron fractions $Y_e\approx 0.1–0.25$. Postprocessing the outflows through a nuclear reaction network shows the production of a robust second- and third-peak $r$ process. Though broadly consistent with the results of previous axisymmetric hydrodynamical simulations, extrapolation of our results to late times suggests that the total ejecta mass from GRMHD disks is significantly higher. Our results provide strong evidence that postmerger disk outflows are an important site for the $r$ process.

Figure 1

Snapshots of rest-mass density, number of grid points per fastest-growing MRI mode, and contours of optical depth to electron neutrino number emission ${\tau }_{{\nu }_e}=0.5,0.1,{10}^{-2},{10}^{-3},{10}^{-4}$ at $t=20\mathrm{ms}$, when the disk has settled into a quasistationary state (the BH interior is masked).

Figure 2

Evolution of the density-averaged ratio of electromagnetic energy to internal energy in the disk.

Figure 3

Snapshots of electron fraction, normalized electron chemical potential, and contours of rest-mass density $\rho =[{10}^7,{10}^8,{10}^9,{10}^{10},{10}^{11}]\mathrm{g}{\mathrm{cm}}^{-3}$ at $t=43\mathrm{ms}$, when the disk has fully self-regulated itself to mild electron degeneracy (the BH interior is masked).

Figure 4

Mass distributions of the unbound disk outflow as measured by tracer particles in terms of electron fraction, specific entropy, expansion time scale (all at $t=t_{5\mathrm{GK}}$), and outflow velocity at $r={10}^3\mathrm{km}$.

Figure 5

Top: Final elemental abundances from individual unbound tracer particles (gray lines) and their mean (blue line), compared to observed solar system abundances [70], scaled to match the mean at $A=130$. Bottom: Comparison of the mean abundances to observed abundances in metal-poor halo stars [71, 72, 73] in terms of $\log \epsilon =\log Y_Z/Y_1+12$, scaled such that $\sum (\log Y_Z/Y_{Z,\mathrm{CS}22892\text{−}052}{)}^2$ is minimized between $55\le Z\le 75$.