Fast neutrino flavor conversion, ejecta properties, and nucleosynthesis in newly-formed hypermassive remnants of neutron-star mergers

Neutrinos emitted in the coalescence of two neutron stars affect the dynamics of the outflow ejecta and the nucleosynthesis of heavy elements. In this work, we analyze the neutrino emission properties and the conditions leading to the growth of flavor instabilities in merger remnants consisting of a hypermassive neutron star and an accretion disk during the first 10 ms after the merger. The analyses are based on hydrodynamical simulations that include a modeling of neutrino emission and absorption effects via the “improved leakage-equilibration-absorption scheme” (ILEAS). We also examine the nucleosynthesis of the heavy elements via the rapid neutron-capture process ($r$-process) inside the material ejected during this phase. The dominant emission of ${\overline{\nu }}_e$ over ${\nu }_e$ from the merger remnant leads to favorable conditions for the occurrence of fast pairwise flavor conversions of neutrinos, independent of the chosen equation of state or the mass ratio of the binary. The nucleosynthesis outcome is very robust, ranging from the first to the third $r$-process peaks. In particular, more than ${10}^{-5}{M}_{\odot }$ of strontium are produced in these early ejecta that may account for the GW170817 kilonova observation. We find that the amount of ejecta containing free neutrons after the $r$-process freeze-out, which may power early-time UV emission, is reduced by roughly a factor of 10 when compared to simulations that do not include weak interactions. Finally, the potential flavor equipartition between all neutrino flavors is mainly found to affect the nucleosynthesis outcome in the polar ejecta within $\lesssim 30°$, by changing the amount of the produced iron-peak and first-peak nuclei, but it does not alter the lanthanide mass fraction therein.

Figure 1

Time evolution of the neutrino energy luminosity and average energy for different species, ${\nu }_e$ (in red), ${\overline{\nu }}_e$ (in green) and ${\nu }_x$ (in blue) of the $1.35+1.35M_{\odot }$ NS-NS merger simulation, with DD2 (solid lines) and SFHo (dashed lines) EoS.

Figure 2

Hydrodynamic and neutrino properties from the $1.35+1.35M_{\odot }$ NS-NS merger remnant simulation with DD2 EoS in the ($x$, $z$) plane for the time snapshot taken at 2.5, 5, 7.5, and 10 ms after the coalescence (from left to right columns). The first, second, and third rows show the baryon mass density $\rho$, temperature $T$, the electron number fraction $Y_e$. The fourth row displays the ratio $(n_{{\overline{\nu }}_e}-n_{{\nu }_e})/(n_{{\overline{\nu }}_e}+n_{{\nu }_e})$. A value of this ratio above (below) 0 implies larger $n_{{\overline{\nu }}_e}$ ($n_{{\nu }_e}$) locally. Also shown with solid lines are the contours of the ${\nu }_e$ (red), ${\overline{\nu }}_e$ (green), and ${\nu }_x$ (blue) emission surfaces.

Figure 3

Same as Fig. 2, but for the simulation with SFHo EoS.

Figure 4

Number density of ${\nu }_e$ (red solid curves), ${\overline{\nu }}_e$ (green dash-dotted curves), and ${\nu }_x$ (blue dash lines) on their respective emission surfaces in the northern hemisphere for the $1.35+1.35M_{\odot }$ merger models with DD2 EoS (top panels) and SFHo EoS (bottom panels) at 2.5, 5, 7.5, and 10 ms post merger. Note that the ${\nu }_x$ number density has been rescaled, see text and Appendix pp1 for details.

Figure 5

Neutrino electron lepton number angular crossings at different positions $(x,z)=(0,25),(10,25),(40,25)\mathrm{km}$ and times $t=2.5$, 5, 7.5, 10 ms, above the remnant for the $1.35+1.35M_{\odot }$ model with DD2 EoS. The red (blue) shaded region represents the region where ${\mathrm{\Phi }}_{{\nu }_e}-{\mathrm{\Phi }}_{{\overline{\nu }}_e}>0$ (${\mathrm{\Phi }}_{{\nu }_e}-{\mathrm{\Phi }}_{{\overline{\nu }}_e}<0$).

Figure 6

Same as Fig. 5, but for the $1.35+1.35M_{\odot }$ simulation with SFHo EoS.

Figure 7

Solutions of $|\mathrm{Im}(\omega )|$ obtained from the DR [Eq. (12)] for $\mathit{k}=(0,0,k)$ at the location $(x,z)=(10,25)\mathrm{km}$ above the merger remnant for different times. The top (bottom) panels show the results for the $1.35+1.35M_{\odot }$ model with the DD2 (SFHo) EoS, while the left (right) panels show the symmetry-preserving (symmetry-breaking) solutions.

Figure 8

Solutions of $|\mathrm{Im}(\omega )|$ obtained from the DR [Eq. (12)] for $\mathit{k}=(0,0,k)$ at 5 ms for different locations labeled by the coordinates $(x,z)$ expressed in km. The top (bottom) panels show the results for the $1.35+1.35M_{\odot }$ model with the DD2 (SFHo) EoS, while the left (right) panels show the symmetry-preserving (symmetry-breaking) solutions.

Figure 9

Contour plots showing $|\mathrm{Im}(\omega )|$ above the ${\nu }_e$ surface for $\mathit{k}=0$ at $t=2.5$, 5.0, 7.5 and 10.0 ms, from left to right, for the $1.35+1.35M_{\odot }$ model. The top (bottom) panels are based on the model with the DD2 (SFHo) EoS. The red and green solid lines represent the locations of ${\overline{\nu }}_e$ and ${\nu }_e$ emission surfaces, respectively.

Figure 10

Histograms of the ejecta mass fraction when the ejecta reach $r=100\mathrm{km}$ at $t(r=100\mathrm{km})$, [panel (a)], ${\theta }_{\mathrm{ej}}$ [panel (b)], radial velocity $v_r$ at $r=100\mathrm{km}$, and $Y_e$ at $r=100\mathrm{km}$ without flavor conversions for the simulations with $1.35+1.35M_{\odot }$ NS binaries. Panel (e) [(g)] and (f) [(h)] show the distributions of each tracer particle in the $t(r=100)\mathrm{km}-{\theta }_{\mathrm{ej}}$ and $(v_r/c)–{\theta }_{\mathrm{ej}}$ planes with the DD2 (SFHo) EoS. The color coding of each tracer particle indicates its $Y_e$ value at $r=100\mathrm{km}$ without considering flavor conversions.

Figure 11

Same as Fig. 10, but for the $1.25+1.45M_{\odot }$ models.

Figure 12

Nucleosynthesis yields, $Y(A)$, as a function of the nuclear mass number $A$ at the time of 1 Gyr for all NS merger models investigated in this work.

Figure 13

Distribution of the ratios of the ${\nu }_e$ absorption rates on neutrons, ${\lambda }_{{\nu }_e}$, to the ${\overline{\nu }}_e$ absorption rates on protons, ${\lambda }_{{\overline{\nu }}_e}$, as a function of ${\theta }_{\mathrm{ej}}$ for all tracer particles at $r=100\mathrm{km}$ in the $1.35+1.35M_{\odot }$ models. Panels (a) and (c) are without flavor conversions, while panels (b) and (d) include flavor equipartition. The color of each point indicates the absolute rate of ${\lambda }_{{\overline{\nu }}_e}$.

Figure 14

Impact of neutrino flavor equipartition on the ejecta $Y_e$ for the $1.35+1.35M_{\odot }$ model with DD2 EoS. Panel (a) shows the distribution $\mathrm{\Delta }{Y}_e=Y_e^{(\mathrm{osc})}-Y_e^{(\text{no osc})}$, at $r=100\mathrm{km}$, as a function of ${\theta }_{\mathrm{ej}}$ for all tracer particles. The color bar represents $Y_e$ without flavor conversions. Panels (b)–(d) show the histograms of the ejecta mass distributions as functions of $Y_e$ at $r=100\mathrm{km}$, for the polar, middle, and equatorial ejecta, respectively.

Figure 15

Same as Fig. 14, but for the $1.35+1.35M_{\odot }$ model with SHFo EoS.

Figure 16

Nucleosynthesis outcome in the polar ejecta for the cases with and without flavor equipartition for the $1.35+1.35M_{\odot }$ model with DD2 EoS (a) and for the SHFo EoS model (b).

Figure 17

Ejecta $Y_e$ histograms computed via post-processing and as from the simulations of Refs. [23, 44].