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Astron. Astrophys. 341, 499-526 (1999)

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4. Summary

We have presented the results of three-dimensional Newtonian hydrodynamic simulations of the merger of equal mass neutron star binary systems. We followed the evolution of two neutron stars containing 1.6 (1.4) [FORMULA] of baryonic matter starting with an initial center of mass distance of 45 km for [FORMULA] 13 ms. In a total of 11 runs we tested the sensitivity of the results to the physical parameters of the binary system and to a number of model assumptions.

In all cases we find a rapidly spinning central object (period [FORMULA] 1 ms) with masses (depending on the initial spins) between 2.5 and 3.1 [FORMULA]. This object might be stable on the simulation time scale, but we suspect it to collapse to a black hole within milliseconds after the merger. Our central objects are surrounded by thick disks containing (depending on the initial spins) [FORMULA] 0.1 to 0.3 [FORMULA]. In addition we find long extended tails for the corotating models. These expand explosively when the neutron star matter gets decompressed by tidal torques in the LS-EOS cases when the adiabatic exponent rises and a few times [FORMULA] erg are released due to the recombination of nucleons into heavy nuclei. For the polytropic EOS ([FORMULA]) the tails remain thin and well-defined. For the other initial spins both the central object and the disk are embedded in a low density cloud of decompressed neutron star matter. In all cases (apart from the one where the stars spin against the orbit) almost baryon free funnels form above the poles of the central object. In these funnels large gradients of radiation pressure could be built up that would be able to accelerate indrifting matter into two jets pointing away from the poles. These funnels would also be an ideal place for relativistic fireballs to form. However, a mass as small as [FORMULA] [FORMULA] within such a fireball would be enough to prevent a GRB from forming. The present resolution is too low to draw conclusions on this point. We find SPH-smoothed temperatures of up to 50 MeV. These are found in macroscopic vortex structures that form along the contact surface of both stars and which we suspect to originate from Kelvin-Helmholtz instabilities.

The main new result is the amount of mass that is ejected into space. We find that, dependent on the initial spins, between [FORMULA] and [FORMULA] [FORMULA] become unbound for the realistic equation of state of Lattimer and Swesty. This result is strongly dependent on the EOS, a stiff polytrope ejects only around one half of this material. In the test case of a soft polytrope ([FORMULA]) we cannot resolve any mass loss, which indicates a strong sensitivity on the stiffness of the EOS. The material gets ejected at very high densities ranging from [FORMULA] to [FORMULA]. The bulk of matter gets ejected with Ye below 0.05 with small contaminations of the neutron star crust ([FORMULA]). Such low [FORMULA], low entropy matter is prone to form r-process nuclei. However, the results concerning [FORMULA] are biased by our simple neutrino treatment. Using recent rates for neutron star mergers we find that [FORMULA] to [FORMULA] of r-process material would have to be ejected to explain the observed abundances exclusively by coalescing neutron stars. Thus our numbers for the amount of ejecta look promising and if, as suggested, large parts of this matter consist of r-process nuclei, neutron star mergers could account for all the observed r-process material in the Galaxy.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998