Nucleosynthesis in neutron star mergers

Recent works suggest that binary mergers of double neutron stars can be major sources of r-process elements in the Galaxy. A long-standing puzzle to be resolved along with the site of the r-process is the origin of the uniqueness ("universality") of the abundance patterns that can be seen in r-process-enhanced stars. We have investigated nucleosynthesis in the dynamical ejecta of mergers by adopting three available nuclear equations of state as well as observational mass ratios of double neutron stars. We find that very similar r-process patterns can be obtained when replacing an equation of state by the others. Furthermore, different mass ratios of neutron stars still lead to robust r-process patterns with small variations of lightest and heaviest components. Our result implies that the origin of the r-process universality can be attributed to the small variation of mass ratios of double neutron stars in nature. (author)