Effects of Symmetry Energy on the Equation of State for Hybrid Neutron Stars

TL;DR

This study explores how symmetry energy influences the phase transitions and properties of hybrid neutron stars using theoretical models, revealing how quark matter and vector interactions affect star mass and structure.

Contribution

It introduces a detailed theoretical analysis of the impact of symmetry energy and vector interactions on the equation of state and structure of hybrid neutron stars.

Findings

Critical density for phase transition varies with symmetry energy and vector coupling.

Maximum neutron star mass is highly sensitive to vector coupling strength.

Presence of quark matter does not alter the canonical 1.4 solar mass neutron star.

Abstract

In this paper, the implications of the symmetry energy on the hadron and quark phase transitions in the compact star, including the properties of the possible configurations of the quark-hadron hybrid stars, are investigated in the frameworks of the energy-density functional (EDF) models and the flavor SU(2) Nambu--Jona-Lasinio (NJL) model with the help of the Schwinger's covariant proper-time regularization (PTR) scheme. In this {theoretical setup}, the equations of states (EoSs) of hadronic matter for various values of symmetry energies obtained from the EDF models are employed to describe the hadronic matter, and the {flavor} SU(2) NJL model with various repulsive-vector interaction strengths are used to describe the quark matter. We then observe the obtained EoS in the mass-radius properties of the hybrid star configurations for various vector interactions and nuclear symmetry energies by solving the Tolman-Oppenheimer-Volkoff equation. We obtain that the critical density at which the phase transition occurs varies over the density (3.6--6.7)$\rho_0$ depending on the symmetry energy and the strength of the vector coupling $G_v$. The maximum mass of the neutron star (NS) is susceptible to $G_v$. When there is no repulsive force, the NS maximum mass is only about $1.5M_\odot$, but it becomes larger than $2.0M_\odot$ when the vector coupling constant is about half of the {attractive} scalar coupling constant. Surprisingly, the presence of the quark matter does not affect the canonical mass of NS ($1.4M_\odot$), so observing the canonical mass of NSs can provide unique constraints to the EoS of hadronic matter at high densities.