Eur. Phys. J. A (2023) 59: 86
https://doi.org/10.1140/epja/s10050-023-01010-x

Regular Article - Theoretical Physics

Impact of symmetry energy on sound speed and spinodal decomposition in dense neutron-rich matter

¹ School of Physics, Southeast University, 211189, Nanjing, China
² Department of Physics and Astronomy, Texas A &M University-Commerce, 75429, Commerce, TX, USA

^b Bao-An.Li@Tamuc.edu

Received: 2 January 2023
Accepted: 12 April 2023
Published online: 25 April 2023

Abstract

Using a meta model for nuclear Equation of State (EOS) with its parameters constrained by astrophysical observations and terrestrial nuclear experiments, we examine effects of nuclear EOS especially its symmetry energy $E_{\text{sym}}(\rho )$ term on the speed of sound squared $C_s^2(\rho )$ and the critical density ${\rho }_t$ where $C_s^2({\rho }_t)$ vanishes (indicating the onset of spinodal decomposition) in both dense neutron-rich nucleonic matter relevant for relativistic heavy-ion collisions and the cold $n+p+e+\mu$ matter in neutron stars at $\beta$-equilibrium. Unlike in nucleonic matter with fixed values of the isospin asymmetry $\delta$, in neutron stars with a density dependent isospin profile $\delta (\rho )$ determined consistently by the $\beta$-equilibrium and charge neutrality conditions, the $C_s^2(\rho )$ almost always show a peak and then vanishes at ${\rho }_t$. The latter strongly depends on the high-density behavior of $E_{\text{sym}}(\rho )$ if the skewness parameter $J_0$ characterizing the stiffness of high-density symmetric nuclear matter (SNM) EOS is not too far above its currently known most probable value of about $-190$ MeV inferred from recent Bayesian analyses of neutron star observables. Moreover, in the case of having a super-soft $E_{\text{sym}}(\rho )$ that is decreasing with increasing density above about twice the saturation density of nuclear matter, the ${\rho }_t$ is significantly lower than the density where the $E_{\text{sym}}(\rho )$ vanishes (indicating the onset of isospin-separation instability and pure neutron matter formation) in neutron star cores.