PRODUCTION OF ALL THE r -PROCESS NUCLIDES IN THE DYNAMICAL EJECTA OF NEUTRON STAR MERGERS

Abstract

Recent studies suggest that binary neutron star (NS-NS) mergers robustly produce the heavy rprocess nuclei above the atomic mass number A ∼ 130 because of their ejecta consisting of almost pure neutrons (electron fraction of Y e < 0.1). However, little production 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 generalrelativistic 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 can be the origin of the Galactic r-process nuclei. Our result also shows that the radioactive heating after ∼ 1 day from the merging, giving 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.

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…Most importantly, the total ejected amount of r-material made of heavy r-nuclei is dramatically reduced. Our study therefore confirms the results of [7] and implies that a proper treatment of the neutrino physics is essential for making quantitative predictions of the elemental yields and especially of the total mass of r-process material that is thrown out by the dynamical merger ejecta.…”

“…[5]). However, it was recently realized that the nucleosynthesis signature of NS mergers is more complex when taking into account ν-interactions in relativistic NS-NS merger simulations and when the ejecta No e ± -capture, no !-interactions for " < " from the merger remnant are consistently included [1,[6][7][8][9][10]. In contrast to NS-BH mergers, in NS-NS mergers the large pressure gradient that is generated by the collision shock building up between both NSs causes a sizable amount of material to become unbound in addition to possible tidal-tail ejecta from the disrupted NS(s).…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…Alternative ways to cure the problems of the 3rd r-process peak discussed above, come from full general relativistic modeling of the merger event. This leads to deeper gravitational potentials, higher temperatures (including neutrino energies), electron-positron pairs, which -via positron captures and neutrino interaction with nuclei/nucleons -increase Y e to values of the order 0.15, comparable to those mentioned above in MHD-supernova (magnetar) jets [18,78], where the 3rd peak shift did not occur.…”

“…Optical and near-infrared observations of such an event, accompanying the short-duration GRB130603B have been reported [71] (see also [4]). After the first detailed nucleosynthesis predictions (following ideas of [33]) of such an event [13], many more and more sophisticated investigations have been undertaken, involving quite a number of authors [5,12,16,17,27,32,39,42,51,56,61,78] as well as the black hole accretion disk system after the formation of a central black hole [27,70,78,82].…”

“…This causes a very strong r-process with fission cycling. The utilized mass model, beta-decay half-lives, fission barriers, and fission yield prescriptions [12,18,29,38,42,55,65,78] have a strong effect on the final abundance distribution. While during the r-process (when reactions are still in n, γ − γ, n equilibrium) the 2nd and 3rd r-process peak are exactly at the right position.…”