Coalescing neutron stars – a step towards physical models

Three-dimensional hydrodynamical, Newtonian calculations of the coalescence of equal-mass binary neutron stars are performed with the “Piecewise Parabolic Method”. The properties of neutron star matter are described by the equation of state of Lattimer & Swesty (1991) which allows us to include the emission of neutrinos and to evaluate our models for the νν̄-annihilation in the vicinity of the merging stars. When the stars have merged into one rapidly spinning massive body, a hot toroidal cloud of gas with a mass of about 0.1–0.2M⊙ forms around the wobbling and pulsating central ∼ 3M⊙ object. At that time the total neutrino luminosity climbs to a maximum value of 1–1.5 ·1053 erg/s of which 90–95% originate from the toroidal gas cloud surrounding the very dense core. The mean energies of νe, ν̄e, and heavy-lepton neutrinos νx are around 12 MeV, 20 MeV, and 27 MeV, respectively. The characteristics of the neutrino emission are very similar to the emission from type-II supernovae, except for the ν̄e luminosity from the merged neutron stars which is a factor 3–6 higher than the luminosities of the other neutrino species. When the neutrino luminosities are highest, νν̄annihilation deposits about 0.2–0.3% of the emitted neutrino energy in the immediate neighborhood of the merger, and the maximum integral energy deposition rate is 3– 4 · 10 erg/s. Since the 3M⊙ core of the merged object will most likely collapse into a black hole within milliseconds, the energy that can be pumped into a pair-photon fireball is insufficient by a factor of about 1000 to explain γ-ray bursts at cosmological distances with an energy of the order of 10/(4π) erg/steradian. Analytical estimates show that the additional energy provided by the annihiSend offprint requests to: M. Ruffert ⋆ e-mail: [email protected] ⋆⋆ e-mail: [email protected] ⋆⋆⋆ e-mail: [email protected] † e-mail: [email protected] lation of νν̄ pairs emitted from a possible accretion torus of ∼ 0.1M⊙ around the central black hole is still more than a factor of 10 too small, unless focussing of the fireball into a jet-like expansion plays an important role. A few 10M⊙ of very neutron-rich, low-entropy matter may be dynamically ejected shortly after the neutron stars have merged, and another 10 up to a few 10M⊙ of strongly neutronized, high-entropy material could be carried away from the accretion torus in a neutrino-driven wind. The contamination with this baryonic material is a severe threat to a relativistic fireball. Aspects of a possible r-processing in these ejecta are discussed.