Role of symmetry energy at subnuclear densities in protoneutron star crusts

TL;DR

This paper investigates how the symmetry energy at subnuclear densities influences protoneutron star evolution, crust formation, and neutrino emissions, highlighting the importance of the nuclear EOS parameters in astrophysical phenomena.

Contribution

It introduces models linking symmetry energy parameters to protoneutron star properties and demonstrates their impact on cooling and crust formation timing.

Findings

Smaller L values lead to longer cooling times and higher neutrino energies.

EOS with smaller L values cause earlier crust formation despite slower cooling.

Crust formation timing is highly sensitive to the symmetry energy slope L.

Abstract

The impact of matter properties at subnuclear densities on the evolution of protoneutron stars is investigated. Several models of nuclear equation of state (EOS) are constructed with varying saturation parameters, particularly the symmetry energy $S_0$ and its density slope $L$. Using the Thomas--Fermi approximation, the mass and proton numbers of heavy nuclei at subnuclear densities are systematically evaluated, along with their dependence on the EOS. Cooling simulations of protoneutron stars reveal that EOSs with smaller $L$ values lead to a longer cooling timescale and higher average neutrino energies. This behavior is attributed to the enhanced neutrino scattering caused by larger mass numbers, which increases the thermal insulation. Furthermore, the crystallization temperature, marking the onset of crust formation, is found to be higher for EOSs with smaller values of $L$. This is due to the enhanced Coulomb energy associated with larger proton numbers. As a result, despite slower cooling, crust formation occurs earlier for smaller-$L$ EOSs. These findings indicate that the timing of crust formation is sensitive to the EOS and highlight the importance of late-time neutrino observations as probes of the matter properties at subnuclear densities.