Equation of state and composition of proto-neutron stars and merger remnants with hyperons

TL;DR

This paper models the equation of state and composition of proto-neutron stars and merger remnants with hyperons, revealing how hyperons appear and influence dense matter under various thermal and lepton conditions.

Contribution

It introduces a covariant density functional approach for hypernuclear matter at finite temperature, detailing hyperon thresholds and composition in merger and supernova environments.

Findings

Hyperons appear in dense matter with temperature-dependent thresholds.

Hyperon fractions are influenced by temperature, lepton fractions, and isospin symmetry.

The equation of state softens with hyperon inclusion, affecting neutron star properties.

Abstract

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, $T\geq 20$ 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 ($\sim$10\%) of electron neutrinos, given that in this case, the $\mu$-on lepton number is zero.