Abstract
We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of σ, ω, ρ, σ*, and ϕ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5 km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system.
Highlights
Neutron stars (NSs) are compact stellar remnants which are born in type-II supernova explosions [1]
The parameter space (V1, G2) of the model has been analyzed by accounting for constraints from Lattice Quantum Chromodynamics (QCD) simulations, the existence of 2 M pulsars, as well as the limits set by the gravitational-wave event GW170817
We expand the (V1, G2) space by accounting for vector interactions and color superconductivity, which introduces the additional parameters Kv and ∆, respectively. To investigate this new parameter space spanned by V1, G2, Kv, ∆, we have chosen V1 = 20 MeV and G2 = 0.009 GeV4, following the results presented in Ref. [34]; these values for V1 and G2 are qualitatively representative of the parameters space
Summary
Neutron stars (NSs) are compact stellar remnants which are born in type-II supernova explosions [1]. Several different theoretical possibilities of the central composition of NSs are being explored, including some which account for a possible hadron-quark phase transition If the hadron-quark phase transition occurs in the cores of NSs, it has been shown that the liberated quarks should form a color superconductor (CSC) [23,24,25] This phase is characterized by the formation of quark Cooper pairs, to the formation of electron Cooper pairs in ordinary condensed matter superconductivity, which is energetically favored since it lowers the energy of the Fermi sea of fermions [26].
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