Three-dimensional GRMHD Simulations of Neutrino-cooled Accretion Disks from Neutron Star Mergers

By D. Siegel, B. Metzger. 2017

168 citations recorded by the source corpus.

Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for r-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales (380 ms). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an ∼20 ms cycle, and the generation of a “hot” disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into α-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low (Ye ∼ 0.1) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of Ye ≈ 0.2 to robustly synthesize third-peak r-process elements. The quasi-spherical outflows are projected to unbind 40% of the initial disk mass with typical asymptotic escape velocities of 0.1c and may thus represent the dominant mass ejection mechanism in NS–NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical α-disk simulations that the entire range of r-process nuclei from the first to the third r-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/d421e241aa8ed6104fea99bb0cdeadf2fbd02e13

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In Astronomy and Astrophysics

Citations in the corpus

A Magnetar Origin for the Kilonova Ejecta in GW170817 (2018, background): “A three-dimensional GR MHD simulation of the post-merger disk evolution demonstrates that ≈40% of the initial mass of the torus will be unbound over a timescale of ∼1 s post-merger at an average velocity of v≈0.1 c (e.g., Siegel & Metzger 2017a, 2017b; see also Metzger et al. 2008a).”
Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis (2018, background): “The viscous ejecta is taken to be M Mej; vis vis diskx= with ξvis=0.2±0.1, a range including the results from most postmerger simulations (e.g., Metzger & Fernández 2014; Just et al. 2015; Fernández et al. 2019; Fujibayashi et al. 2018; Siegel & Metzger 2018).”
Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts (2018, background): “Nevertheless, neutrino absorption is crucial for determining a reliable composition and therefore detailed r-process yields for comparison with observations (as demonstrated by Siegel & Metzger 2018 who assess the effect of neutrino absorption in post-processing).”
Origin of the heaviest elements: The rapid neutron-capture process (2019, background): “merger accretion disk ejecta given their smaller velocities and larger masses (Siegel and Metzger, 2017; Siegel and Metzger, 2018; Kasen et al., 2017; Perego et al., 2017a; Fernández et al., 2019; Siegel, 2019) (see section VI.”