Comprehensive study of mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars

Abstract

By performing a set of numerical relativity simulations for the merger of binary neutron stars with several mass ratios, the properties of ejected matter in dynamical and post-merger phases are investigated for the cases in which the remnant massive neutron star collapses into a black hole in 20 ms after the onset of merger. The dynamical mass ejection is investigated in three-dimensional general relativistic hydrodynamics simulations with an approximate neutrino-radiation transfer. The resulting post-merger systems are then mapped onto the axisymmetric ones and used as the initial conditions of axisymmetric long-term radiation-hydrodynamics simulations supposing that the effective viscosity should arise as a result of magnetohydrodynamical activity in the post-merger system. We show that the typical electron fraction of the dynamical ejecta is lower for the merger of more asymmetric binaries, and hence, heavier r-process nuclei are dominantly synthesized. We also show that the post-merger ejecta has only a mild neutron-richness, which results in the production of lighter r-process nuclei, irrespective of the binary mass ratio and that the ejecta mass is larger for the merger of more asymmetric binaries due to the larger disk mass. Thus, for the asymmetric merger case, the underproduction of lighter r-process nuclei can be compensated by the post-merger ejecta. As a result, by summing up both ejecta components, the solar residual r-process pattern is approximately reproduced irrespective of the binary mass ratio. Implications of our results associated with the mass distribution of compact NS binaries and the magnetar scenario of short gamma-ray bursts are discussed.

Constraining inputs to realistic kilonova simulations through comparison to observed r-process abundances

“…In this work, we have considered fiducial initial conditions for the outflow, including composition, without allowing for correlations induced by the fact that both the composition and outflows are initialized by BNS mergers. In future work, we will explore self-consistent initialization from merger properties, in particular exploring the effects of binary mass ratio and NS remnant lifetime, which should have significant impact on the ejecta amount and composition [53][54][55].…”

The Fast Flavor Instability in Hypermassive Neutron Star Disk Outflows

“…cases, the mean energies of electron antineutrinos emitted by the disk are 20-50% higher than the mean energies of electron neutrinos, with values becoming close to one another only before a sharp drop at t ∼ 0.5 s. The drop in mean energies is a consequence of energy luminosities decreasing faster with time than number luminosities as the disk transitions to a radiatively inefficient state with lower temperature and density. Due to enhanced neutrino irradiation and suppressed mass loss through the inner boundary, a longer HMNS lifetime correlates with more mass ejected as well as an overall higher average electron fraction and velocity of the unbound ejecta [49,55,56,78,79], which in turn translates into a lower yield of heavy r-process elements [51,80,81] (Table I). Our model t010-ab00 has a slightly lower average Y e than the prompt BH model due to a relative increase in the ejecta with Y e < 0.25 material (Figure 2)…”

The production of actinides in neutron star mergers

“…Earlier r-process nucleosynthesis studies that adopted either the parametrized BNSM ejecta properties or the outflow trajectories extracted from numerical simulations modeling the post-merger evolution of BNSMs, revealed that a significant amount of actinides with mass fraction X act 10 −3 is only produced in ejecta with Y e 0.2 [47][48][49][50][51][52][53][54][55][56][57][58][59][60]. This can be qualitatively understood from relating the averaged nuclear mass number at the end of the r-process ( A f ) to the averaged initial seed nuclear mass number ( A s ) and the abundance ratio of free neutrons to seed nuclei (R n/s ) before the onset of r-process by…”

“…Broadly, abundance of SLRIs can be measured in three distinct regimes. The first is the measurement of live SLRIs present in the ISM using γ -ray telescopes that detect the emitted γ -rays following radioactive decay of SLRIs such as 26 Al and 60 Fe. Such measurements have been used to directly probe ongoing star formation in the Galaxy [139,140], and can in principle be used to probe the BNSM history as well (see Section 3).…”