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 Λ, Ξ−, and Ξ0 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 Λ hyperon survives in the deep subnuclear regime. The triplet of Σs 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 μ-on neutrinos are close and are about 1%, whereas in the supernova case, we only find a significant fraction (∼10%) of electron neutrinos, given that in this case, the μ-on lepton number is zero.
Highlights
Several astrophysical scenarios lead to the formation of hot, neutrino-rich compact objects which contain nuclear and hypernuclear matter at finite temperature
We explored the finite-temperature equation of state (EoS) of nuclear and hypernuclear matter within the covariant density functional (CDF) formalism
The abundances of particles in a baryon–lepton mixture in a merger remnant and a supernova were explored within the CDF formalism
Summary
Several astrophysical scenarios lead to the formation of hot, neutrino-rich compact objects which contain nuclear and hypernuclear matter at finite temperature. In the “hot” stage of evolution of these objects the thermodynamics of the matter is characterized by several parameters, for example, density, temperature and lepton fraction This is in contrast to the case of cold (essentially zero-temperature) compact stars whose thermodynamics is fully determined by a oneparameter EoS relating pressure to energy density under approximate β-equilibrium. We adopt the DDME2 parameterization [38] which is based on the version of the theory that uses density-dependent coupling constants for the meson-baryon interactions [41].
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