Unequal mass binary neutron star simulations with neutrino transport: Ejecta and neutrino emission

We present 12 new simulations of unequal mass neutron star mergers. The simulations are performed with the spec code, and utilize nuclear-theory-based equations of state and a two-moment gray neutrino transport scheme with an improved energy estimate based on evolving the number density. We model the neutron stars with the SFHo, LS220, and DD2 equations of state (EOS) and we study the neutrino and matter emission of all 12 models to search for robust trends between binary parameters and emission characteristics. We find that the total mass of the dynamical ejecta exceeds $0.01M_{\odot }$ only for SFHo with weak dependence on the mass ratio across all models. We find that the ejecta have a broad electron fraction ($Y_e$) distribution ($\approx 0.06–0.48$), with mean 0.2. $Y_e$ increases with neutrino irradiation over time, but decreases with increasing binary asymmetry. We also find that the models have ejecta with a broad asymptotic velocity distribution ($\approx 0.05–0.7c$). The average velocity lies in the range $0.2c-0.3c$ and decreases with binary asymmetry. Furthermore, we find that disk mass increases with binary asymmetry and stiffness of the EOS. The $Y_e$ of the disk increases with softness of the EOS. The strongest neutrino emission occurs for the models with soft EOS. For (anti) electron neutrinos we find no significant dependence of the magnitude or angular distribution or neutrino luminosity with mass ratio. The heavier neutrino species have a luminosity dependence on mass ratio but an angular distribution which does not change with mass ratio.

Figure 1

M-R curves for each of the equations of state used in this work.

Figure 2

The evolution of rest-mass density ${\rho }_0$ for the D144144 model. For the first $\sim 18\mathrm{ms}$ the stars orbit each other before merging at $\sim 0\mathrm{ms}$. At around $\sim 3\mathrm{ms}$ we can see a postmerger remnant almost fully formed with two tidal tails. At $\sim 7.5\mathrm{ms}$ the postmerger remnant has largely settled and most of the unbound material has left the grid.

Figure 3

Density (${\rho }_0$), temperature ($T$), and electron fraction ($Y_e$) at 3 ms postmerger for the $1.2M_{\odot }+1.44M_{\odot }$ models. For the temperature and electron fraction plots we threshold on densities above $\sim {10}^7\mathrm{g}/{\mathrm{cm}}^3$ to remove the atmosphere; points below this threshold are colored white. At this time, DD2 and LS220 have much more defined tidal tails than SFHo, but SFHo has a hotter core and higher $Y_e$ in many regions.

Figure 4

Density (${\rho }_0$), temperature ($T$), and electron fraction ($Y_e$) at 7.5 ms for the $1.2M_{\odot }+1.44M_{\odot }$ models. At this time, the tidal tail and its associated ejecta has left the grid, leaving behind a remnant core with an accretion torus. SFHo has a hotter core than the other two EOSs and a much higher $Y_e$ in the accretion torus.

Figure 5

The electron fraction $Y_e$ for the $1.2M_{\odot }+1.32M_{\odot }$ models at 7.5 ms postmerger. Matter that is flagged unbound is outside of the black contours, e.g., the matter in the polar regions at the top and bottom of the grid. The white contours encapsulate material that is above densities ${10}^9,{10}^{11},{10}^{13}\mathrm{g}/{\mathrm{cm}}^3$ respectively, with the higher-density contours appearing closer to the remnant core.

Figure 6

$v_{\infty }$ distribution of the polar and equatorial ejecta across different EOSs for the $1.2M_{\odot }+1.32M_{\odot }$ models.

Figure 7

$Y_e$ distribution of the polar and equatorial ejecta across different EOSs for the $1.2M_{\odot }+1.32M_{\odot }$ models.

Figure 8

$v_{\infty }$ distribution of the polar and equatorial ejecta across different mass ratios for the SFHo EOS.

Figure 9

$Y_e$ distribution of the polar and equatorial ejecta across different mass ratios of the SFHo EOS.

Figure 10

Disk masses at 7.5 ms postmerger or just before collapse if the remnant is a black hole.

Figure 11

Antielectron neutrino energy density (first moment) for the $1.44M_{\odot }+1.44M_{\odot }$ DD2 model at 3 ms postmerger. Top panel: An xz-slice of the antielectron neutrino energy density $E_{{\overline{\nu }}_e}$ with the arrows showing the effective neutrino transport velocity $v_{\nu }^i=\alpha \frac{E_{{\overline{\nu }}_e}}{F_{{\overline{\nu }}_e}^i}-{\beta }^i$. We see a large energy density near the polar regions of the remnant, where the density is less and the neutrinos are free to stream (while an overdensity of neutrinos in the polar region is expected to be qualitatively correct, we note that the use of the approximate Minerbo closure leads us to overestimate the neutrino density at the poles [61]). Bottom panel: An xy-slice of the antielectron neutrino energy density $E_{{\overline{\nu }}_e}$ with the arrows showing the effective neutrino transport velocity. We see the neutrinos advecting with the fluid in the dense regions near the core.

Figure 12

Electron neutrino luminosity for the 12132 models up to around 7.5 postmerger. We see that for the softer SFHo EOS, there is a larger luminosity. Similar trends hold for the other neutrino species.

Figure 13

Neutrino luminosity for DD2 runs up to around 7.5 ms across neutrino species.

Figure 14

Neutrino flux density moment as a function of the angle for different EOSs at 7.5 ms postmerger. The angle is defined with respect to the equatorial plane with 0° being the equator, and $\pm 90°$ being the north and south poles. The different neutrino species are color coded.

Figure 15

Electron fraction distribution for the off-grid ejecta of the $1.2M_{\odot }+1.32M_{\odot }$ SFHo model at 7.5 and 10 ms post-merger. One can clearly see that more matter is being ejected at higher $Y_e$ fractions due to neutrino irradiation.