Strange Stars Research Articles

Influence of three parameters on maximum mass and stability of strange star under linear f(Q) − action

ABSTRACT This study simulates strange stars in f(Q) gravity with an additional source under an electric field using gravitational decoupling by means of the complete geometric deformation (CGD) technique. By employing the Tolman ansatz and the MIT bag model equation of state (EOS), we explore bounded star configurations derived from the $\theta _0^0 = \rho$ and $\theta _1^1 = p_r$ sectors within the CGD formalism. Our models are subjected to physical viability tests, and we analyse the impact of anisotropy and the electric charge parameter E0 as well as the coupling parameters α and β1. Comparisons are made with observational constraints, including GW190814, neutron stars PSR J1614-2230, PSR J1903 + 6620, Cen X-3, and LMC X-4. Notably, we achieve the presence of a lower ‘mass gap’ component by adjusting parameters α and β1. Our models exhibit well-behaved mass profiles, internal regularity, and stability, along with the absence of gravitational collapse verified through the Buchdahl–Andréasson’s limit. In addition, we present a detailed physical analysis based on three parameters, α (decoupling strength), β1 (f(Q)–coupling), and Q (surface charge). This study provides insights into the behaviour of compact objects in f(Q) gravity and expands our understanding of strange star configurations within this framework.

Maximum mass of charged strange quark star in presence of strange quark mass (m s )

In this manuscript, we present an approach to calculate maximum mass of strange quark star having net charge inside. For this purpose we took the modified MIT bag model equation of state in presence of non-zero strange quark mass (m s ). The general solution of Einstein field equations in presence of charge is obtained by considering a specific form of the g rr component of the line element according to Vaidya & Tikekar. Such metric ansatz describes a homogeneous fluid distribution which has a departure from the spherical geometry determined by the two parameters: spheroidal (λ) and curvature (R). In this approach, we find that maximum mass as well as radius both decreases with the increase of strange quark mass (m s ). Also maximum mass increases with charge and obtained from our model is as high as 4.383 M ⊙ for maximum allowed value of charge with m s = 0. The stability is also studied in this model and note that our model is stable for the constraint value of parameters.

Three-layered compact star in modified Buchdahl-I spatial metric with distinct equations of state

We present a singularity free and smoothly connected complete solution for a three-layered compact star with deconfined quark matter core surrounded by coherent quantum fluid of a Bose-Einstein condensate, all enclosed under a thin envelope of a repulsive neutron-Coulomb fluid. A modified form of Buchdahl-I type spatial metric is used as a seed to obtain the explicit temporal metric potential for each layer using Einstein field equation. The process allows us to obtain insights into the pressure-density profile and associated thermodynamic properties within stellar interior, shedding light on its structural stability, mass-radius relation and physical plausibility. One of the key finding of our work is the close similarity between the M–R curve of our three-phase model star VelaX-1 and that of a strange quark star model.

Anisotropic strange stars in the spotlight: unveiling constraints through observational data

ABSTRACT Motivated by the recent suggestions that very massive pulsar (PSR J0952−0607) and very light compact object (HESS J1731−347) exist, in this article, we revisit the possibility of such objects being strange stars instead of the standard hadronic neutron stars. We study the possible presence of local anisotropy and how it affects the macroscopic properties of strange stars and compare our results with the recent constraints presented in the literature. We found that the presence of anisotropy increases the maximum mass, the radius of the canonical star, and its tidal deformability for positive values of λBL and the opposite for negative values. We also show that although we cannot rule out the possibility of very compact objects being standard hadronic neutron stars, strange stars easily fulfill most of the observational constraints.

EOS-dependent millihertz quasi-periodic oscillation in low-mass X-ray binary

ABSTRACT We studied the frequency and critical mass accretion rate of millihertz quasi-periodic oscillations (mHz QPOs) using a one-zone X-ray burst model. The surface gravity is specified by two kinds of equation of states: neutron star (NS) and strange star (SS). The base flux, Qb, is set in the range of 0–2 MeV nucleon−1. It is found that the frequency of mHz QPO is positively correlated to the surface gravity but negatively to the base heating. The helium mass fraction has a significant influence on the oscillation frequency and luminosity. The observed 7–9 mHz QPOs can be either explained by a heavy NS/light SS with a small base flux or a heavy SS with a large base flux. As base flux increases, the critical mass accretion rate for marginally stable burning is found to be lower. Meanwhile, the impact of metallicity on the properties of mHz QPOs was investigated using one-zone model. It shows that both the frequency and critical mass accretion rate decrease as metallicity increases. An accreted NS/SS with a higher base flux and metallicity, combined with a lower surface gravity and helium mass fraction, could be responsible for the observed critical mass accretion rate ($\dot{m}\simeq 0.3\dot{m}_{\rm Edd}$). The accreted fuel would be in stable burning if base flux is over than ∼2 MeV nucleon−1. This finding suggests that the accreting NSs/SSs in low-mass X-ray binaries showing no type I X-ray bursts possibly have a strong base heating.

Charged strange star coupled to anisotropic dark energy in Tolman–Kuchowicz spacetime

The concept of dark energy can be used as a possible option to prevent the gravitational collapse of compact objects into singularities. It affects the universe on the largest scale, as it is responsible for our universe’s accelerated expansion. As a consequence, it seems possible that dark energy will interact with any compact astrophysical stellar object [Phys. Rev. D 103, 084042 (2021)]. In this work, our prime focus is to develop a simplified model of a charged strange star coupled to anisotropic dark energy in Tolman–Kuchowicz spacetime (Tolman in Phys Rev 55:364, 1939; Kuchowicz in Acta Phys Pol 33:541, 1968) within the context of general relativity. To develop our model, here we consider a particular strange star object, Her X-1 with observed values of mass =(0.85 pm 0.15)M_{odot } and radius = 8.1_{-0.41}^{+0.41} km. respectively. In this context, we initially started with the equation of state (EoS) to model the dark energy, in which the dark energy density is proportional to the isotropic perfect fluid matter-energy density. The unknown constants present in the metric have been calculated by using the Darmois–Israel condition. We perform an in-depth analysis of the stability and force equilibrium of our proposed stellar configuration as well as multiple physical attributes of the model such as metric function, pressure, density, mass–radius relation, and dark energy parameters by varying dark energy coupling parameter alpha . Thus after a thorough theoretical analysis, we found that our proposed model is free from any singularity and also satisfies all stability criteria to be a stable and physically realistic stellar model.

Compact stars with dark energy in general relativity and modified gravity

We investigate realistic models of compact objects, focusing on neutron and strange stars, composed by dense matter and dark energy in the form of a simple fluid or scalar field interacting with matter. For the dark energy component, we use equations of state compatible with cosmological observations. This requirement strongly constrains possible deviations from the simple Λ-Cold Dark-Matter model with EoS pd=−ρd at least for small densities of the dark component. But we can propose that the density of dark energy interacting with matter can reach large values in relativistic stars and affects the star parameters such as the mass and radius. Simple models of dark energy are considered. Then we investigated possible effects from modified gravity choosing to study the R2 model combined with dark energy. Finally, the case of dark energy as scalar field non-minimally interacting with gravity is considered.

Strange quark mass (m s ) dependent model of anisotropic strange quark star* *A fellowship has been provided to A. Hakim by Government of West Bengal (G.O. No. 52-Edn(B)/5B-15/2017 dated June 7, 2017, read with 65-Edn(B)/5-15/2017 dated July 11, 2017) and to K.B. Goswami by Council of Scientific and Industrial Research, India (vide no. 09/1219 (0004)/2019-EMR-I)

This article presents the configuration of strange quark stars in hydrostatic equilibrium considering the Vaidya-Tikekar metric ansatz. The interior of such stars comprises strange quark matter (henceforth SQM), whose equation of state () is described by the MIT EoS , where B is the difference between perturbative and non-perturbative vacuum. We have included the mass of the strange quark into the EoS and studied its effect on the overall properties of the strange quark star in this work. It is observed that the maximum mass reaches its highest value when . We have evaluated the range of the maximum mass of the strange quark star by solving the TOV equation for necessary for stable strange quark matter at a zero external pressure condition with respect to neutrons. Maximum mass lies within the range of to when B ranges from to and . It is noted that the maximum mass decreases with an increase in . Our model is found suitable for describing the mass of pulsars such as PSR J1614-2230 and Vela X-1 and the secondary objects in the GW170817 event. The model is also useful in predicting the radius of the recently observed pulsars PSR J0030+0451, PSR J0740+6620, and PSR J0952-0607 and the secondary objects in the GW170817 and GW190814 events. Our model is found to be stable with respect to all stability criteria of the stellar configurations and is also stable with respect to small perturbations.

Compact Stars in the vBag Model and Its f-Mode Oscillations

Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced bag model (vBag model) is employed to investigate strange and hybrid stars’ properties. The parameters of the vBag model are constrained using multi-messenger observations, revealing that strange stars are incompatible with current observations. In contrast, hybrid stars can exhibit a substantial mixed phase region and a thin hadronic shell. Furthermore, we present the frequencies and damping time of fundamental mode (f-mode) oscillations of hybrid stars and test their universal relations with compactness and tidal deformability. The findings indicate that the presence of mixed phase components leads to larger frequencies and shorter damping time of the f-mode oscillation of hybrid stars, and the softer equation of state (EoS) affects this behavior more significantly. The universal relations of hybrid stars in the vBag model can be described by fourth-order/seventh-order polynomials, which do not break the previous results.

Fluid pulsation modes and tidal deformability of anisotropic strange stars in light of the GW170817 event

The effects of anisotropy on the fluid pulsation modes when adopting the so-called Cowling approximation and the tidal deformability of strange-quark stars are investigated by numerically integrating the hydrostatic equilibrium, nonradial oscillation, and tidal deformability equations, all of which are modified from their standard form to include the anisotropic effects. The fluid matter inside compact stars is described by the MIT bag model equation of state. For the anisotropy profile, we consider a local anisotropy that is both regular at the center and null at the star's surface. We find that the effect of anisotropy is reflected in the fluid pulsation modes and tidal deformability. Finally, we analyze the correlation between the tidal deformability of the GW170817 event and anisotropy.

Properties of strange quark matter and strange star in a new mass scaling

Previous research studies observed that quark mass scalings typically neglect the inclusion of asymptotic freedom. However, we have introduced a Woods–Saxon-like factor to incorporate the effects of asymptotic freedom into our new mass scaling. Our findings indicate that the equation of state and sound velocity for strange quark matter exhibit different behaviors at zero temperature when using this new mass scaling. This suggests the presence of novel properties in the phase transition and structure of strange stars. Additionally, through numerical calculations, we have successfully obtained a strange star with a mass two times that of the Sun, aligning with astronomical observations. In a parameter group considering first-order perturbation effects, characterized by large C and small D, we have made an interesting discovery: the surface density of the strange star can be lower than that of normal nuclear matter. This observation serves as a possible signal of a phase transition from quark matter to nuclear matter.

Stellar features of strange dark energy stars

Dark energy stars are theoretical objects that could potentially provide an explanation for the observable properties of black holes, including their event horizons and emission of the Hawking radiation. Because of this, they are considered to be an important area of research in modern theoretical physics and could help to answer fundamental questions about the Universe and the nature of dark energy. So, identifying and characterizing dark energy stars are a theoretically active and significant field in literature. In this context, for a theoretical model describing a new static symmetric singularity-free anisotropic dark energy star in view of the Rastall theory of gravity, which is an alternative to the general theory of relativity and is consistent with a wide range of astronomical observations, we focus our awareness in the present study on the modified field equations as well as the hydrostatic equilibrium expression and then discuss the corresponding solutions. It is shown that this new-type dark energy star meets all the necessary astronomical requirements, including stability and feasibility can potentially represent a compact cosmic structure.

Charged strange star model in Tolman–Kuchowicz spacetime in the background of 5D Einstein–Maxwell–Gauss–Bonnet gravity

In this article, we provide a new model of static charged anisotropic fluid sphere made of a charged perfect fluid in the context of 5D Einstein–Maxwell–Gauss–Bonnet (EMGB) gravity theory. To generate exact solutions of the EMGB field equations, we utilize the well-behaved Tolman–Kuchowicz (TK) ansatz together with a linear equation of state (EoS) of the form p_r=beta rho -gamma , (where beta and gamma are constants). Here the exterior space-time is described by the EGB Schwarzschild metric. The Gauss–Bonnet Lagrangian term mathcal {L}_{GB} is coupled with the Einstein–Hilbert action through the coupling constant alpha . When alpha rightarrow 0, we obtain the general relativity (GR) results. Here we present the solution for the compact star candidate EXO 1785-248 with mass=(1.3 pm 0.2)M_{odot }; radius = 10_{-1}^{+1} km. respectively. We analyze the effect of this coupling constant alpha on the principal characteristics of our model, such as energy density, pressure components, anisotropy factor, sound speed etc. We compare these results with corresponding GR results. Moreover, we studied the hydrostatic equilibrium of the stellar system by using a modified Tolman–Oppenheimer–Volkoff (TOV) equation and the dynamical stability through the critical value of the radial adiabatic index.The mass-radius relationship is also established to determine the compactness factor and surface redshift of our model. In this way, the stellar model obtained here is found to satisfy the elementary physical requirements necessary for a physically viable stellar object.

Strange stars within bosonic and fermionic admixed dark matter

In this work, we study dark matter (DM) admixed strange quark stars exploring the different possibilities about the nature of the DM and their effects on the macroscopic properties of strange stars, such as maximum masses, radii, as well the dimensionless tidal parameter. We observe that the DM significantly affects the macroscopic properties that depend on its mass, type, and fraction inside the star.

Confronting Strange Stars with Compact-Star Observations and New Physics

Strange stars ought to exist in the universe according to the strange quark matter hypothesis, which states that matter made of roughly equal numbers of up, down, and strange quarks could be the true ground state of baryonic matter rather than ordinary atomic nuclei. Theoretical models of strange quark matter, such as the standard MIT bag model, the density-dependent quark mass model, or the quasi-particle model, however, appear to be unable to reproduce some of the properties (masses, radii, and tidal deformabilities) of recently observed compact stars. This is different if alternative gravity theory (e.g., non-Newtonian gravity) or dark matter (e.g., mirror dark matter) are considered, which resolve these issues. The possible existence of strange stars could thus provide a clue to new physics, as discussed in this review.

A light strange star in the remnant HESS J1731−347: Minimal consistency checks

Context. Recently, Doroshenko and collaborators reported a very low-mass compact star, a Central Compact Object named XMMU J173203.3−344518 inside the supernova remnant HESS J1731−347. Its tiny mass is at odds with all calculations of minimum masses of neutron stars generated by iron cores, therefore (and even if not compellingly) it has been suggested to be a “strange star”. In addition to the mass, the radius and surface temperature were extracted from the data, and the whole body of information should ultimately reveal whether this object is truly consistent with an exotic composition. Aims. Our aim is to understand the status of the compact object XMMU J173203.3−344518 in HESS J1731−347 within the existing models of strange stars, including its prompt formation. Methods. The information obtained on the mass, radius and surface temperature are compared to theoretical calculations performed within usual theoretical models using General Relativity as the assumed theory of gravitation and a handful of cooling scenarios. A qualitative discussion showing the consistency of the strange-matter driven supernova scenario with a low-mass compact star is provided. Results. We found that the object HESS J1731−347 fits within the same quark star models recently employed to explain the masses and radii of the NICER objects PSR J040+6620 and PSR J0030+0451, in which both quantities were simultaneously determined. It is also remarkable to find that a simple cooling scenario devised 30 yr ago with superconducting quarks provides an overall good explanation of the surface temperature. Conclusions. We conclude that XMMU J173203.3−344518 in the remnant HESS J1731−347 fits into a strange star scenario that is also consistent with heavier compact stars, which can also belong to the same class and constitute an homogeneous type of self-bound objects produced in Nature.

Charged compact stars with color–flavor-locked strange quark matter in f(R,T) gravity

Due to the high density of neutron stars, there is still an open question on the complete structure of the inner core of compact object. It is a great challenge to propose a model describing the highly-dense matter of neutron stars since there are neither experimentally nor observationally adequate results. In theoretical physics, Quantum Chromodynamics (QCD) predicts that at high densities and low temperatures the inner core of neutron stars could be made of exotic quark matter in the color-flavor-locked (CFL) phase of color superconductivity. Regarding the equation-of-state, we predict the existence of electrically charged strange quark stars in the background of the modified theory of gravity f(R,T). In our discussion, we assume a linear relation between charge density and the fluid energy density, and also utilize the f(R,T)=R+2βT model to numerically compute the generalized Tolman–Oppenheimer–Volkoff (TOV) structure equations. In order to compute the properties of quark stars, we discuss the mass–radius profile, the effect of the charge in the stellar interior, factor of compactness, and mass-central energy density relation for stellar stability. Finally, we provide a set of basic differences between the standard Einstein-Maxwell gravity and the modified f(R,T) gravity theory.

Strange magnetars admixed with fermionic dark matter

We discuss strange stars admixed with fermionic dark matter in the presence of a strong magnetic field using the two-fluid Tolman-Oppenheimer-Volkov equations. We describe strange quark matter using the MIT bag model and consider magnetic fields in the range ∼ 1017 - 1018 G. For the fermionic dark matter, we consider the cases of free particles and strongly self-interacting particles, with dark fermion masses m = 5, 100, 500 GeV. We discuss the effects of dark matter and a strong magnetic field on the masses and radii of the stars, as well as on its tidal deformability. Even though strong magnetic fields contribute to decreasing the total mass of the star, they attenuate the rate of decrease in the maximum mass brought about by increasing the dark matter fraction in the admixed system. The most intensely affected observable, however, is the tidal deformability, with variations on the range of 70%-90% for reasonable values of the magnetic field or dark matter central energy density.

Analytical model of low-mass strange stars in $$2+1$$ space–time

It is quite fascinating to study the low-mass compact stars because of their enigmatic behaviour. In this paper, we have modelled this kind of low-mass strange stars based on the Heintzmann ansatz in $$(2+1)$$ dimensions. Attractive anisotropic force plays a significant role in restricting the upper mass limit (which is comparatively low) of the strange star. We have applied our model to some low-mass strange stars. Our model is useful to predict important parameters of the low-mass strange stars.

Strange quark star models from Rastall gravity

We study the possible existence of strange matter in the interior of compact stars in Rastall gravity. The main feature of the Rastall gravity is the covariant derivative of the energy–momentum tensor does not vanish and depends on the curvature R and a free parameter η. We consider the MIT bag model in the present work and numerically solve the modified Tolman–Oppenheimer–Volkoff (TOV) equations. We investigate the effect of the Rastall parameter η on the mass–radius and the mass-central energy density relation of quark stars. From the quark star masses and radii obtained, we conclude that η has significant consequences on the structure of stellar objects.