http://arxiv.org/abs/2109.01919

Finite-temperature equation of state (EoS) and composition of dense nuclear and hypernuclear matter under conditions characteristic for neutron star binary merger remnants and supernovas are discussed. We consider both the neutrino free-streaming and the 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 the 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 there exists a special isospin degeneracy point 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 find only a significant fraction ($\sim$10\%) of electron neutrinos, given that in this case, the $\mu$-on lepton number is zero.

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A. Sedrakian and A. Harutyunyan
Tue, 7 Sep 21
44/89

Comments: 17 pages, 6 figures, submitted to “Universe”