Recent progresses in strange quark stars (2404.00363v3)

Abstract: According to the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, strange quark stars should be stable and could exist in the Universe. It is possible that pulsars may actually be strange stars, but not neutron stars. Here we present a short review on recent progresses in the field of strange quark stars. First, three popular phenomenological models widely used to describe strange quark matter are introduced, with special attention being paid on the corresponding equation of state in each model. Combining the equation of state with the Tolman-Oppenheimer-Volkov equations, the inner structure and mass-radius relation can be obtained for the whole sequence of strange stars. Tidal deformability and oscillations (both radial and non-radial oscillations), which are sensitive to the composition and the equations of state, are then described. Hybrid stars as a special kind of quark stars are discussed. Several other interesting aspects of strange stars are also included. For example, strong gravitational wave emissions may be generated by strange stars through various mechanisms, which may help identify strange stars via observations. Especially, close-in strange quark planets with respect to their hosts may provide a unique test for the existence of strange quark objects. Fierce electromagnetic bursts could also be generated by strange stars. The energy may come from the phase transition of neutron stars to strange stars, or from the merger of binary strange stars. The collapse of the strange star crust can also release a huge amount of energy. It is shown that strange quark stars may be involved in short gamma-ray bursts and fast radio bursts.

• The paper reviews diverse phenomenological models (MIT, NJL, quasi-particle) to characterize the equation of state for strange quark stars.
• It identifies gravitational signatures, including tidal deformability and oscillation modes, as key observational discriminants.
• The study explores hybrid stars and phase transitions, linking them to potential gravitational wave and electromagnetic burst phenomena.

Recent Advances in the Study of Strange Quark Stars

The hypothesis that strange quark matter (SQM) may constitute the true ground state of matter at exceedingly high densities has generated significant interest, leading to the theoretical proposition that strange quark stars (SQS) should be inherently stable and present in the universe. This paper provides a comprehensive review of recent advancements in the theoretical and observational paper of strange quark stars, exploring the implications of SQM as a fundamental astrophysical phenomenon.

Phenomenological Models and Equations of State

The authors begin by delineating three prominent phenomenological models that describe strange quark matter: the MIT bag model, the Nambu-Jona-Lasinio (NJL) model, and the quasi-particle model. Each of these models provides a unique equation of state (EOS), offering insights into the behavior of SQM under extreme conditions:

1. MIT Bag Model: This model posits that quarks are confined within a "bag," with the bag constant $B$, serving as a key parameter defining the pressure at which confinement occurs. Despite its simplicity and single free parameter, it effectively models quark confinement without addressing the asymptotic freedom characteristic of Quantum Chromodynamics (QCD).
2. NJL Model: This model incorporates the spontaneous breaking of chiral symmetry, essential for understanding nucleon mass dynamics. Unlike the MIT bag model, the NJL model derives confinement dynamics as emergent rather than imposed but similarly lacks features of QCD asymptotic freedom.
3. Quasi-particle Model: Here, quarks and gluons are treated as quasi-free particles with temperature and density-dependent effective masses, providing an EOS that aligns well with lattice QCD calculations while reflecting quark confinement dynamically.

These models, in conjunction with the Tolman-Oppenheimer-Volkoff (TOV) equations, inform the theoretical framework for determining the mass-radius relationship of strange stars and their internal composition.

Gravitational Signatures and Oscillations

The paper discusses tidal deformability and Love numbers as crucial gravitational wave signatures that could discriminate between neutron stars and SQSs. The dimensionless tidal deformability parameter ($\Lambda$) and Love number ($k_2$) display sensitivity to the star's EOS, providing potential observational avenues for identifying strange stars through gravitational wave detections from binary mergers.

Oscillations, both radial and non-radial, are explored concerning their stability implications for strange stars. Radial oscillation modes reveal insights about the internal pressure and density distribution, while non-radial modes, such as the f-, p-, and g-modes, offer additional observational characteristics linked to gravitational emission.

Hybrid Stars and Phase Transitions

The research expands into hybrid stars where quark and hadronic matter coexist, exploring both Maxwell and Gibbs constructions to model phase transitions within such stars. These constructions provide frameworks for understanding first-order transitions, latent heat, and smooth phase changes, all potentially observable through associated gravitational phenomena.

Gravitational Wave Emissions

Highlighting gravitational wave emissions, the authors propose multiple scenarios where strange stars or their hybrid counterparts could serve as significant gravitational wave sources. Binary systems involving strange stars (SQS-SQS) or interactions with black holes (BH-SQS) are considered, including discussions on GW190814 and implications of low-mass strange objects like quark planets. Such systems could provide definitive observational evidence for the existence of strange quark objects, particularly if events occur within detectable ranges.

Electromagnetic Bursts: GRBs and FRBs

The potential role of strange stars in generating electromagnetic bursts such as gamma-ray bursts (GRBs) and fast radio bursts (FRBs) is assessed. The conversion of neutron stars to strange stars, the accretion-induced collapse of SQS crusts, and interactions with smaller celestial bodies (e.g., asteroids) are explored as potential mechanisms for producing these high-energy bursts.

Conclusion

This review underscores recent developments in understanding strange quark stars and their possible prevalence and observational signatures. By merging theoretical advancements with gravitational and electromagnetic observational methods, the paper emphasizes the importance of continued investigation into SQM and its implications for astrophysics. Future research, particularly in gravitational wave astronomy, could further elucidate the existence and nature of these exotic states, potentially redefining our understanding of compact stellar objects in the universe.