Shinya Wanajo¹, Yuichiro Sekiguchi², Nobuya Nishimura³, Kenta Kiuchi², Koutarou Kyutoku⁴ and Masaru Shibata²

¹iTHES Research Group, RIKEN, Wako, Saitama 351-0198, Japan
²Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
³Astrophysics, EPSAM, Keele University, Keele ST5 5BG, UK
⁴Department of Physics, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA

Recent studies suggest that binary neutron star (NS-NS) mergers robustly produce heavy r-process nuclei above the atomic mass number A ~ 130 because their ejecta consist of almost pure neutrons (electron fraction of Y_e < 0.1). However, the production of a small amount of the lighter r-process nuclei (A  90-120) conflicts with the spectroscopic results of r-process-enhanced Galactic halo stars. We present, for the first time, the result of nucleosynthesis calculations based on the fully general relativistic simulation of a NS-NS merger with approximate neutrino transport. It is found that the bulk of the dynamical ejecta are appreciably shock-heated and neutrino processed, resulting in a wide range of Y_e (0.09-0.45). The mass-averaged abundance distribution of calculated nucleosynthesis yields is in reasonable agreement with the full-mass range (A  90-240) of the solar r-process curve. This implies, if our model is representative of such events, that the dynamical ejecta of NS-NS mergers could be the origin of the Galactic r-process nuclei. Our result also shows that radioactive heating after ~1 day from the merging, which gives rise to r-process-powered transient emission, is dominated by the β-decays of several species close to stability with precisely measured half-lives. This implies that the total radioactive heating rate for such an event can be well constrained within about a factor of two if the ejected material has a solar-like r-process pattern.

Reference
Wanajo S, Sekiguchi Y, Nishimura N, Kiuchi K, Kyutoku K and Shibata M (2014) Production of All the r-process Nuclides in the Dynamical Ejecta of Neutron Star Mergers. The Astrophysical Journal Letters 789:L39.
[doi:10.1088/2041-8205/789/2/L39]

Link to Article