Quark-nuclear hybrid star equation of state with excluded volume effects

Abstract

A two-phase description of the quark-nuclear matter hybrid equation of state that takes into account the effect of excluded volume in both the hadronic and the quark-matter phases is introduced. The nuclear phase manifests a reduction of the available volume as density increases, leading to a stiffening of the matter. The quark-matter phase displays a reduction of the effective string-tension in the confining density-functional from available volume contributions. The nuclear equation of state is based upon the relativistic density functional model DD2 with excluded volume. The quark-matter EoS is based upon a quasiparticle model derived from a relativistic density-functional approach and will be discussed in greater detail. The interactions are decomposed into mean scalar and vector components. The scalar interaction is motivated by a string potential between quarks, whereas the vector interaction potential is motivated by higher-order interactions of quarks leading to an increased stiffening at high densities. As an application, we consider matter under compact star constraints of electric neutrality and β-equilibrium. We obtain mass-radius relations for hybrid stars that form a third family, disconnected from the purely hadronic star branch, and fulfill the 2M constraint.

Mixed phase within the multi‐polytrope approach to high‐mass twins

“…The elucidation of the interior composition of compact stars is of special importance when considering the quantum chromodynamics (QCD) phase diagram. In case of a strong first‐order phase transition from hadronic matter into any type of exotic matter, such as quark deconfinement, the so‐called high‐mass twin (HMT) phenomenon (Alvarez‐Castillo & Blaschke , ; Alvarez‐Castillo et al ; Benic et al ; Blaschke et al ; Kaltenborn et al ) predicts particular characteristics of macroscopic observables in compact stars: disconnected sequences (families) in the mass–radius (M–R) diagram featuring compact star branches with overlapping ranges in the gravitational mass M but with different ranges of radii so that the radius difference at equal mass can vary from one half to a couple of kilometres depending on the model description. HMTs allow the resolution of several problems in the description of dense nuclear matter and its relation to compact stars: the masquerade effect, the reconfinement equation of state (EoS) case, and the hyperon puzzle (Blaschke & Alvarez‐Castillo ).…”

Strange matter in compact stars

“…However, choosing a finite constant as offset to the quark pressure can alter this result [17] and render neutron stars with strange matter core stable (the thermodynamic properties of matter are described in terms of pressure derivatives, hence a constant offset keeps them intact). Another way to stabilize strange core configurations is a strongly density dependent stiffening of the strange matter EoS following the transition [28,29]. This can result in situations where stable two flavor and three flavor quark core neutron star configurations are separated by a population gap at intermediate central neutron star densities.…”

Strange matter prospects within the string-flip model

“…At low densities (a) the field lines are compressed to thin flux tubes by the dual Meissner effect while at high densities (b) this pressure is reduced and consequently the effective string tension is lowered. Figure from [13].…”

“…This goes far beyond the conditions encountered in astrophysical applications, e.g., neutron stars and supernovae. Recently, a new approach has been developed, which starts from an effective relativistic density-functional [13] that implements quark confinement based on the string-flip model (SFM) [14,15]. According to the SFM, the color interactions between quarks are saturated among nearest neighbors.…”

“…In analogy to the Walecka model of nuclear matter [19], the relativistic densityfunctional approach to interacting quark matter can be obtained from the path integral approach based on the partition function [13],…”