A. Bauswein, S. Goriely, H. -T. Janka
Neutron star (NS) mergers can eject considerable amounts of neutron-rich matter, allowing for r-processing. The radioactive decay of the products heats the ejecta and makes them potentially observable as a source of thermal electromagnetic radiation. We investigate systematically the dynamical mass ejection during the NS collision in dependence on uncertain properties of the nuclear equation of state (EoS) by using 40 representative, microphysical EoSs in relativistic merger simulations. The NS compactness, characterized by the radius R_1.35 of nonrotating NSs of 1.35 Msun, is the crucial parameter that determines the ejecta mass. NSs with smaller radii R_1.35 ("soft"' EoS) collide more violently and eject systematically higher masses. These range from ~10^-3 Msun to ~10^-2 Msun for symmetric 1.35-1.35 Msun binaries with R_1.35 between 16 km and 11 km, and from ~5*10^-3 Msun to ~2*10^-2 Msun for asymmetric 1.2-1.5 Msun binaries. Correspondingly, the bolometric peak luminosities of the optical transients vary between 3*10^41 erg/s and 14.5*10^41 erg/s and are reached with effective spectral temperatures of 13.000-19.000 K on timescales between ~2h and ~12h. The correlation of their properties with those of the merging NSs might provide constraints on the high-density EoS. The r-process nucleosynthesis exhibits a remarkable robustness. In all investigated cases >93% of the ejecta mass are converted to n-rich nuclei with nuclear mass numbers A>~130 and a uniform, nearly solar abundance pattern. By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4*10^-5/yr (4*10^-4/yr) for a soft (stiff) NS EoS, if NS mergers are the main source of heavy r-nuclei. The production ratio of 232Th to 238U attains a stable value of ~1.65, which does not exclude NS mergers as sources of heavy r-material in the most metal-poor stars.
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http://arxiv.org/abs/1302.6530
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