Dynamical mass ejection from binary neutron star mergers: Radiation-hydrodynamics study in general relativity

Abstract

We perform radiation-hydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese "K" supercomputer, taking into account neutrino cooling and heating by an updated leakage-plus-transfer scheme for the first time. Neutron stars are modeled by three modern finite-temperature equations of state (EOS) developed by Hempel and his collaborators. We find that the properties of the dynamical ejecta of the merger such as total mass, average electron fraction, and thermal energy depend strongly on the EOS. Only for a soft EOS (the so-called SFHo), the ejecta mass exceeds 0.01M⊙. In this case, the distribution of the electron fraction of the ejecta becomes broad due to the shock heating during the merger. These properties are well-suited for the production of the solar-like r-process abundance. For the other stiff EOS (DD2 and TM1), for which a long-lived massive neutron star is formed after the merger, the ejecta mass is smaller than 0.01M⊙, although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.

Nuclear robustness of therprocess in neutron-star mergers

“…Importantly, we found that these features do not depend sensitively on the astrophysical conditions for the majority of the ejecta (but see Ref. [16][17][18] for a possible increase of the initial electron fraction which may have an impact on the abundance patterns). We have also shown that these features do not depend in general on the nuclear mass model used.…”

“…We note that such a robust scenario is quite attractive as it might arXiv:1409.6135v2 [astro-ph.HE] 1 Oct 2015 explain the occurrence of r-process elements above the second peak in solar proportion as observed in very old metal-poor stars [15]. Recent simulations suggest that the electron fraction of part of the ejecta may be raised by neutrino processes [16][17][18]. However, the sensitivity of these results to the treatment of neutrino radiation transport in NS mergers simulations remains to be explored.…”

“…We hence conclude that dynamical ejecta of NS mergers show a robust r-process pattern, as already concluded in refs. [12][13][14], provided that Y e in the ejecta remains low, see refs [16][17][18], as expected in dynamical ejecta of NS-black hole and very asymmetric NS-NS mergers. We will now discuss the origin of the robustness.…”

“…This simplification was a reasonably good approximation in Newtonian models (e.g. [43]), but the impact of neutrinos in relativistic merger models requires further investigation [16][17][18]. NS matter is modelled with the TM1 EoS [44][45][46], which leads to a NS radius of 14.49 km for a 1.35 M NS and a maximum gravitational mass of 2.21 M for non-rotating NSs.…”

“…Korobkin et al have performed rprocess simulations for a set of NS mergers consisting of various combinations of neutron stars in the relevant mass range between 1 and 2 M [13], while Bauswein et al have explored the influence of various equations of state on the merger dynamics and nucleosynthesis [14] (see also ref. [17]). It turns out that the specific treatment of the merger dynamics, e.g.…”