R-process Nucleosynthesis of Subminimal Neutron Star Explosions

TL;DR

This study shows that a minimum-mass neutron star can undergo a delayed explosion after surface-mass removal when modeled with Newtonian hydrodynamics and a large nuclear reaction network. The ejecta experience robust r-process nucleosynthesis, producing lanthanides and heavy elements near the second and third r-process peaks, with yields sensitive to the assumed fission-fragment asymmetry. An electron antineutrino burst accompanies the explosion, and nuclear heating sustains high ejecta temperatures for seconds, powering a soft gamma-ray electromagnetic signal. The results imply that subminimal neutron star explosions could be a plausible site for solar-system heavy elements and expand the class of known r-process events. The work also maps how variations in surface-mass removal and fission-fragment asymmetry influence the nucleosynthetic outcomes.

Abstract

We show that a minimum-mass neutron star undergoes delayed explosion after mass removal from its surface. We couple the Newtonian hydrodynamics to a nuclear reaction network of $\sim4500$ isotopes to study the nucleosynthesis and neutrino emission during the explosion. An electron antineutrino burst with a peak luminosity of $\sim3\times10^{50}$ erg s$^{-1}$ is emitted while the ejecta is heated to $\sim10^{9}$ K. A robust $r$-process nucleosynthesis is realized in the ejecta. Lanthanides and heavy elements near the second and third $r$-process peaks are synthesized as end products of nucleosynthesis, suggesting that subminimal neutron star explosions could be an important source of solar chemical elements.

Figures (10)

• Figure 1: Initial configuration of the minimum-mass neutron star at hydrostatic and $\beta$-equilibrium for the EoS by Schneider, Roberts, and Ott PhysRevC.96.065802PhysRevC.100.025803 at uniform temperature $T=10^8$ K. The density (solid line) and electron fraction (dashed line) are plotted vs. the radial coordinates in logarithm scale.
• Figure 2: Isotopes included in the nuclear reaction network. The colored region covers the isotopes considered in the network, and the grids in red further illustrate the stable isotopes in the solar system lodders2019solar. Extremely neutron-rich isotopes with proton number $Z>80$ are not included because some of the nuclear reaction rates are not available to form linkages with other isotopes included.
• Figure 3: Initial mass fractions of isotopes at NSE for the neutron star crust-like matter. Ni62 and Fe56 are the major isotopes near the surface of the neutron star. The electron fraction decreases gradually as density increases, and neutron-rich isotopes appear correspondingly. Free neutrons are dripped when the density is above $\sim4\times10^{11}$ g cm$^{-3}$ and become the predominant composition near the threshold density $\rho_{\text{th}}\equiv10^{13}$ g cm$^{-3}$.
• Figure 4: Mass fractions of isotopes at NSE and threshold density $\rho_{\text{th}}\equiv10^{13}$ g cm$^{-3}$. The solutions at different temperatures and electron fractions $Y_e$ are shown. The mass fraction of the free neutron is $\sim 0.9$ (not shown in the figure) for the conditions concerned. For $Y_e$ between $0.016$ and $0.030$, the solutions at NSE are alike.
• Figure 5: Radial positions (upper left panel), densities (lower left panel), temperatures (upper right panel), and velocities (lower right panel) of mass elements vs. time in logarithm scale in the model L-40-50. The temperatures are indicated by dashed-dotted lines prior to the activation of the nuclear reaction network to signal potentially large uncertainty due to the EoS construction ambiguity (see footnote \ref{['footnote:phase transition']}). After the mass removal, the mass elements near the surface of the exposed neutron star core-like matter start expanding promptly, though most of them remain gravitationally bound. They fall back onto the neutron star at $\sim0.01$ s and undergo radial oscillation. A new equilibrium of the subminimal neutron star with lower central density is temporarily found between $\sim0.01-0.05$ s. Nonetheless, the nuclear reaction network is activated as the density of the mass elements is lower than the threshold density $\rho_{\text{th}}\equiv10^{13}$ g cm$^{-3}$. The heating effect and leptonization caused by the nuclear reactions alter the structure of the neutron star in a quasi-equilibrium state. The star loses stability after $\sim0.05$ s, leading to a delayed explosion.