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In [arXiv:2306.13709], Astrophysical Journal in press, we performed the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling time scale. We found that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (∼20 ms postmerger). Electron flavor anti-neutrino luminosity dominates over electron flavor neutrino luminosity at early times, resulting in a secular increase of the electron fraction in the outer layers of the remnant. However, the two luminosities become comparable ∼20−40 ms postmerger. A dense gas of electron anti-neutrinos is formed in the outer core of the remnant at densities ∼10^14.5 g cm⁻³, corresponding to temperature hot spots. The neutrinos account for ∼10% of the lepton number in this region. Despite the negative radial temperature gradient, the radial entropy gradient remains positive and the remnant is stably stratified according to the Ledoux criterion for convection. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible stable rigidly rotating configurations. The remnant is differentially rotating, but it is stable against the magnetorotational instability. Other MHD mechanisms operating on longer timescales are likely responsible for the removal of the differential rotation. Our results indicate that the remnant massive neutron star is qualitatively different from a protoneutron stars formed in core-collapse supernovae.

In a recent work (arXiv:2310.06025) we explore avenues for the detectability of QCD phase transitions to deconfined quarks in mergers of neutron stars. In particular we show that employing the sensitivities of next-generation gravitational wave (GW) detectors, at postmerger signal to noise ratios (SNRs) as low as 10, we can detect phase transitions but only if they strongly violate the so-called quasi-universal relations by more than 1.6 σ. We also show that at the same SNR, we can reliably recover the postmerger signal and differentiate between GW models on the basis of shifts in their postmerger peak frequencies.

Our group has received a grant from the Department of Energy’s Office of Science program Scientific Discovery through Advanced Computing (SciDAC) for studying the role of neutron star mergers in the creation of heavy elements in the universe. The project is called Exascale Nuclear Astrophysics for FRIB (ENAF). FRIB or Facility for Rare Isotope Beams is a DOE-supported research, teaching, and training center located at Michigan State University. This collaboration comprises Principal Investigators (PIs) from six institutions: Argonne National Laboratory, North Carolina State University, Oak Ridge National Laboratory, Penn State University, the University of California, Berkeley and the University of Notre Dame.

ENAF is developing sophisticated numerical simulation codes to study the life cycle of stars, including studying the role neutrinos play in this process. The study of the interaction of neutrinos with matter, along with the phenomenon of neutrino flavor oscillations forms one of the key goals of this project. This will provide valuable insights into the behavior of neutrinos and their interactions with matter, the composition of the universe and the energy production processes in stars.

In a recent study accepted for publication in The Astrophysical Journal Letters (see arXiv:2302.11359), we explore finite-temperature effects in numerical-relativity simulations of binary neutron star mergers with microphysical equations of state and full neutrino transport. By increasing the specific heat through controlled changes to the effective nucleon masses, we find that merger remnants become colder and more compact due to the reduced thermal pressure support.

This effect leaves an imprint on the post-merger gravitational wave signal via a shift in the peak frequency. We use a full Bayesian analysis to demonstrate that these shifts in frequency are distinguishable with next-generation gravitational-wave observatories at signal-to-noise ratios of 15.

In 2210.11491 we present a systematic numerical relativity study of the impact of different treatment of microphysics and grid resolution in binary neutron star mergers. We consider series of simulations at multiple resolutions comparing hydrodynamics, neutrino leakage scheme, leakage augmented with the M0 scheme and the more consistent M1 transport scheme. Additionally, we consider the impact of a sub-grid scheme for turbulent viscosity. We find that viscosity helps to stabilise the remnant against gravitational collapse but grid resolution has a larger impact than microphysics on the remnant’s stability. The gravitational wave (GW) energy correlates with the maximum remnant density, that can be thus inferred from GW observations. M1 simulations shows the emergence of a neutrino trapped gas that locally decreases the temperature a few percent when compared to the other simulation series. This out-of-thermodynamics equilibrium effect does not alter the GW emission at the typical resolutions considered for mergers. Different microphysics treatments impact significantly mass, geometry and composition of the remnant’s disc and ejecta. M1 simulations show systematically larger proton fractions. The different ejecta compositions reflect into the nucleosynthesis yields, that are robust only if both neutrino emission and absorption are simulated. Synthetic kilonova light curves calculated by means of spherically-symmetric radiation-hydrodynamics evolutions up to 15 days post-merger are mostly sensitive to ejecta’s mass and composition; they can be reliably predicted only including the various ejecta components. We conclude that advanced microphysics in combination with resolutions higher than current standards appear essential for robust long-term evolutions and astrophysical predictions.