Narrow Results

The discovery of pulsars indicates the possibility for heavy neutron stars and also opened a new field of research for understanding the behavior of matter at ultrahigh densities and temperatures. Therefore, the equation of state (EoS) governing these compact objects is a key entity in describing their global properties. In this study, we investigate the potential of quark stars, which are composed of interacting quark matter with color superconductivity and perturbative QCD corrections. By using the EoS with the Tolman–Oppenheimer–Volkoff (TOV) equations, we explore the structural properties of quark stars in a modified theory of gravity known as energy-momentum squared gravity (EMSG). We impose observational limits to constrain the allowed values of the parameters appearing in EMSG, taking into account some astrophysical constraints related to the mass-radius $(M-R)$ relation of pulsars. Consequently, we study the quark star (QS) properties with different model parameters and determine the stability of stars by analyzing their sound velocity profiles, adiabatic index, static stability criteria and surface gravitational redshift.

The evolution of compact stars is believed to be able to produce various violent phenomena in our universe. In this paper, we discuss the possibility that gamma-ray bursts (GRBs) might result from the phase transition of a neutron star to a quark star and calculate the energy released from the conversion. In our study, we utilize the relativistic mean field (RMF) theory to describe the hadronic phase of neutron stars, while an improved quasi-particle model is adopted to describe the quark phase of quark stars. With quark matter equation-of-state (EOS) more reliable than models used before, it is found that the energy released is of the order of 10${}^{52}$ erg, which confirms the validity of the phase transition model.

The phase transition from a neutron star to a quark star and its relation to gamma-ray bursts are investigated. A new model: the 2 + 1 flavor Nambu–Jona-Lasinio (NJL) model with the method of proper-time regularization (PTR) is utilized for the quark phase; while the Relativistic Mean Field (RMF) theory is used for the hadronic phase. The process of phase transition is studied by considering the chemical potential, paying special attention to the phase transition point and the emergence of strange quark matter. Characteristics of compact stars are illustrated, and the energy release during the phase transition is found to be $\sim 10^{52}$ erg.

This paper is aimed to study spherically symmetric anisotropic dissipative quark star for tilted observer. The corresponding field equations as well as dynamical equations are formulated. We consider the MIT bag model for quark stars and investigate numerical solution of the field equations by imposing the shear-free condition. It is found that all the expected physical properties are present related to stellar fluid distribution. Finally, we analyze stability of the compact object by analyzing splitting of the fluid distribution. It is found that radial velocity does not change its sign during evolution of the system implying that the system does not undergo splitting and remains stable.

In this paper, we explore the behavior and anisotropic structure of quark stellar models in the framework of massive Brans–Dicke gravity. The system of field equations, representing a static sphere, is formulated by incorporating the MIT bag model. We use the Karmarkar condition for embedding class-one to formulate a relativistic model corresponding to a well-behaved radial metric function. The values of unknown parameters are determined through the matching of internal and external space–times at the hypersurface. The observed masses and radii of the strange star candidates (RXJ 1856-37, Her X-1 and PSR J1614-2230) specify the solution. Further, we evaluate the impact of the massive scalar field on state parameters and investigate the viability as well as stability of the self-gravitating objects. It is found that the obtained values of the bag constant (corresponding to each star) lie within the accepted range. Moreover, the anisotropic structure meets the necessary viability and stability criteria.

It is believed that quark stars or neutron stars with mixed phase in the core have smaller radii compared to ordinary compact stars. With the recent observation of several low radius objects, typically a radius of <10km for star of mass <1 M_⊙ in low mass X-ray binaries (LMXB), it has become very important to understand the nature of these objects. An accurate determination of the mass–radius relationship of these objects provide us with a physical laboratory to study the composition of high density matter and the nature of phase transition. We study the effect of quark and nuclear matter mixed phase on the mass radius relationship and radial oscillations of neutron stars. We find that the effect of the mixed phase is to decrease the maximum mass of a stable neutron star and to decrease the radial frequencies.

In contrast to the widely held belief that a degenerate star that has exhausted its nuclear fuel will, if sufficiently massive, unremittingly collapse to a singularity in space (unless the contraction is prevented by some unknown process of quantum gravity acting at the scale of the Planck length), I present a heuristic argument, based on known quantum processes, for the existence of stable equilibrium states of neutron stars and quark stars with macroscopic radii and masses unconstrained by the Chandrasekhar and Oppenheimer–Volkoff limits. The processes that stabilize the star against gravitational contraction involve strong magnetic coupling of the constituent fermions and fermionic pair production at the expense of gravitational potential energy.

It is generally agreed on that the tremendous densities reached in the centers of neutron stars provide a high-pressure environment in which several intriguing particles processes may compete with each other. These range from the generation of hyperons to quark deconfinement to the formation of kaon condensates and H-matter. There are theoretical suggestions of even more exotic processes inside neutron stars, such as the formation of absolutely stable strange quark matter. In the latter event, neutron stars would be largely composed of strange quark matter possibly enveloped in a thin nuclear crust. This paper gives a brief overview of these striking physical possibilities with an emphasis on the role played by strangeness in neutron star matter, which constitutes compressed baryonic matter at ultra-high baryon number density but low temperature which is not accessible to relativistic heavy ion collision experiments.

We review the stability of magnetized strange quark matter (MSQM) within the phenomenological MIT bag model, taking into account the variation of the relevant input parameters, namely, the strange quark mass, baryon density, magnetic field and bag parameter. A comparison with magnetized asymmetric quark matter in β-equilibrium as well as with strange quark matter (SQM) is presented. We obtain that the energy per baryon for MSQM decreases as the magnetic field increases, and its minimum value at vanishing pressure is lower than the value found for SQM, which implies that MSQM is more stable than non-magnetized SQM. The mass–radius relation for magnetized strange quark stars is also obtained in this framework.

The adiabatic index of pulsars formed by hadronic or quarkionic matter, with and without trapped neutrinos, is calculated for two values of the entropy per baryon S = 0, 2 k_B/baryon. The nonlinear Walecka model¹ is used to describe hadronic stars, and the MIT bag model² to describe quark stars. Particle fractions for each case are obtained and the appearance of particles are shown to coincide with drops in the adiabatic index.

This paper inspects the impact of minimal matter-geometry coupling present in $R+\alpha R^2+\gamma T$ model of $f(R,T)$ theory on the physical attributes of anisotropic quark stars. The geometry of considered stellar candidates is modeled via spherically symmetric static spacetime whose metric functions are influenced by Heintzmann solutions. The inner matter distribution of the stellar system is assumed as anisotropic with the phenomenological MIT bag model equation of state. The expressions of unknown parameters that appear in Heintzmann solution are evaluated in terms of mass and radius by the continuity of interior and exterior geometries. Further, insertion of masses and radii of some particular observed stellar models will yield their numerical values. In order to discuss the physical acceptability as well as stability of the quarks stars based on the considered solutions, we have checked the physical behavior of matter variables, mass and related quantities, energy conditions, equilibrium of forces, adiabatic index and Herrera’s cracking concept. The energy conditions are fulfilled ensuring the compatibility of assumed matter and geometry of quark stars. It is also found that all compact star candidates exhibit stable structures corresponding to the proposed values of the model parameters. Hence, the considered $f(R,T)$ model shows consistency with all the physical conditions and presents a viable study to the nature of anisotropic massive stellar system.

The aim of this paper is to develop the isotropic and anisotropic quark stars configurations in the context of $f(T,{𝒯})$ gravity in the static spherically symmetric background. To explore the combined effects of torsion scalar $T$ and the trace of energy–momentum tensor (EMT) ${𝒯}$ on relativistic astrophysics, we use diagonal as well as non-diagonal tetrad fields. By considering the conformal Killing vectors along with the MIT bag model, the interior solutions of the field equations corresponding to the linear $f(T,{𝒯})=\alpha T(r)+\beta {𝒯}(r)+\varphi$ model (in which $\alpha ,\beta$ are the constants and $\varphi$ indicates the cosmological constant) are calculated. The feasibility of the obtained solutions is confirmed by implementing several physical tests. The model parameters are constrained subject to the existence and stability of the quark star models. We formulate the energy constraints, stability equations, mass function, compactness and redshift factor, and present the graphical analysis of all physical quantities. It is found that the derived solutions for both diagonal and non-diagonal tetrad exhibit well-behaved profiles in the framework of modified teleparallel gravity.

We investigate the thermal evolution of isolated neutron stars, including the transition of nuclear matter into quark matter at some time during the cooling stage. We show cooling curves by changing the transition periods in parametric manner. If there would be observational data which fit to our results, it suggests the existence of quark stars.

This book chapter explores key aspects of neutron stars, pulsar glitches, tidal deformability, fast pulsars, the equation of state, and strange quark matter stars. Challenges in directly measuring neutron star radius have led to reliance on spectroscopic and timing techniques, with uncertainties addressed through careful source selection and theoretical modeling. Pulsar glitches reveal insights into the equation of state through angular momentum transfer within the neutron star. Tidal deformability is crucial in gravitational-wave astronomy, exemplified by the GW170817 event. Fast pulsars, instrumental in astrophysical testing, are classified into ordinary pulsars, millisecond pulsars, and magnetars. The EOS is vital for understanding neutron star internal structure, explored through various models. The chapter delves into the theoretical framework for rotating neutron stars, addressing uniform and differential rotation scenarios and their impacts on mass and radius. Additionally, the intriguing concept of quark stars and strange dwarfs is investigated. The various topics discussed in this book chapter contribute to a broader understanding of dense matter physics, astrophysical phenomena, and the potential for transformative discoveries through advanced observational techniques and technologies like gravitational wave detectors, radio telescopes, and X-ray telescopes.