ar X iv : a st ro - p h / 98 04 33 2 v 2 1 1 M ay 1 99 8 Coalescing Neutron Stars : A Solution to the R - Process Problem ?

1.1 Introduction Most recent nucleosynthesis parameter studies [3, 4, 11] place questions on the ability of high entropy neutrino wind scenarios in type II supernovae to produce r-process nuclei for A < 110 in correct amounts. In addition, it remains an open question whether the entropies required for the nuclei with A > 110 can actually be attained in type II supernova events. Thus, an alternative or supplementary r-process environment is needed, leading possibly to two different production sites for r-process nuclei: a high entropy, high Y e (neutrino wind in type II supernovae) and a low entropy, low Y e (decompression of neutron star (ns) material) scenario. Further indications for a production site possibly different from SN II arise from observations of low metallicity stars [9]. It seems that the production of r-process nuclei is delayed in comparison with the major SN II yields, a fact that would be consistent with the merging scenario of two neutron stars. The tidal disruption of a ns by a black hole and possible consequences for nucleosynthesis has first been studied by Lattimer and Schramm [6, 7], the merging of a neutron star binary has been discussed by Symbalisty and Schramm [16]. The related decompression of the neutron star material has been investigated by Lattimer et al. [5], Eichler et al. [2], who also discussed various other aspects of such a merging scenario, and by Meyer [10]. In the context of numerical simulations the merging event nucleosynthesis has been discussed by Davies et al. [1] and by Ruffert et al. [15]. To investigate the possible relevance of neutron star mergers for the r-process nucleosynthe-sis we perform 3D Newtonian SPH calculations of the hydrodynamics of equal (1.6 M ⊙ of baryonic) mass neutron star binary coalescences. Starting with an initial separation of 45 km we follow the evolution of matter for 12.9 ms. We use the physical equation of state of Lattimer and Swesty [8] to model the microphysics of the hot neutron star matter. To test the sensitivity of our results to the chosen approaches and approximations we perform in total 10 different runs where we test each time the sensitivity to one property of our model [14]. We vary the resolution (∼ 21000 and ∼ 50000 particles), the equation of state (polytrope), the artificial viscosity scheme [12], the stellar masses (1.4 M ⊙ of baryonic matter), we include neutrinos (free-streaming limit), switch …