Relativistic neutron star merger simulations with non-zero temperature equations of state. I. Variation of binary parameters and equation of state

By R. Oechslin, Hans-Thomas Janka, Andreas Marek. 2006

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An extended set of binary neutron star (NS) merger simulations is performed with an approximative treatment of general relativity to systematically investigate the influence of the nuclear equation of state (EoS), the NS masses, and the NS spin states prior to merging. The general relativistic hydrodynamics simulations are based on a conformally flat approximation to the Einstein equations and a Smoothed Particle Hydrodynamics code for the gas treatment. We employ the two non-zero temperature EoSs of Shen et al. (1998a, Nucl. Phys. A, 637, 435; 1998b, Prog. Theor. Phys., 100, 1013) and Lattimer & Swesty (1991, Nucl. Phys. A, 535, 331), which represent a “harder” and a “softer” behavior, respectively, with characteristic differences in the incompressibility at supernuclear densities and in the maximum mass of nonrotating, cold neutron stars. In addition, we use the cold EoS of Akmal et al. (1998, Phys. Rev. C, 58, 1804) with a simple ideal-gas-like extension according to Shibata & Taniguchi (2006, Phys. Rev. D, 73, 064027), in order to compare with their results, and an ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS. We estimate the mass sitting in a dilute “torus” around the future black hole (BH) by requiring the specific angular momentum of the torus matter to be larger than the angular momentum of the ISCO around a Kerr BH with the mass and spin parameter of the compact central remnant. The dynamics and outcome of the models is found to depend strongly on the EoS and on the binary parameters. Larger torus masses are found for asymmetric systems (up to ~$0.3~M_\odot$ for a mass ratio of 0.55), for large initial NSs, and for a NS spin state which corresponds to a larger total angular momentum. We find that the postmerger remnant collapses either immediately or after a short time when employing the soft EoS of Lattimer& Swesty, whereas no sign of post-merging collapse is found within tens of dynamical timescales for all other EoSs used. The typical temperatures in the torus are found to be about $3{-}10$ MeV depending on the strength of the shear motion at the collision interface between the NSs and thus depending on the initial NS spins. About $10^{-3}{-}10^{-2}~M_\odot$ of NS matter become gravitationally unbound during or right after the merging process. This matter consists of a hot/high-entropy component from the collision interface and (only in case of asymmetric systems) of a cool/low-entropy component from the spiral arm tips.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/589d489cce921ffb9941f66091a173f4c1139b81

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Citations in the corpus

Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis (2018, background): “…for the gravitational field of the NSs (Ruffert et al. 1996; Rosswog et al. 1999, 2003, 2013; Rosswog & Davies 2003; Rosswog & Liebendoerfer 2003; Oechslin et al. 2007; Korobkin et al. 2012; Bauswein et al. 2013), or included GR effects, but compromised on the treatment of NS matter (Shibata &…”
NEUTRINO-DRIVEN WINDS IN THE AFTERMATH OF A NEUTRON STAR MERGER: NUCLEOSYNTHESIS AND ELECTROMAGNETIC TRANSIENTS (2015, background): “For example, the dynamic ejecta have been recognized to consist of a cold “tidal component” that is launched via gravitational torques and a hotter “interaction component” that comes from the hydrodynamical interaction of the neutron stars (Oechslin et al. 2007; Korobkin et al. 2012).”
The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta (2017, background): “As simulations suggest that this mass is close to the maximum amount of dynamical polar ejecta that can result from a BNS merger (Oechslin et al. 2007; Bauswein et al. 2013; Hotokezaka et al. 2013), this could point to a compact neutron star and therefore a soft equation of state.”
NEUTRINO SIGNATURES AND THE NEUTRINO-DRIVEN WIND IN BINARY NEUTRON STAR MERGERS (2008, methodology): “Oechslin et al. (2007), using a conformally-flat approximation to GR and an SPH code, find that BNS mergers of the type discussed here and modeled with the Shen EOS, avoid the general-relativistic gravitational instability for many tens of milliseconds after the neutron stars first come into…”
r-Process Nucleosynthesis in Hot Accretion Disk Flows from Black Hole-Neutron Star Mergers (2008, background): “However, given that current estimates of mass ejected from compact object mergers tend to be not more than 10 2 M, (Ruffert et al. 1997; Oechslin et al. 2007), unless the rate is higher there is an additional source of weak r-process material beyond BH-NS mergers.”
ELECTROMAGNETIC TRANSIENTS POWERED BY NUCLEAR DECAY IN THE TIDAL TAILS OF COALESCING COMPACT BINARIES (2011, methodology): “For double NS binaries there are one or two such structures, depending on the mass ratio and equation of state (EoS) of the NSs (Oechslin et al. 2007).”
SYSTEMATICS OF DYNAMICAL MASS EJECTION, NUCLEOSYNTHESIS, AND RADIOACTIVELY POWERED ELECTROMAGNETIC SIGNALS FROM NEUTRON-STAR MERGERS (2013, methodology): “The simulations of our study are performed with a relativistic Smooth Particle Hydrodynamics (SPH) code, i.e. the hydrodynamical equations are evolved in a Lagrangian manner (Oechslin et al. 2002, 2007; Bauswein et al. 2010a).”
r-PROCESS NUCLEOSYNTHESIS IN DYNAMICALLY EJECTED MATTER OF NEUTRON STAR MERGERS (2011, methodology): “Recently, special attention has been paid to NS mergers because hydrodynamic simulations of NS-NS and NS-BH mergers showed that a non-negligible amount of matter may be ejected (e.g., Janka et al. 1999; Rosswog et al. 2004; Oechslin et al. 2007).”
FORMATION OF STABLE MAGNETARS FROM BINARY NEUTRON STAR MERGERS (2013, background): “…allow for shock heating during merger.4 This EOS has been chosen since it fits very well the Shen nuclear EOS (Shen et al. 1998a,b) at high densities (Oechslin et al. 2007), and hence it provides a more accurate description of the evolution of the plasma in the high-density regions than the…”
Europium production: neutron star mergers versus core-collapse supernovae (2013, background): “Rosswog et al. (1999, 2000) and more recently Oechslin et al. (2007), Bauswein et al. (2013), Rosswog (2013), Hotokeza a et al. (2013), Kyutoku et al. (2013) showed that up to 10−2 M⊙ of r-process matter may be ejected in a single coalescence event.”
EFFECT OF A HIGH OPACITY ON THE LIGHT CURVES OF RADIOACTIVELY POWERED TRANSIENTS FROM COMPACT OBJECT MERGERS (2013, background): “Simulations of NSMs (e.g. Rosswog et al. 1999; Oechslin et al. 2007; Goriely et al. 2011; Hotokezaka et al. 2013) and of neutron star-black hole mergers (e.g. Janka et al. 1999; Lee 2001; Rosswog 2005) have found that the amount of material ejected is sensitive to binary type, mass ratio, nuclear…”
On the astrophysical robustness of the neutron star merger r-process (2012, background): “As already pointed out by Oechslin et al. (2007), the ejecta stem from two different regions: a hot component from the interaction region between the stars (subsequently called “interaction-” or i-component for short) and a colder one where matter is simply flung out by tidal torques, see the first…”
OPACITIES AND SPECTRA OF THE r-PROCESS EJECTA FROM NEUTRON STAR MERGERS (2013, background): “…simulations suggest that a small fraction of mass is ejected when two neutron stars (or a black hole and neutron star) collide or merge (Janka et al. 1999; Rosswog et al. 1999; Lee 2001; Rosswog 2005; Oechslin et al. 2007; Chawla et al. 2010; Shibata & Taniguchi 2011; Hotokezaka et al. 2013).”