Rapidly rotating hot nuclear and hypernuclear compact stars: integral parameters and universal relations

TL;DR

The paper investigates how the symmetry-energy slope $L_{ m sym}$ and finite-temperature isentropy affect global properties of hot, rotating neutron stars with nucleonic and hyperonic matter. Using covariant density functionals and three $L_{ m sym}$ values, the authors compute static and Keplerian equilibria with fixed $s/k_B$ and $Y_e$, analyzing mass–radius, moments of inertia, and Kepler frequencies, and they test several universal relations among $I$, $Q$, and $ar{Q}$ across the EoS variants. They find that finite-temperature sequences obey state-dependent quasi-universal relations with coefficients tied to $s/k_B$ and $Y_e$; universality across different $L_{ m sym}$ and compositions holds only within the same thermodynamic state, while hot remnants do not map universally to cold TOV configurations. The results underscore the need to account for thermal and compositional states when interpreting post-merger observations (e.g., GW170817) and provide fitted relations and coefficients for practical astrophysical modeling of hot compact objects.

Abstract

In this work, we investigate hot, isentropic compact stars in the limiting cases of static and maximally rotating configurations, focusing on how variations in the symmetry energy of the equation of state derived from covariant density functional theory affect stellar properties. We consider both nucleonic and hyperonic matter with systematically varied symmetry energy slopes, fixed entropies per baryon $s / k_B=1$ and 3, and electron fractions $Y_e=0.1$ and $Y_e=0.4$, representative of conditions in binary neutron star mergers and proto-neutron stars. We compute and analyze mass--radius and moment--of--inertia--mass relations, as well as the dependence of the Keplerian (mass-shedding) frequency on mass, angular momentum, and the ratio of kinetic to gravitational energy. Furthermore, we show that several universal relations between global properties remain valid across both nucleonic and hyperonic equations of state with varying symmetry energy, both in the static and Keplerian limit, and for various combinations of the fixed entropy and electron fraction.

Figures (13)

• Figure 1: Pressure versus energy for the nucleonic and hyperonic EoS models for different $L_{\rm sym}$, and for fixed values of $s/k_B = 1$ and $s/k_B = 3$ and electron fractions $Y_{e}=0.1$ and 0.4.
• Figure 2: Mass-radius relations for static and Keplerian stellar sequences defined through combinations of fixed values of entropy per baryon and electron fraction with nucleonic and hyperonic compositions and for various values of $L_{\rm sym}$.
• Figure 3: Dependence of mass on central energy density for static and Keplerian sequences for fixed combinations of entropy per baryon and electron fractions and different values of $L_{\rm sym}$ in the case of nucleonic and hyperonic EoS.
• Figure 4: Dependence of moment of inertia on the gravitational mass for static and Keplerian sequences for fixed combinations of entropies per baryon and electron fractions and different values of $L_{\rm sym}$ in the case of nucleonic and hyperonic EoS.
• Figure 5: Keplerian frequency as a function of stellar mass (top panels), angular momentum (middle panels), and the ratio of kinetic to gravitational energies $T/W$ (bottom panels) for nucleonic and hyperonic EoS for fixed combinations of entropies per baryon and electron fractions.