Equation of State and Composition of Proto-Neutron Stars and Merger Remnants with Hyperons

Finite-temperature equation of state (EoS) and the composition of dense nuclear and hypernuclear matter under conditions characteristic of neutron star binary merger remnants and supernovas are discussed. We consider both neutrino free-streaming and trapped regimes which are separated by a temperature of a few MeV. The formalism is based on covariant density functional (CDF) theory for the full baryon octet with density-dependent couplings, suitably adjusted in the hypernuclear sector. The softening of the EoS with the introduction of the hyperons is quantified under various conditions of lepton fractions and temperatures. We find that Lambda, Xi(-), and Xi(0) hyperons appear in the given order with a sharp density increase at zero temperature at the threshold being replaced by an extended increment over a wide density range at high temperatures. The Lambda hyperon survives in the deep subnuclear regime. The triplet of sigma s is suppressed in cold hypernuclear matter up to around seven times the nuclear saturation density, but appears in significant fractions at higher temperatures, T >= 20 MeV, in both supernova and merger remnant matter. We point out that a special isospin degeneracy point exists where the baryon abundances within each of the three isospin multiplets are equal to each other as a result of (approximate) isospin symmetry. At that point, the charge chemical potential of the system vanishes. We find that under the merger remnant conditions, the fractions of electron and mu-on neutrinos are close and are about 1%, whereas in the supernova case, we only find a significant fraction (similar to 10%) of electron neutrinos, given that in this case, the mu-on lepton number is zero.

Automated summary

The finite-temperature equation of state and composition of dense nuclear and hypernuclear matter in conditions characteristic of neutron star binary merger remnants and supernovas are discussed. The introduction of Lambda, Xi (-), and Xi (0) hyperons softens the equation of state under various conditions of lepton fractions and temperatures. A special isospin degeneracy point is identified, and the abundance of electron and muon neutrinos under different conditions, such as merger remnants and supernovas, is compared.