Strange Star Candidates Research Articles

Pulsars with the masses 2.14M⊙ and 2.27M⊙ as strange star candidates

Strange stars with maximal masses, more than the values of recently precisely measured masses of two radio pulsars: SRJ0740+6620, with the mass 2.14М⊙, and PSRJ2215+5135, with the mass 2.27М⊙, have been studied. For the examination of strange quark matter (SQM), the bag model, developed in Massachusetts Technological Institute was chosen. The observed bag model depends on vacuum pressure B, quark-gluon interaction constant αc, and strange quark mass ms. Since the transition to SQM state takes place at the energy density, not exceeding double density in atomic nuclei, neutron stars with small mass and configuration, consisting of SQM, form one family in the dependence curve of the mass M, of the equilibrium super-dense configurations, on the energy central density ρc (curve M(ρc)). The state equation of strange quark matter was studied when the vacuum pressure was constant. Groups of the values of these parameters were determined; their application in the state equation of SQM leads to the maximal mass of the equilibrium quark configurations Mmax, which are heavier than the aforementioned radio pulsars. For such configurations, the values of mass, radius, the entire number of baryons, red shift from the strange star surface were calculated, depending on the energy central density ρc. For each series with Mmax>2.14M⊙ and Mmax>2.27M⊙ the values of the mentioned integral parameters were calculated as well for super-dense configurations with 2.27and2.14 solar masses that were precisely determined from observations. According to the obtained state equations, these two radio pulsars can be the possible candidates for the strange stars.

Impact of energy–momentum conservation violation on the configuration of compact stars and their GW echoes

This work investigates the impacts of energy–momentum conservation violation on the configuration of strange stars constraint with gravitational wave (GW) event GW190814 as well as eight recent observations of compact objects. The GW echoes from these interesting classes of compact objects are also calculated. To describe the matter of strange stars, we have used two different equations of state (EoSs): first an ad-hoc exotic EoS, the stiffer MIT Bag model and next realistic CFL phase of quark matter EoS. We choose Rastall gravity as a simple model with energy–momentum conservation violation with a set of model parameter values. Our results show that this gravity theory permits stable solutions of strange stars and the resulting structures can foster GW echoes. We illustrate the implication of the gravity theory and found that the negative values of the Rastall parameter result in more compact stellar configurations and lower GW echo frequency. With an increase in the Rastall parameter, both the compactness of the stellar configurations and echo time decrease. It is worth mentioning here that with the chosen set of some probable strange star candidates from observational data and also in light of GW 190814, we have evaluated the radii of stellar models. Also, the GW echo frequencies associated with strange stars are found to be in the range of ≈ 9–27 kHz for both cases. From this work, it is also inferred that the assumption regarding the equivalence of Rastall’s theory to Einstein’s theory is refuted as we have noticed many deviations in the physical properties of the considered compact stars.

Quark stars with 2.6 M_odot in a non-minimal geometry-matter coupling theory of gravity

This work analyses the hydrostatic equilibrium configurations of strange stars in a non-minimal geometry-matter coupling (GMC) theory of gravity. Those stars are made of strange quark matter, whose distribution is governed by the MIT equation of state. The non-minimal GMC theory is described by the following gravitational action: f(R,L)=R/2+L+sigma RL, where R represents the curvature scalar, L is the matter Lagrangian density, and sigma is the coupling parameter. When considering this theory, the strange stars become larger and more massive. In particular, when sigma =50 km^2, the theory can achieve the 2.6 M_odot , which is suitable for describing the pulsars PSR J2215+5135 and PSR J1614-2230, and the mass of the secondary object in the GW190814 event. The 2.6 M_odot is a value hardly achievable in General Relativity, even considering fast rotation effects, and is also compatible with the mass of PSR J0952-0607 (M = 2.35 pm 0.17 ~M_odot ), the heaviest and fastest pulsar in the disk of the Milky Way, recently measured, supporting the possible existence of strange quark matter in its composition. The non-minimal GMC theory can also give feasible results to describe the macroscopical features of strange star candidates.

Decoupled quark stars relativistic models in the regime of self-interacting Brans–Dicke gravity

In this work, we generate two static anisotropic solutions for a sphere containing quark matter in the framework of self-interacting Brans–Dicke theory. For this purpose, we add an anisotropic source in the seed distribution and decouple the field equations through deformation in the radial metric function. As a result of this transformation, the field equations are disintegrated into two systems which separately include the effects of isotropic and anisotropic sources. The system related to the additional source is solved via the MIT bag model equation of state. We consider Tolman V spacetime and Karmarkar condition to formulate two solutions for the isotropic sector which are extended to the anisotropic domain via decoupling technique. The junction conditions at the boundary determine the unknown parameters in terms of mass and radius of the spherical object. We investigate the viability and stability of the constructed strange star models in the presence of massive scalar field corresponding to three strange star candidates: Her X-1, PSR J1614-2230 and 4U 1608-52. It is concluded that the anisotropic models are well-behaved as they fulfill the necessary requirements for lower as well as higher values of the decoupling parameter. • We generate two anisotropic spherical solutions with quark matter in self-interacting Brans–Dicke theory. • Viability and stability of the constructed strange star models are checked for three strange star candidates. • Anisotropic models are well-behaved for lower as well as higher values of the decoupling parameter.

Buchdahl quark stars within f(Q) theory

In the present paper, authors study the strange stars with the MIT Bag EoS admitting Buchdahl symmetry (non-singular and physically viable metric potential) in the both linear $$f(Q)=aQ+b$$ and nonlinear $$f(Q)=Q+aQ^b$$ forms of modified symmetric teleparallel gravitation. In order to obtain the correct form of Buchdahl metric coefficients, they matched interior strange star space-time with the exterior Schwarzschild vacuum space-time. As a strange star candidate, PSRJ1416-2230 has been used with mass $$M=1.69M_\odot$$ and radius $$R=9.69R_\odot$$ . For either linear and nonlinear f(Q) gravities, they probed Null, Dominant and Strong Energy Conditions as well as the radial, tangential Equation of State (EoS), gradients of the perfect fluid energy-momentum tensor elements, Tolman–Oppenheimer–Volkoff equilibrium condition, relativistic adiabatic index and causality conditions, surface redshift. It was found that strange stars in the linear and nonlinear f(Q) gravity show physically viable behavior, respect energy and causality conditions, has EoS in the bounds $$0\le \omega \le 1$$ and surface redshift does not exceed 2, as expected.

Strange stars in f(ℛ) gravity palatini formalism and gravitational wave echoes from them

The compact stars are promising candidates associated with the generation ofgravitational waves (GWs). In this work, we study a special type of compactstars known as strange stars in the f(ℛ) gravity Palatiniformalism. Here we consider three promising f(ℛ) gravity modelsviz., Starobinsky, Hu-Sawicki and Gogoi-Goswami models in the domain of MITBag model and linear equations of state (EoSs). We compute the stellarstructures numerically and constrained the f(ℛ) model parameterswith a set of probable strange star candidates. The study shows that theconsideration of stiffer MIT Bag model and linear EoSs within a favourableset of f(ℛ) gravity model parameters may result in strange starswith sufficient compactness to produce echoes of GWs. Thus, we have computedthe GWs echo frequencies and characteristic echo times for such stars. It isfound that in compliance with the experimentally obtained possible strange starcandidates, the obtained GW echo frequencies for all the models are in therange of 65 - 85 kHz.

Charged anisotropic gravitational decoupled strange stars via complexity factor

This paper develops a charged anisotropic spherically symmetric strange star model by constructing an exact solution of the field equations through the gravitational decoupling approach. We employ minimal geometric deformation through which the initial decoupled system is divided into Einstein–Maxwell and quasi-Einstein systems. We choose a known solution for isotropic spherical matter distribution with MIT bag equation of state including electromagnetic field and generalize it to an anisotropic model by solving quasi-Einstein field equations using complexity factor. We study the physical viability of the resulting charged anisotropic solution by plotting metric functions, density, pressure, anisotropy parameter, energy conditions, and stability criterion for the strange star candidates SAXJ1808.4-658,SMCX-1,4U1538-52 as well as HERX-1. It is noticed that all the physical aspects are well-behaved, ensuring the physical acceptability of the developed models.

Model of hybrid star with baryonic and strange quark matter in Tolman–Kuchowicz spacetime

The purpose of our work is to investigate some new features of a static anisotropic relativistic hybrid compact star composed of strange quark matter (SQM) in the inner core and normal baryonic matter distribution in the crust. Here we apply the simplest form of the phenomenological MIT bag model equation of state [Formula: see text] to correlate the density and pressure of strange quark matter within the stellar interior, whereas radial pressure and matter density due to baryonic matter are connected by the simple linear equation of state [Formula: see text]. In order to obtain the solution of the Einstein field equations, we have used the Tolman–Kuchowicz ansatz [R. C. Tolman, Static solutions of Einstein’s field equations for spheres of fluid, Phys. Rev. 55 (1939) 364–373; B. Kuchowicz, Acta Phys. Pol. 33 (1968) 541] and further derivation of the arbitrary constants from some physical conditions. Here, we examine our proposed model graphically and analytically in detail for physically plausible conditions. In particular, for this investigation, we have reported on the compact object Her [Formula: see text] [[Formula: see text]; [Formula: see text] km] in our paper as a strange quark star candidate. In order to check the physical validity and stability of our suggested model, we have performed various physical tests both analytically and graphically, namely, dynamical equilibrium of applied forces, energy conditions, compactness factor and surface redshift. Finally, we have found that our present model meets all the necessary physical requirements for a realistic model and can be studied for strange quark stars (SQS).

Anisotropic stars of class one space–time in [formula omitted] gravity under the simplest linear functional of the matter–geometry coupling

In the present paper, we have explored the existence of compact structures describing anisotropic matter distributions within the framework of f(R,T) theory, where R is Ricci scalar and T is a trace of energy–momentum tensor. We have chosen f(R,T) as a linear function of R and T, i.e., f(R,T)=R+2χT with χ as matter–geometry coupling constant. We employed the embedding class one technique in order to obtain a full space–time description inside the stellar configuration. After specifying space–time geometry, we determined the complete solution of modified field equations by using the MIT-Bag model equation of state, i.e., pr=13(ρ−4Bg) that describes the strange quark matter distribution inside the stellar system, where Bg represents a bag constant. We checked the physical viability of the solution through different physical tests. Moreover, using observed mass values for the various strange star candidates, we have predicted the exact radii by taking distinct values for χ and Bg. Accordingly, we thoroughly show that with decreasing the value of χ, the stellar systems under examinations become progressively massive and larger in size, transforming them into less dense compact objects. It additionally uncovers that for χ<0 the fR,T gravity arises as an adequate theory for explaining the magnetars, super-Chandrasekhar stars and massive pulsars, which are assumed to be the reason behind the overluminous SNeIa e.g. SN 2003fg, SN 2006gz, SN 2007if, SN 2009dc and remain mostly unexplained in the background of general relativity.

Study on anisotropic strange stars in Rastall gravity

The strange quark matter (SQM) whose distribution is governed by the simplified MIT bag model equation of state has been the subject of a series of investigations within the framework of Rastall’s theory. We obtain an exact solution of the modified form of the Tolman–Oppenheimer–Volkoff (TOV) equation in the Rastall gravity theory and study the dependence of different physical properties (the total mass, radius, energy density, and pressure) for the chosen values of the Rastall parameter $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }$$ . To examine physical acceptability of the proposed stellar model, we conduct different tests in detail: the energy conditions, the the mass–radius relation, the Compactification factor, the redshift, the system stability, the modified TOV equation, the causality condition, and the adiabatic index in terms of $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }$$ . We precisely explain the effects arising due to the Rastall parameter and geometry on the compact stellar system. We find that as the factor $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }$$ decreases, the strange star candidates become gradually massive and larger in size with a less dense stellar configuration. But when $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }$$ increases, the stars shrink gradually and become less massive, turning into a more compact stellar system. For $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }>0$$ , our proposed model is therefore suitable for explaining the ultradense compact stars well within the observational limits; for $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }<0$$ , it allows representing the recent massive pulsars and super-Chandrasekhar stars. For $$\lambda_{ \scriptscriptstyle{\mathrm{Ras}} }=1$$ , we retrieve the standard results of general relativity.

Analysis of strange quark stars in massive Brans–Dicke gravity

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.

Anisotropic charged strange stars in Krori-Barua spacetime under [formula omitted] gravity

We present spherically symmetric anisotropic charged strange stars under the framework of f(R,T) gravity. For this we consider that the Lagrangian density is a linear function of R and T where R is the Ricci Scalar and T represents the trace of the energy–momentum tensor. These stars are considered to be built with Strange Quark Matter (SQM) distribution. To describe this SQM distribution inside the stellar bodies, we incorporate the simplest form of phenomenological MIT bag model equation of state (EOS) pr=13(ρ−4Bg), where pr is the radial pressure, ρ is the energy density and Bg is the bag constant. To determine density and pressure from the Einstein Field Equations (EFE), we use Krori-Barua (KB) metric and using the observed values of mass and radius for strange star candidates LMCX−4, we calculate the value of Bg from our model. Comparing the interior metric with exterior Reissner-Nordström metric at the surface, we calculate the unknown parameters incorporating the observed values of the mass and radius, we find that the coupling constant χ affects the bag value significantly. The model shows consistency with all the physical conditions and presents a viable study to the nature of charged anisotropic massive stellar system.

Charged analogues of isotropic compact stars model with buchdahl metric in general relativity

In this work, we examine a spherically symmetric compact body with isotropic pressure profile, in this context we obtain a new class of exact solutions of Einstein's-Maxwell field equation for compact stars with uniform charged distributions on the basis of Pseudo-spheroidal space-time with a particular form of electric field intensity and the metric potential g_rr. Indicating these two parameters takes into account further examination to be done in deciding unknown constants and depicts the compact strange star candidates likes PSR J1614-2230, 4U 1608-52, SAX J1808.4-3658, 4U 1538-52, SMC X-1, Her X-1, and Cen X-3. By the isotropic Tolman-Oppenhimer-Volkoff(TOV)equation, We explore the equilibrium among hydrostatic, gravitational and electric forces. Then, we analyze the stability of the model through the adiabatic index(gamma) and velocity of sound (0 <dp/c^2dr< 1). we additionally talk about other physical features of this model like, for example, the pressure, redshift, density, energy conditions and mass-radius of the stars in detail and demonstrate that our results are satisfying all the basic prerequisites of a physically legitimate stellar model. We have seen the measurement of basic physical parameter such as pressure, density, energy and redshift are satisfied the reality condition. All the physical quantities such as density, pressure pressure-density ratio and speed of sound is monotonically decreasing. The outcomes acquired are valuable in exploring the strength of other compact objects like white dwarfs, gravastars and neutron stars. Finally, we have shown that the obtained solutions are compatible with observational data for compact objects.

Anisotropic models for compact star with various equation of state

We model anisotropic compact stars by solving Einstein field equation with a generalized form of equation of state (EoS) and a physically reasonable metric potential $$g_{rr}$$ which allows us to extract models with various type of EoSs such as linear, quadratic, polytrope, Chaplygin and Color-flavor-locked. The generated interior metrics match smoothly with the exterior Schwarzschild metric. The radius $$R = 8.301$$ km and mass $$M=1.0359M_{\odot }$$ of the strange star candidate SMC X-1 has been used as reference to check the physical validity of the generated models. The physical quantities emanating from the analysis for all generated models are regular and well behaved throughout the interior of the star and satisfy the necessary criterion of a realistic anisotropic star. The significance of EoSs in modeling a realistic star with regard to its gross physical behavior has been illustrated by graphical analysis. A complete physical analysis has been performed for the main physical observables, such as the density $$\rho $$ , the radial and tangential pressures as well as the anisotropy factor. Moreover, the stability of the star has been checked by means of the velocities of the pressure waves and the relativistic adiabatic index. The generated models satisfy the Bondi’s stability condition and Tolman–Oppenheimer–Volkoff equation for static equilibrium for all type of EoSs. The effect of anisotropy on cracking condition has been discussed.

Charged strange stellar model describing by Tolman V metric

This paper deals with the existence of a compact stellar object, precisely strange (quark) star, in the framework of Einstein’s General Theory of Relativity with Tolman V metric potential, which is one of the simplest forms of potential among his proposals. The potential is given by eν=Kr2n, where K is the constant and n is a parameter [R.C. Tolman, Phys. Rev. 55, 364 (1939)]. Considering charged, static, spherically symmetric, isotropic fluid sphere we have studied different physical features of some strange star candidates namely EXO1785-248,LMCX-4,SMCX-1,SAXJ1808.4-3658,4U1538-52 and HerX-1. To represent the strange quark matter (SQM) distribution we have employed the simplest form of MIT Bag equation of state (EOS), which provides a linear relationship between pressure and density of the matter through Bag constant B. Several tests are done for the stability criteria and physical acceptability of the proposed model. The results show consistency with energy condition, TOV equation, adiabatic index, etc. We have also calculated different physical parameters of our model for the three different consecutive values of Bag constant B which are 83MeV/fm3,90MeV/fm3 and 100MeV/fm3. Among them with B=90MeV/fm3 we have analyzed different properties of the proposed strange star candidates.

Hybrid star model with quark matter and baryonic matter in minimally coupled [formula omitted] gravity

The present article deals with some new features of static anisotropic relativistic compact object with two fluids distribution (strange quark matter (SQM) and normal baryonic matter) along with Karori–Barua (KB) type metric within the framework of so called f(R) theory of gravity. We have considered the MIT bag model equation of state (EoS) to describe the relationship between pressure and matter density for the quark matter distribution inside the celestial object. For this investigation, we have formulated the field equations in f(R) theory with MIT bag EoS together with KB ansatz in the background of two specific viable f(R) models (f(R)=R+αR2 and f(R)=R+α(R2+ζR3)), respectively. Further, the physical credibility of the obtained solutions have been analyzed in the form of matter variables, energy conditions, hydrostatic equilibrium equation and stability criterion by using the graphical representation for different choices of free parameter α. Moreover, we have evaluated that the matter density and radial pressure are central singularity free and remain finite throughout the interior of the fluid sphere. The numerical values of such parameters and bag constant have been calculated corresponding to different compact star candidates (SAXJ1808.4-3658 (x=7.07Km), 4U1820-30 (x=10Km) and PSRJ1614-2230 (x=10.3Km)). Finally, we have observed that our proposed compact star candidate with two viable f(R) models, satisfies all the necessary and sufficient conditions for potentially stable configuration, which are quite suitable to test the strange star candidates.

Charged strange star with Krori–Barua potential in f(R, T) gravity admitting Chaplygin equation of state

In present paper a new compact star model in f(R,T) gravity is obtained where R and T denote the Ricci scalar and the trace of energy-momentum tensor $T_{\mu \nu}$ respectively. To develop the model we consider the spherically symmetric space-time along with anisotropic fluid distribution in presence of electric field with $f(R,T) = R + 2\gamma T$ where $\gamma$ is a small positive constant. We have used the Chaplygin equation of state to explore the stellar model. The field equations for $f(R,T)$ gravity have been solved by employing the Krori-Barua ansatz already reported in literature [J. Phys. A, Math. Gen. 8:508, 1975]. The exterior spacetime is described by Reissner-Nordstrom line element for smooth matching at the boundary. It is worthwhile to mention here that the values of all the constants involved with this model have been calculated for the strange stars 4U 1538-52 for different values of {\gamma} with the help of matching conditions. The acceptability of the model is discussed in details both analytically and graphically by studying the physical attributes of matter density, pressures, anisotropy factor, stability etc. We have also obtained the numerical values in tabular form for central density, surface density, central pressure and central adiabatic index for different values of $\gamma$. The solutions of the field equations in Einstein gravity can be regained by simply putting $\gamma = 0$ to our solution. Moreover, the proposed model is shown to be physically admissible and corroborate with experimental observations on strange star candidates such as 4U 1538-52.

Quark Stars in Massive Brans–Dicke Gravity with Tolman–Kuchowicz Spacetime

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the stellar model to evaluate the unknown constants in terms of mass and radius of the star. The radius of the strange star candidate PSR J1614-2230 is predicted by assuming maximum anisotropy at the surface of the star for different values of the coupling parameter, mass of the scalar field and bag constant. We examine various properties as well as the viability and stability of the anisotropic sphere. We conclude that the astrophysical model agrees with the essential criteria of a physically realistic model for higher values of the coupling parameter as well as mass of the scalar field.

Hybrid compact stars model in Rastall gravity: a comparative study

The purpose of present work is to provide a new mathematical model for hybrid compact stars composed of strange quark matter ( $\mathit{SQM}$ ) and normal baryonic matter distribution in the modified Rastall theory of gravitation. The simplest form of phenomenological $\mathit{MIT}$ bag model equation of state ( $P_{q}=\frac{1}{3}(\rho _{q}-4B_{g})$ ) has been used to correlate the density and pressure of strange quark matter in the stellar interior, whereas radial pressure and density are associated by the simple linear equation of state ( $p_{r}=m\rho $ ) for normal matter distribution. In order to attain the solution of field equation, we used the static and spherically symmetric Krori and Barua ( $\mathit{KB}$ ) type geometry and then arising arbitrary constants are obtained from some physical conditions. It has been manifested that for reasonable choice of the Rastall coupling parameter (i.e., $\chi =0$ ), the original results in the counterpart General Relativity ( $\mathit{GR}$ ) can be reobtained. We examined the obtained model graphically and analytically in detail for physically plausible conditions and compare with the observational facts and counterpart $\mathit{GR}$ . For the investigation of obtained results, we incorporate three different compact stars namely, $\mathit{Her} X-1$ , $\mathit{SAX} J1808-3658 (\mathit{SSI})$ and $4U 1820-30$ with radii $7.7\mbox{ km}$ , $7.07\mbox{ km}$ , and $10\mbox{ km}$ , respectively as strange quark star candidates. In order to check the validity of present model, we perform various physical tests analytically and graphically, namely, stability test, energy conditions, compactification and Buchdahl condition, dynamical equilibrium of applied forces and surface redshift etc. We found that our model meets necessary physical requirements for realistic model and more consonant with the observational facts as compared to the counterpart $\mathit{GR}$ which deviates from essential physical requirements for $\mathit{Her} X-1$ . Therefore our model is more realistic and can be studied for strange quark stars.

Anisotropic strange stars through embedding technique in massive Brans–Dicke gravity

This paper investigates the existence and properties of anisotropic strange quark stars in the context of massive Brans–Dicke theory. The field equations are constructed in Jordan frame by assuming a suitable potential function with MIT bag model. We employ the embedding class-one approach as well as junction conditions to determine the unknown metric functions. Radius of the strange star candidate, LMC X-4, is predicted through its observed mass for different values of the bag constant. We analyze the effects of coupling parameter as well as mass of scalar field on state determinants and execute multiple checks on the stability and viability of the spherical system. It is concluded that the resulting stellar structure is physically viable and stable as it satisfies the energy conditions as well as essential stability criteria.