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

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 $0.01M_{\odot }$. 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 $0.01M_{\odot }$, although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.

Figure 1

Mass (upper panel) and characteristic velocity (lower panel) of the ejecta as functions of time for SFHo (red solid), DD2 (blue dashed), and TM1 (green dotted dashed). $t_{M-6}$ approximately denotes the time at the onset of merger (see text).

Figure 2

Contours of the electron fraction, $Y_e$ (left half), and the entropy per baryon, $s$ (right half), in $x\text{−}y$ (lower) and $x\text{−}z$ (upper) planes. Left panel: for DD2 at 8.5 ms after the merger. Middle panel: for SFHo at 5.0 ms after the merger. Right panel: for SFHo at 15.0 ms after the merger.

Figure 3

Luminosity curves of ${\nu }_e$ (red solid), ${\overline{\nu }}_e$ (blue dashed), and heavy (green dotted dashed) neutrinos for TM1 (top), DD2 (middle), and SFHo (bottom).

Figure 4

Upper panel: The time evolution of the averaged value of $Y_e$ for SFHo (red solid), DD2 (blue dashed), and TM1 (green dotted dashed). Lower panel: The mass-distribution histograms of $Y_e$ normalized by the total mass of ejecta measured at $\approx 25\mathrm{ms}$ after the onset of merger for SFHo, DD2, and TM1.

Figure 5

Same as Fig. 4 but for simulations with and without (denoted as no heat) the neutrino heating for SFHo [red and magenta (no heat)] and DD2 [blue and light blue (no heat)].