Scalarized Hybrid Neutron Stars in Scalar Tensor Gravity

TL;DR

This paper investigates spontaneous scalarization of hybrid neutron stars within scalar-tensor gravity using a piecewise polytropic hadronic EoS and three MIT bag-model descriptions for quark matter. By solving the Einstein-frame field equations with boundary conditions for static, spherically symmetric stars, it quantifies how the scalar field, stellar mass, radius, and scalar charge depend on the quark-core density jump $\eta$, bag constants $B$ or $B_{eff}$, strange-quark mass $m_s$, and QCD correction $a_4$ through the coupling parameter $\beta$. The study finds that larger $\eta$ and denser quark cores generally increase the maximum mass and extend the scalarization range, while higher bag constants, $m_s$, or $B_{eff}$ soften the EoS and suppress scalarization; the onset of scalarization occurs at a nearly universal critical value $\beta_{cr} \approx -4.35$, and the scalar charge grows with increasing compactness and density discontinuities. Overall, the results show scalarized hybrid neutron stars can be compatible with current observations, providing potential observational signatures for scalar-tensor theories in compact-star systems.

Abstract

Hybrid neutron stars, the compact objects consisting hadronic matter and strange quark matter, can be considered as the probes for the scalar tensor gravity. In this work, we explore the scalarization of hybrid neutron stars in the scalar tensor gravity. For the hadronic phase, we apply a piecewise polytropic equation of state constrained by the observational data of GW170817 and the data of six low-mass X-ray binaries with thermonuclear burst or the symmetry energy of the nuclear interaction. In addition, to describe the strange quark matter inside the hybrid neutron star, different MIT bag models are employed. We study the effects of the value of bag constant, the mass of s quark, the perturbative quantum chromodynamics correction parameter, and the density jump at the surface of quark-hadronic phase transition on the scalarization of hybrid neutron stars. Our results confirm that the scalarization is more sensitive to the value of bag constant, the mass of s quark, and the density jump compared to the perturbative quantum chromodynamics correction parameter.

Figures (19)

• Figure 1: EoS of HNS in the massless quark approximation considering different values of the bag constant, $B$, and the density jump, $\eta$.
• Figure 2: EoS of HNS considering the massive strange quark matter with the QCD coupling constant. Different values of the bag constant, $B$, the mass of the strange quark, $m_s$, and the density jump, $\eta$, have been applied.
• Figure 3: EoS of HNS in the third model considering different values of $B_{eff}$ and $a_4$ with fixed values of the strange quark mass $m_s = 100\ MeV$ and the density jump $\eta=0$.
• Figure 4: HNS mass in the first model for the HNS EoS applying the scalar tensor gravity and GR. Different values of the bag constant, $B$, the density jump, $\eta$, and the coupling constant, $\beta$, have been considered.
• Figure 5: Same as Fig. \ref{['M1']} but for the second model of the HNS EoS. We have applied different values of the bag constant, $B$, the mass of the strange quark, $m_s$, and the density jump, $\eta$.