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This paper is devoted to observe the physical attributes of static spherically symmetric isotropic compact stellar candidates in the context of Rastall theory of gravity. In order to inspect the structural composition of compact objects, the Heintzmann ansatz is taken into account. The unknown parameters associated with Heintzmann ansatz are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition, stellar equation and adiabatic index for several choices of Rastall parameter. It is observed that the stars under consideration manifest stable structures corresponding to Heintzmann ansatz in this framework. Moreover, it is shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved. [ABSTRACT FROM AUTHOR]

Using modified f(T) gravity and the diagonal tetrad, we propose a new kind of analytical solution to describe anisotropic charged stellar structures in this study. We obtain precise solutions by applying the linear form of f(T) as f(T) = γT + δ in the framework of Tolman–Kuchowicz physically sound metric potentials. The stellar structures of the compact star EXO 1785-248 are then investigated using the model by incorporating a linear equation of state relating the radial pressure pr and the matter density ρ. We computed the closed-form expressions for the model parameters and illustrated their characteristics. The comprehensive graphical analysis demonstrates the scientific plausibility, causality, and dynamical stability of the model describing charged anisotropic stars. Finally, using the M-R curve, we estimate the numerical values of mass for different values of γ. The observational data of the neutron star candidates 4U 1820-30, PSR J1614-2230, and PSR J2215 + 5135 are covered by the maximum allowable masses for different values of γ additionally, when γ = 2.5, the charged compact star’s maximum permissible mass is calculated to be 2.56M⊙, which could indicate the secondary component of the GW190814 event discovered by LIGO/VIRGO experiments. [ABSTRACT FROM AUTHOR]

The primary objective of this study is to conduct a thorough examination of relativistic structures within the framework of f (ℛ ,) gravity, where ℛ is the Ricci scalar and is the trace of the energy–momentum tensor. We investigate the emerging characteristics of compact stars, focusing on the ℛ + 2 ξ model, where ξ is the coupling constant. In the initial sections of the paper, we concentrate on the examination of exact interior solutions involving perfect fluid. With this aim, we introduce the metric coefficient L = S (1 + 2 a r 2) d / 2 , where a and S are arbitrary constants to be computed from the matching conditions and d is a positive odd integer. It is worth noticing that we select a group of 10 significant compact stars from recent studies, namely, Vela X − 1 , EXO 1 7 8 5 − 2 4 8 , 4 U 1 8 2 0 − 3 0 , SMC X − 4 , Her X − 1 , PSR J 1 6 1 4 − 2 2 3 0 , PSR J 1 9 0 3 + 3 2 7 , Cen X − 3 , 4 U 1 5 3 8 − 5 2 and LMC X − 4. To investigate the viability of our model, we conduct various checks, including the analysis of energy density and pressure components, equilibrium, and energy conditions, stability analysis and the adiabatic index. An important part of this work provides the slowly rotating property of stars, accompanied by the derivation of the moment of inertia. The relationship of mass, energy density, radius and moment of inertia for stellar objects are observed. Our findings demonstrate that obtained results are physically acceptable and consistent with our considered model. [ABSTRACT FROM AUTHOR]

This paper investigated the anisotropic quintessence stars in the f (R , ϕ) theory of gravity, where R is the Ricci scalar and ϕ is the scalar potential. We investigated the field equations of the f (R , ϕ) theory of gravity by using the Krori–Barua technique. We have determined that all the obtained solutions are free from central singularity and potentially stable. The observed values of mass and radius of the different strange stars Her X1, SAXJ1808.4-3658, and 4U1820-30 have been used to calculate the values of unknown constants in the Krori and Barua metric. The physical phenomena of stars, such as energy density, pressure components, anisotropy, sound speeds, equation of state parameters, energy conditions and redshift have been investigated in detail. [ABSTRACT FROM AUTHOR]

This paper is devoted to investigating the behavior of charged compact stars in the f(R,ϕ,X) theory of gravity, where R denotes the Ricci Scalar, ϕ is a scalar potential, and X is a kinetic term. For this purpose, we consider spherically symmetric spacetime with Bardeen geometry as exterior spacetime to investigate various properties of compact stars, including energy density and pressure components. Matching conditions are used to find model parameters. We present a detailed analysis, including a discussion of equations of state parameters, and stability criteria. In this analysis, we discuss the energy conditions to check the viability of our model. Furthermore, an equilibrium condition can be visualized through the modified Tolman–Oppenheimer–Volkov equation. It is concluded that Bardeen’s geometry supports the existence of compact stars in the background of the f(R,ϕ,X) theory of gravity. [ABSTRACT FROM AUTHOR]

The primary objective of this paper is to examine singularity-free solutions within the framework of anisotropic solutions for the Chaplygin equation of state in f (R , ϕ , X) modified gravity theory. Herein, R signifies the Ricci scalar, ϕ denotes the scalar field, and X represents the kinetic term associated with ϕ. The investigation employs the Krori–Barua metric to explore the characteristics of an anisotropic compact star. In the course of our analysis, we specifically consider observational data about three distinct stellar structures: Her X-I, LMC X-4, and 4U 1538-52. Our inquiry extends to the graphical examination of various physical parameters, encompassing energy density, pressure components, anisotropic components, equation of state parameters, equilibrium conditions, stability analysis, adiabatic index, and energy conditions. Furthermore, we delve into the intricacies of the mass-radius function, compactness factor, and surface redshift–integral features of charged compact stars. The culmination of our findings suggests that the f (R , ϕ , X) theory of gravity when subjected to observational scrutiny, manifests as realistic, stable, and devoid of singularities. [ABSTRACT FROM AUTHOR]

In this study, our main focus is to investigate the mass–radius relation and several important properties of massive neutron stars to realize the nature, behavior and evolution of these kinds of compact objects at present time. Also, we want to understand the equation-of-state of the core nuclear matter precisely with their stable equilibrium configuration. We have chosen a few massive binary pulsars and investigated on maximum attainable mass and lowest possible radius of them. We have considered Einstein–Hilbert action as f (R , T) = R + α T , where R is the Ricci scalar and T , the trace of energy–momentum tensor with α as the coupling parameter and also used modified TOV equations. The interior space-time of the spherical neutron star is matched to the exterior Schwarzschild line element at the surface of the star. The M – R curve can predict the maximum achievable mass is about 3. 5 1 M ⊙ with lowest possible radius of around 1 0. 5 km for the massive compact stars under stable equilibrium. We have obtained a clear picture of structural evolution of massive neutron stars through accelerating space-time and can put some constraints on several quantities related to them. It is depicted from our present investigation that all the derived outcomes are compatible with physically adopted regimes which reveal the viability of our current model in the context of f (R , T) modified gravity. [ABSTRACT FROM AUTHOR]

Observational evidence revealed that in the composition of the Universe, the dark energy is in heap proportion which is mainly responsible for the accelerated expansion of the Universe. The astrophysicists hypothesized that every astrophysical object is likely interact with the dark energy Sakti and Sulaksono [Phys. Rev. D 103 (2021) 084042]. Therefore, researchers are intended to study the astrophysical objects with dark energy interaction. In this paper, we proposed a dark energy stars (composed of with dark matter and ordinary matter) model in the Rastall theory of gravity. The Rastall field equations are obtained in the Tolman–Kuchowicz-type metric. To solve the field equations, the arbitrary constants involved in the field equations (due to Tolman–Kuchowicz ansatz) are obtained by matching the interior geometry with the exterior Schwarzschild geometry. The obtained solution is tested physically by computing some analytical results and plotting graphs of different physical parameters like pressure, density for ordinary matter, density for dark matter, mass function, compactness and surface redshift. Stability of the model is also analyzed by different tests like velocity of sound, adiabatic index and TOV equations. The influence of the Rastall parameter on the model has been studied by plotting graphs and computing numerical values of model's physical parameters for different values of the Rastall parameter. We infer that our propounded model fulfills all the necessary requirements for a physically plausible model. [ABSTRACT FROM AUTHOR]

This paper examines the viable characteristics of anisotropic compact stars in f(ℜ,T2) theory (ℜ is the Ricci scalar and T2 = T αβTαβ). In this perspective, we use Tolman–Kuchowicz solutions (μ = wr2 + 2ln z and ν =ln(1 + xr2 + yr4)) to examine the configurations of static spherically symmetric structures. The unknown constants are found by the first fundamental form of Darmois junction conditions. We analyze the behavior of fluid parameters in the interior of 4U 1538-52, PSRJ 1614-220, EXO 1785-248 and SAXJ 1808.4-3658 compact stars correspond to different models of this theory. Furthermore, the stability of the proposed compact stellar objects is examined through sound speed and adiabatic index methods. The satisfaction of requisite conditions ensures that stable compact objects exist in this framework. [ABSTRACT FROM AUTHOR]

This paper is devoted to observe the physical attributes of isotropic relativistic spherical objects in the context of Rastall theory of gravity. In order to inspect the structural composition of spherical objects, the Tolman IV solution is taken into account. The unknown parameters associated with Tolman IV solution are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition and stellar equation for several choices of Rastall parameter. It is observed that the considered solution does not admit singularities in the matter variables. Moreover, in accordance with the graphical analysis, the stellar models satisfy all physical requirements anticipated in a realistic star. It is also shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to obtain physically stable stellar models coupled with anisotropic matter distribution in the context of f (ℛ , T 2) theory. For this, we consider a static spherical geometry and formulate modified field equations containing various unknowns such as matter determinants and metric potentials. We then obtain a unique solution to these equations by employing Durgapal–Fuloria ansatz possessing a constant doublet. We also use matching criteria to calculate the values of these constants by considering the Schwarzschild exterior spacetime. Two different viable models of this modified theory are adopted to analyze the behavior of effective matter variables, anisotropy, energy conditions, compactness and redshift in the interiors of Her X-1, PSR J0348-0432, LMC X-4, SMC X-1, Cen X-3, and SAX J 1808.4-3658 star candidates. We also check the stability of these models by using three different physical tests. It is concluded that our considered stars satisfy all the physical requirements and are stable in this modified gravity for the considered parametric values. [ABSTRACT FROM AUTHOR]

A new solution to the Einstein–Maxwell field equations for a charged anisotropic compact star is generated. The presence of both a conformal Killing vector and a quadratic equation of state give rise to a new solution with astrophysical significance. The incorporation of the conformal Killing vector leads to an equation relating the gravitational potentials. By combining the quadratic equation of state with the Einstein–Maxwell field equations, an equation for electric field intensity is obtained. Subsequently, one of the gravitational potentials is chosen on physical merits to obtain the second gravitational potential and other matter variables. The generated solution undergoes a thorough analysis to ensure acceptance of physical conditions for realistic compact star models. A deep analysis of the physical aspects reveals the well-behaved nature of the gravitational potentials and matter variables. A smooth matching of the interior and exterior metrics is achieved at the boundary. The gravitational potentials are free from physical and geometric singularities. Also, the energy conditions are satisfied, and the ratio of mass to radius and the surface redshifts remain within reasonable ranges. The equilibrium is attained by the balanced natural interior physical forces. The stability of the model against gravitational collapse is satisfied. Interestingly, the use of quadratic equation of state in conformal stars is missing in treatments performed in the past. [ABSTRACT FROM AUTHOR]

This work mainly focuses on modeling the charged compact stars and investigating their properties in the framework of f (T) modified gravity, under the accelerated phase of the universe. In this study, we have introduced the modified gravity and dark matter to culture the evolution of the charged neutron stars by considering the present situation. True knowledge and proper study on the macroscopic parameters of the compact stars are very much needed to realize the microscopic properties and behavior of the core nuclear matter at very high density and pressure. The existence of modified Chaplygin gas as an exotic fluid inside the core of these stars in f (T) gravity plays a very important role inside the stars. We have found out that exotic fluid has a great impact on the equation of state of the core nuclear matter and even on the stability of the compact stars. Pressure anisotropy reduces the tidal deformability of the charged stars in an appreciable amount and helps to get a more compact structure. But surprisingly, the compact stars are still able to maintain their spherically stable and equilibrium configuration. Further, we can put constraints on several macroscopic parameters of the compact stars and can investigate their evolution from our present investigation. [ABSTRACT FROM AUTHOR]

The construction of compact star models with quintessence and cloud of strings anisotropic fluid lays the foundation for the research presented in this paper, which focuses on the ansatz of f(Q) gravity. We utilized a spherically symmetric spacetime to derive the field equations incorporating quintessence and cloud of strings anisotropic matter fields. The well-known Karmarkar condition was employed to obtain the gravitational components of the spherically symmetric spacetime. The constant parameters used in the study were obtained by applying junction conditions on the interior and exterior spacetimes. We used the mass and radius of P S R J 1 6 1 4 − 2 2 3 0 & S M C X − 1 , which are already present in the literature. Afterward, we calculated the important properties of quintessence stars in the presence of cloud of strings to ensure the stability and physical presence of compact models. To achieve this goal, we studied features such as energy density, quintessence density, pressure profiles, gradients, anisotropic factor, energy conditions, sound speeds, TOV forces, EoS components, mass function, compactification and redshift. [ABSTRACT FROM AUTHOR]

In this study, we investigate the literature on anisotropic stellar spheres in f (R , ϕ) gravity utilizing Tolman–Kuchowicz (TK) spacetime. Due to the existence of anisotropic matter distribution, we derive the equations of motion for spherically symmetric spacetime by taking into account physically accurate formulations of the metric potentials. For this purpose, we develop the matching conditions by comparing them with the exterior solution to determine the TK constants. We have also discussed the graphical analysis of energy density, pressure components, equation of state components, equilibrium condition, and stability analysis. Moreover, other properties of a compact star, like the mass–radius function, compactness, and redshift function, have also been investigated. It is also noticed that energy conditions are satisfied, which justifies that our considered stars are viable. The validity of our proposed model has been proven by studying the development of various physical characteristics, and it is determined that our system is physically stable and free of any singularities. [ABSTRACT FROM AUTHOR]

In this paper, we obtain a new class of stationary axisymmetric spacetimes by using the Gürses–Gürsey metric with an appropriate mass function in order to generate a rotating core of matter that may be smoothly matched to the exterior Kerr metric. The same stationary spacetimes may be obtained by applying a slightly modified version of the Newman–Janis algorithm to a nonrotating spherically symmetric seed metric. The starting spherically symmetric configuration represents a nonisotropic de Sitter-type fluid whose radial pressure p r satisfies an state equation of the form p r = − ρ , where the energy density ρ is chosen to be the Tolman-type-VII energy density [R. C. Tolman, Phys. Rev.55, 364 (1939)]. The resulting rotating metric is then smoothly matched to the exterior Kerr metric, and the main properties of the obtained geometries are investigated. All the solutions considered in this study are regular in the sense they are free of curvature singularities. Depending on the relative values of the total mass m and rotation parameter a, the resulting stationary spacetimes represent different kinds of rotating compact objects such as regular black holes, extremal regular black holes, and regular starlike configurations. [ABSTRACT FROM AUTHOR]

This work deals with the investigation of several physical elements that contribute to the construction of stellar objects in the f (R , G) theory, where R and G symbolize the Ricci and Gauss–Bonnet scalars, respectively. In the presence of an anisotropic source, we offer the fundamental formalization of this modified theory. To establish the physical viability of our models, we employ various methods. Energy conditions (ECs) will be derived to ensure the physical validity of our models within the context of f (R , G) gravity. Moreover, we examine the physical viability through the assessment of radial and tangential sound speeds. Additionally, we analyze the equation of state (EoS) parameter and the anisotropic factor in the presence of the electromagnetic field's influence. Dense stars are shown to be stable under anisotropy measures and exhibit their physical behavior under anisotropic stresses. We identify regions where the systems occupy high compactness and maintain stable ECs with a particular electric charge contribution. [ABSTRACT FROM AUTHOR]

In this paper, we investigated anisotropic solution for a self-bound compact star model. The solution is obtained via Vaidya–Tikekar metric function 1 / W (r) = K + C r 2 K (1 + C r 2) and the vanishing complexity factor condition. The physical analysis of the solution has been discussed in detail which includes mass-radius ratio, and stability analysis via adiabatic index for a secondary component of the GW190814 event for different values K. Furthermore, we predicted the radii and obtained the compactness factor of different self-bound compact objects for different K. Moreover, the hydrostatic equilibrium condition of the solution is also checked. The physical result shows that the solution so obtained is well behaved and may be useful for modeling the self-bound compact objects as far as observational data is concerned. [ABSTRACT FROM AUTHOR]

The main objective of this study is to investigate the stability and viability of anisotropic compact stellar objects using the Krori–Burua solutions in f () gravity, where is a nonmetricity scalar that explains the gravitational effects. We use a static spherical metric in the inner region and Schwarzschild spacetime in the outer region of the Her X-I, SAX J 1808.4-3658 and 4U1820-30 compact stars to investigate their physical properties. By using observed values of the radius and mass of the considered compact stars, the unknown parameters are determined. We use a particular model of this theory to examine the behavior of energy density, pressure components, anisotropy, equation of state parameters and energy constraints in the interior of the proposed stellar objects. The Tolman–Oppenheimer–Volkoff equation is used to analyze the equilibrium state of these stars and causality condition, Herrera cracking method, adiabatic index methods are used to determine their stability. It is found that revolutionary technique sheds new light on the development of observationally accurate gravity model. [ABSTRACT FROM AUTHOR]

The main purpose of this research is to examine a number of interior configurations of stable anisotropic objects formed in the shape of spherical charged stars within the gravity region described by f (R , G) , where G represents the Gauss–Bonnet invariant and R defines the Ricci Scalar in this scenario. The formation of these charged stars is investigated using the solutions found by Karori and Barua, among several feasible models within the theory of f (R , G) gravity. The study explores the behavior of these realistically charged compact stars, analyzing various physical components through the use of graphs. Furthermore, the feasibility of our model is assessed by evaluating it against different energy conditions. Additionally, topics such as density modification, stress evolution, various forces, star stability, anisotropy measurement, state parameter equations, and charge distribution are discussed. [ABSTRACT FROM AUTHOR]

This study is devoted to investigate the formation of compact stars using Tolman–Kuchowicz space-time in f (,) gravity. By taking into account the physically reliable formulations of metric potentials, ξ = B r 2 + 2 ln C and η = ln (1 + a r 2 + b r 4) , we investigate the equation of motion for spherically symmetric space-time in the presence of an anisotropic matter distribution. Furthermore, matching conditions are employed to compute the unknown constants. By making use of dynamical equations, the pivotal relevant aspects, including energy density, radial and tangential pressures, dynamical equilibrium, anisotropy effect, energy conditions and stability, are physically tested in order to determine the physical acceptability of yielding celestial model, which are thoroughly compared with experimental facts and figures of ten different compact stars. Finally, we observe that obtained anisotropic outcomes are physically viable, free from geometrical and physical singularities. Moreover, these outcomes also provide circumstantial evidence for the existence of super-massive compact stars. [ABSTRACT FROM AUTHOR]

The aim of this paper is to obtain new physically admissible exact anisotropic stellar models via gravitational decoupling technique. This will be done by taking Korkina and Orlyanskii model solution as a seed solution and extend it to an anisotropic domain via gravitational decoupling through the minimal geometric deformation (MGD) approach. We analyze the physical properties of four compact star models Her X − 1 (Mass M = 0. 8 5 M Θ , radius = 8. 1 km), LMC X − 4 (M = 1. 0 4 M Θ , radius = 8. 3 0 1 km), PSR J 1 9 0 3 + 3 2 7 (M = 1. 6 6 7 M Θ , radius = 9. 4 3 8 km) and PSR J 1 6 1 4 − 2 2 3 0 (M = 1. 9 7 M Θ , radius = 9. 6 9 km). We demonstrate the stability conditions of the obtained solution by plotting energy conditions, velocity of sound, adiabatic index and all the thermodynamic parameters and equilibrium conditions via TOV equation, etc. We found that the results are physically well behaved and also fulfill the requirement of stability criteria inside the anisotropic fluid sphere. We study the physical features of the solutions in detail, analytically and graphically for the four compact stars Her X − 1 , LMC X − 4 , PSR J 1 9 0 3 + 3 2 7 and PSR J 1 6 1 4 − 2 2 3 0. Based on the results we realized that the extended gravitational decoupling technique is very effective for the analysis of the interior of the compact stellar structures. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of tidal Love numbers on the equation-of-state (EoS) governing the interior matter distribution of a compact star within the framework of classical general relativity. By invoking a linear EoS for a spherically symmetric anisotropic matter configuration, we solve the Einstein field equations for an arbitrary EoS parameter. We subject our m odel to rigorous regularity, causality, and stability tests. In particular, we show that our model gives a very good approximation of the pulsar 4U 1608–52. We further demonstrate the intrinsic connection between the EoS parameter which describes the nature of the matter making up the stellar fluid and the tidal Love numbers. [ABSTRACT FROM AUTHOR]

In this work, the analysis of the behavior of an interior solution in the frame of Einstein's general theory of relativity is reported. Given the possibility that, for greater densities than the nuclear density, the matter presents anisotropies in the pressures and that these are the orders of density present in the interior of the compact stars, the solution that is discussed considers that the interior region contains an anisotropic fluid, i.e. P r ≠ P t . The compactness value, where u = G M c 2 R , for which the solution is physically acceptable is u ≤ 0. 2 3 5 7 7 as such the graphic analysis of the model is developed for the case in which the mass M = (0. 8 5 ± 0. 1 5) M ⊙ and the radius R = 8. 1 ± 0. 4 1 km which corresponds to the star Her X-1, with maximum compactness u max = 0. 1 9 1 9 , although for other values of compactness u ≤ 0. 2 3 5 7 7 the behavior is similar. The functions of density and pressures are positive, finite and monotonically decreasing, also the solution is stable according to the cracking criteria and the range of values is consistent with what is expected for these type of stars. [ABSTRACT FROM AUTHOR]

This paper inspects the impact of minimal matter-geometry coupling present in R + α R 2 + γ 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. [ABSTRACT FROM AUTHOR]

The aim of this work is to discuss the evolution of compact stars from the view point of a string fluid in Rastall theory using Krori–Barua (KB) metric as interior geometry. The exterior spacetime is considered as Schwarzschild metric while matching of interior and exterior spacetimes lead to coefficients of KB ansatz. The field equations and dynamical variables of the string fluid are explored. We found the values of Rastall parameter η for which the dynamical variables satisfy the energy conditions which shows the existence of physical matter. The model is applied to specific physical features including energy conditions, anisotropy, stability, Tolman–Oppenheimer–Volkoff equation, mass function, compactness and redshift of compact stars, in particular, SAX J1808.4-3658, Vela X-12 and Hercules X-1. It is found that the presented model fulfills all the physical requirements and thus, is realistic. We conclude that the string fluid is responsible for the evolution of compact stars in the cosmos. [ABSTRACT FROM AUTHOR]

In this work, we present an exact interior solution to a physically acceptable Einstein–Maxwell equation system, assuming a static and spherically symmetric spacetime with a distribution of matter from a perfect charged fluid to represent a generalization of a model for a perfect chargeless fluid. The charge parameter modifies the mass function, its compactness rate and the comportment of the speed of sound. The behavior analysis of the functions of density, pressure and charge shows that the solution is applicable for the description of relativistic compact stars. In particular, we analyze the behavior of these functions for the values of observed mass M ∈ [ 0. 9 5 , 1. 1 3 ] M ⊙ and the theoretical radius interval estimated previously R ∈ [ 8. 1 0 1 , 8. 5 0 1 ] km from the star LMC X-4. Thus, the biggest charge value of maximum charge Q = 1. 7 3 × 1 0 2 0 C occurs for the maximum compactness u = 0. 2 0 5 9. [ABSTRACT FROM AUTHOR]

The main emphasis of this paper is to find the viable solutions of Einstein Maxwell field equations of compact star in context of modified f (R) theory of gravity. Two different models of modified f (R) gravity are considered. In particular, we choose isotropic matter distribution and Bardeen's model for compact star to find the boundary conditions as an exterior space-time geometry. We use the conformal Killing geometry to compute the metric potentials. We discuss the behavior of energy density and pressure distribution for both models. Moreover, we analyze different physical properties such as behavior of energy density and pressure, equilibrium conditions, equation of state parameters, causality conditions and adiabatic index. It is noticed that both f (R) gravity models are suitable and provide viable results with Bardeen geometry. [ABSTRACT FROM AUTHOR]

Recent research works have shown the existence of super-massive neutron stars (NSs) with mass about 2. 2 M ⊙ or even more. The query about those super-massive NSs inspires the researchers to analyze their features and structures immensely. Here, we have inspected the behavior and properties of some of those super-massive NSs in f (T) modified gravity with T = T + α T 2 , where T is the torsional scalar and α is a regulatory parameter. In this framework of teleparallel formalism of modified gravity, we obtain the equations of motion by considering quintessence field, modified Chaplygin gas (MCG) and electromagnetic field. For our model, we use matching conditions under spherical symmetry, in order to find out the numerical values of different unknown constants of our model. This helps us to acquire various physical quantities thoroughly and to understand about the nature of those super-massive NSs deeply and quite clearly. Moreover, from our work, we can also explain the role of quintessence field and MCG in case of massive compact stars. The mass–radius relationship curve of this model can effectively describe the mass of the heaviest NS (about 2. 6 M ⊙ ) ever detected via gravitational wave detection. Again, we overall investigate the anisotropic behavior, density profile, pressure profile, core repulsive force, stability, equilibrium and energy conditions of those massive compact objects. We further analyze different important parameters like anisotropic stress, surface redshift, adiabatic behavior, compactness factor, sound speed, etc. in case of super-massive NSs for better realization and future study. [ABSTRACT FROM AUTHOR]

Motivated by the increasing interest in finding physically viable rotating sources, we present a new class of anisotropic rotating solutions. The energy–momentum tensor compatible with the metric is composed of anisotropic matter with a nonvanishing energy flow around the symmetry axis and vanishing viscosity. The new class of solutions can be used to find new possible sources for the Kerr metric, to obtain new regular black hole solutions and to study galaxies with a central rotating black hole and an halo of dark matter. As an example, we obtain a five-parameter class of solutions representing a two-way traversable wormhole smoothly matched to the Kerr one and satisfying all energy conditions outside the wormhole for a wide range of parameters, in particular for compact objects. Finally, with a simple modification of the aforementioned solution, we obtain a source for Kerr metric with a throat geometry, nonrepresenting a two-way traversable wormhole and satisfying all energy conditions. [ABSTRACT FROM AUTHOR]

We study the impact of nonlocal modifications of General Relativity on stellar structure. In particular, assuming an analytic distortion function and specific equations of state, we made use of remnant stars to put qualitative constraints on a parameter not directly restricted by solar system tests. Using current data sets available for white dwarfs and strange quark stars candidates, we find that the most stringent bounds come from the objects displaying the highest core densities, namely strange quark stars. Specifically, the constraints obtained from this class of stars are three to four orders of magnitude tighter than those obtained using white dwarfs. [ABSTRACT FROM AUTHOR]

There are a number of different theories which tend to explain the concept of universe's accelerated expansion. Among these theories, modified gravity is the most promising one. This paper elaborates matter distribution along with different characteristics of anisotropic compact stars in f (R , G) gravity background. Specifically, to profoundly understand physical behavior of the compact stars, we considered six compact stars, namely: Her X-1 ( CS 1 ), SAXJ1808.4-3658 ( CS 2 ), 4U1820-30 ( CS 3 ), PSR J 1614 2230 ( CS 4 ), VELA X-1 ( CS 5 ) and Cen X-3 ( CS 6 ) and calculated the corresponding quantities such as energy density (ρ), radial pressure ( p r ) and tangential pressure ( p t ) using three distinct models of f (R , G) gravity. For simplicity, f (R , G) gravity is divided into two parts as: f (R , G) = f 1 (R) + f 2 (G). The first part f 1 (R) is considered Hu–Sawicki like model, while the second part f 2 (G) is considered logarithmic like for Model 1 and power law like for Model 2 and Model 3. Predominantly, measurements of anisotropy, the energy conditions (ECs) and stability aspects of models for considered compacts stars are presented using graphical techniques. Furthermore, we also established that for the f (G) model parameter (n > 0), all the six stars show conventional behavior. [ABSTRACT FROM AUTHOR]

This work will proceed to examine the dynamics of collapsing charged string cloud in a spherically symmetric rainbow metric. This is the framework that allows new stars and compact objects to emerge. The dynamical variables (mass density, string tension) and their physical existence as energy conditions are computed and visually depicted. We also calculated the duration and radius of development of the collapsing string cloud's apparent horizons. The mass density, string tension, expected time and radius of apparent horizon creation are dependent on the Rainbow parameter η. At the center of the configuration R = 0 , the mass density, string tension and electric field intensity are at their maximum leading to the formation of a compact object (black hole). The fluid's mass density and string tension are reduced by the electric field. Furthermore, the presence of an electric field delays the formation of a compact object (black hole). [ABSTRACT FROM AUTHOR]

There are many ways to probe alternative theories of gravity, namely, via experimental tests at solar system scale, cosmological data and models, gravitational waves and compact objects. In this paper, we consider a model of gravity with torsion f (T) applied to compact objects such as neutron stars (NSs) for a couple of realistic equations of state (EOS). To do so, we follow our previous articles, in which we show how to model compact stars in f (T) gravity by obtaining its corresponding Tolman–Oppenheimer–Volkof equations. In this modeling of NS in f (T) gravity presented here, we calculate, among other things, the maximum mass allowed for a given realistic EOS, which would also allow us to evaluate which models are in accordance with the observations. The results already known to General Relativity must be reproduced to some extent and, eventually, we can find models that allow higher maximum masses for NSs than Relativity itself, which could explain, for example, the secondary component of the event GW190814, if this star is a massive NS. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate and analyze the behavior of charged compact stars in the modified f (R , ϕ , χ) theory of gravity. When solving the field equations, we assume that Krori– Barua space-time describes the geometry of the inner space [K. Krori and J. Barua, A singularity-free solution for a charged fluid sphere in general relativity, J. Phys. A Math. Gen.  8 (1975) 508]. We further analyzed various physical characteristics for our model, including energy density, pressure components, anisotropy parameter, mass function, compactness element, and redshift through their graphical representations. Additionally, we have checked the equilibrium position by taking Tolman–Oppenheimer–Volkoff equation and stability by using Abreu's criterion [H. Abreu, H. Hernandez, and L. Nunez, Sound speeds, cracking and the stability of self-gravitating anisotropic compact objects, Class. Quantum Grav.  24 (2007) 4631] and relativistic adiabatic index. For our study, we choose Hercules XI, Centaurus XIII, EXO 1785-248, LMC XIV stars for analysis. [ABSTRACT FROM AUTHOR]

In this paper, we have provided a new family of solutions to Einstein–Maxwell field equations of charged spherically symmetric anisotropic matter distribution, related to the description of strange stars, in the background of Rastall-Rainbow gravity. We have constructed the field equations along with cosmological constant (Λ) and quintessence dark energy described by the equation of state w q satisfying the range − 1 < w q < − 1 / 3. We have solved the field equations by the proper choice of some relations like radial pressure with energy density and tangential pressures with the energy density and also a useful form of energy density. Next, we have discussed about the smooth connection between the interior spherically symmetric spacetime and the exterior Reissner–Nordstrom-de Sitter spacetime. To demonstrate the physical viability of our considered model, we have discussed a detailed physical analysis of different parameters analytically and graphically. Our whole graphical analysis has been performed for three compact stars like VelaX-1, SAXJ1808.4-3658 and 4U1820-30 in the context of our solutions. With the help of various suitable conditions for stability, we have examined the stability of our model by checking the radial and tangential sound speeds and the adiabatic index values. [ABSTRACT FROM AUTHOR]

In recent years, a class of compact objects called gravastars have drawn immense interest as regular solutions to end state stellar collapse. Since the energy density involved in collapse process is expected to be high, it is a natural choice to study such compact objects in context of modified gravity theories which reduce to General Relativity (GR) in the low energy regime. We have already framed a model of gravastar in such a modified gravity framework involving extra dimensional Randall–Sundrum (RS) single brane gravity [R. Sengupta, S. Ghosh, S. Ray, B. Mishra and S. K. Tripathy, Phys. Rev. D 102, 024037 (2020)]. As a sequel in this paper, we substantially improve our previous model by choosing the Kuchowicz function as one of the metric potentials, which leads to many new interesting results and physical features from our analysis as discussed in this paper. Also, we provide essential additional stability checks on our gravastar model to investigate the possibility of any instability creeping in due to the higher-dimensional framework. Our present improved gravastar model is found to clear all the stability checks successfully. Very interestingly, the static spherically symmetric matter distributions are found to accommodate both classes of solutions obeying and violating the modified energy conditions on the RS brane as we find in this work. We can conclude from our analysis that the Kuchowicz metric potential is very effective for describing regular solutions to compact objects at substantially high energies on the three-brane. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to study the physical characteristics and stability of anisotropic compact stars in energy–momentum squared gravity. We use Tolman solution and consider specific models of this theory to study the viable features of SAX J1808.4-3658, 4U 1538-52 and Her X-1 stars. We explore the behavior of energy density, pressure components, anisotropic factor, energy conditions and equation of state parameter in the interior of considered stars. To check the stability of these stars, we examine the behavior of causality conditions, Herrera cracking approach and adiabatic index. It is found that all the required conditions are satisfied for the proposed models, thus the considered stars are viable as well as stable in this modified theory. [ABSTRACT FROM AUTHOR]

This paper provides a simplified description for simulating the coupling of dark energy with baryonic matter by considering a super-dense pulsar PSRJ1614-2230 as the model star. The starting equation of state for modeling the dark energy is motivated by the MIT Bag model for spherically confined hadrons and the observational evidences of its repulsive nature. Einstein field equations are solved in the stellar interior using the generalized framework of Tolman–Kuchowicz spacetime metric. The solutions are then analyzed for various physical parameters such as metric potential, pressure, density, energy conditions, etc. The physical analysis of multiple parameters indicates stable star formation. The proposed stellar model is free from all singularities and also meets the requisite stability criteria. The numerical results obtained for relativistic adiabatic index indicate that the model star is stiff and stable against radially induced adiabatic perturbations. [ABSTRACT FROM AUTHOR]

The commitment behind this work is to find the solutions of compact stellar models by using the spherically symmetric static space-time and anisotropic matter distribution in the framework of f (T ,) gravity. In this study, we used the new type of metric functions ν (r) = 2 log (c 1 + r 2) + log (c 2) and evaluated other functions λ (r) = log (1 + 1 6 c 2 F r 2) by the well-known Karmarkar condition. In this work, we use the rotated and spinning tetrad named as off-diagonal and most generic function of f (T ,) gravity. We find the strange stellar solutions by incorporating the MIT bag equation of state p r = 1 3 (ρ − 4 B g) to evaluate cosmological constant. Aiming at the simplicity of solutions we utilize the given data for masses and radii of compact objects PSR J1416-2230, 4 U 1608-52, Cen X-3, EXO 1785-248, SMC X-1. By graphical presentation of the core properties of compact models, we show by concluding that our solutions are graphically fit and physically admissible and interesting by having the compatibility with the compact stars study. [ABSTRACT FROM AUTHOR]

One of the possible potential candidates for describing the universe's rapid expansion is modified gravity. In the framework of the modified theory of gravity f(R, G), this work features the materialization of anisotropic matter such as compact stars. Specifically, to learn more about the physical behavior of compact stars, the radial, and tangential pressures as well as the energy density of six stars namely H e r X − 1 , SAXJ1808.4-3658, 4U1820-30, PSR J 1614 2230, VELA X-1, and Cen X-3 are calculated. Herein, the modified theory of gravity f(R, G) is disintegrated into two parts i.e. the tanh hyperbolic f(R) model and the three different f(G) models. The study focuses on graphical analysis of compact stars wherein the stability aspects, energy conditions, and anisotropic measurements are mainly addressed. Our calculation revealed that for the positive value of parameter n of the model f(G), all the six stars behave normally. [ABSTRACT FROM AUTHOR]

In this work, we have discussed the anisotropic compact stars in the framework of modified Gauss–Bonnet gravity, named by f(G) theory of gravity. We have found the equations of motion of stars with respect to spherically symmetric metric, describing interior geometry of the compact stellar object, anisotropic mode of the matter distribution in the presence of electromagnetic field and quintessence field. We have taken the metric coefficients in the form μ = Br α + Cr β and λ = Ar γ as well as a particular form of f (G) = G 2 , where the powers and coefficients of r are constants. The unknown constants are evaluated by matching between interior and exterior Reissner–Nordström–de Sitter metric. We have analyzed some physical aspects like energy density, radial and tangential pressures, anisotropic parameter of matter distribution, energy conditions, mass-radius relation, compactness factor, surface redshift, and stability of our resulting solutions through graphical behavior by comparing to the observational data of different types of compact stars like 4U1820-30, Vela X-1 and SAXJ1808.4-3658. We conclude that all physical attributes maintain their experimental ranges which deliver our anisotropic compact star models are viable and also stable upto certain region in f (G) gravity theory. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to examine the physical features and stability of anisotropic compact stellar objects in energy–momentum squared gravity. For this purpose, we apply Krori–Barua metric solutions and consider three different models of this modified theory to examine the physical characteristics of Her X-1, 4U 1538-52 and SAX J1808.4-3658 compact stars. We analyze the behavior of various physical quantities such as energy density, pressure components, energy conditions, equation of state parameter and anisotropic factor in the interior of these stars. We then use the estimated values of the sound speed and of the adiabatic index to explore the stability of these compact stars. It is found that all the required conditions are satisfied corresponding to all the selected models. We are therefore allowed to conclude that this modified theory provides viable and stable anisotropic compact stars. [ABSTRACT FROM AUTHOR]

It is found difficult to construct a complete general relativistic theory for an accretion disc around a relativistic compact star. There exists a trend to reproduce the effects of general relativistic attraction by using a pseudo-Newtonian approach. One such pseudo-Newtonian force for a spinning black hole sitting in quintessence background is constructed in this paper. Comparison with general relativity at different parametric values is picturised thoroughly. Next, an accretion model using this newly constructed pseudo-Newtonian force is built. The radial inward speed gradient is found to be expressed as a ratio of two functions. It can be realized easily that among these two functions, the denominator might vanish for a particular value of radial speed in the interval zero to speed of light. To make the flow physical, the numerator should vanish at the same radial distance. This will ultimately generate two speed profiles starting from the particular radial distance. This is the cause of generation for accretion and wind branches. It has been followed that the disc is truncated at a finite distance due to the fact that the specific angular momentum becomes larger than the Keplerian angular momentum beyond that. Effects of the spin of the central gravitating object are followed along with the intense effect of the quintessence parameters concerned. Accretion and wind density profiles are also studied to find the density variations near the central engine. Ratio of shear viscosity to entropy density is analyzed in the presence of quintessence for viscous accretion flows. [ABSTRACT FROM AUTHOR]

In this paper, we develop a new relativistic compact stellar model for a spherically symmetric anisotropic matter distribution. The model has been obtained through generating a new class of solutions by invoking the Tolman ansatz for one of the metric potentials g r r and a physically reasonable selective profile of radial pressure. We have matched our obtained interior solution to the Schwarzschild exterior spacetime over the bounding surface of the compact star. These matching conditions together with the condition of vanishing the radial pressure across the boundary of the star have been utilized to determine the model parameters. We have shown that the central pressure of the star depends on the parameter p 0 . We have estimated the range of p 0 by using the recent data of compact stars 4U 1608-52 and Vela X-1. The effect of p 0 on different physical parameters, e.g. pressure anisotropy, the subliminal velocity of sound, relativistic adiabatic index, etc., has also been discussed. The developed model of the compact star is elaborately discussed both analytically and graphically to justify that it satisfies all the criteria demanded by a realistic star. From our analysis, we have shown that the effect of anisotropy becomes small for higher values of p 0 . The mass–radius (MR) relationship which indicates the maximum mass admissible for observed pulsars for a given surface density has also been investigated in our model. Moreover, the variation of radius and mass with central density has been shown which allows us to estimate central density for a given radius (or mass) of a compact star. We have explored the tidal deformabilities by calculating the Love numbers and showed the variation of tidal Love numbers with the central pressure of a star. [ABSTRACT FROM AUTHOR]

In this paper, we formulate exact anisotropic interior solutions of the Einstein field equations preserving spherically symmetric geometry in the context of f (T ,) gravity. To make the system close, we consider a linear function in torsion scalar (T) and trace of energy–momentum tensor () , and employ the well-known Karmarkar condition with metric potential of the form e λ (r) = 1 + c r 2 (erf (1 + a r 2)) 2 . We consider standard data of some well-known compact star models (Vela X-1, PSR J1614-2230, Cen X-3 and EXO 1785-248) to estimate the unknown model parameters. We perform physical analysis of the developed model in detail, which includes density profile, pressure profile, anisotropy factor, mass function, gravitational/surface redshift, compactness factor, energy bounds, stability criteria, and the equilibrium condition. The analysis leads us to conclude that solutions are well-behaved and physically viable. [ABSTRACT FROM AUTHOR]

We present a model of compact astrophysical object under General Theory of Relativity using the anisotropic extension of Tolman IV solution. The anisotropy function, derived from the model, remains well behaved throughout the interior of the star. The model satisfies several necessary conditions for a physically realistic compact star. Physical viability of the model is verified specifically by plugging in the estimated parameter values of the Low Mass X-ray Binary (LMXB) candidate 4U 1608–52. Our stability analysis of this star, by using various criteria for stability, provides satisfactory results. In connection to anisotropy, we compute the Tidal Love Number (TLN) for the compact stellar model and compare the calculated values with existing literature. [ABSTRACT FROM AUTHOR]

General relativistic effects around massive astrophysical objects can be captured using a test gyro orbiting the object in a circular geodesic. This paper discusses how the tidal field due to a companion object affects the spin precession frequency and orbital angular velocity of a spinning gyro orbiting around a compact astrophysical object. The precession frequency is studied in a region of space around the central object using a perturbative approach. The central object is either a neutron star or a white dwarf in this study. The test gyro is any planetary or asteroid-like object orbiting a neutron star or a white dwarf. Moreover, the companion object that causes the tidal field can be a neutron star, white dwarf or a stellar black hole. It is seen that the tidal effect significantly affects the spacetime around the central object, which affects the gyro precession frequency and the orbital angular velocity. Slow rotation approximation has been considered for the central object, which creates negligible deformation. The change in the gyro's precession frequency and the orbital angular velocity due to the tidal field increases with an increase in the companion object's mass and decreases as the separation between the central star and the companion star increases. The tidal effect also varies with the stiffness of the equation of state of matter describing the host star. The lower the compactness of the host star, the greater is the tidal response; thus the greater is the change in the gyro's precession and angular velocity of the geodesic. [ABSTRACT FROM AUTHOR]

Inspired by various astrophysical phenomenons, it is suggested that pulsar-like compact stars are comprised entirely of strangeons (quark-clusters with three-light-flavor symmetry) and a small amount of electrons. In order to better constrain the properties of strangeon stars, we propose a linked bag model to describe the condensed matter by the strong interaction (i.e., strong condensed matter) in both 2-flavored (nucleons) and 3-flavored (hyperons, strangeons, etc.) scenarios. The model parameters are calibrated to reproduce the saturation properties of nuclear matter, which are later applied to hyperon matter and strangeon matter. Compared with baryon matter, the derived energy per baryon of strangeon matter is reduced if the strangeon carries a large number of valence quarks, which stiffens the equation of state and consequently increases the maximum mass of strangeon stars. In a large parameter space, the maximum mass and tidal deformability of strangeon stars predicted by the linked bag model are consistent with the current astrophysical constraints. It is found that the maximum mass of strangeon stars can be as large as ∼ 2. 5 M ⊙ , while the tidal deformability of a 1. 4 M ⊙ strangeon star lies in the range of 1 8 0 ≲ Λ 1. 4 ≲ 3 4 0. [ABSTRACT FROM AUTHOR]

In this study, we have explored an anisotropic stellar configuration by applying the Karmarkar condition in the framework of F (T ,) [where T and stand for torsion and trace of energy–momentum tensor (EMT)] gravity. We have developed field equations of F (T ,) gravity for a spherically symmetric line element and considered a viable F (T ,) model, i.e. F (T ,) = α T (r) 2 + β (r) (where α and β are arbitrary constants) for meaningful results. Following [S. K. Maurya, B. S. Ratanpal and M. Govender, Ann. Phys.382 (2017) 36], a spherically symmetric solution of embedding class I has been explored which resulted into a generalized compact stellar model. For the evaluation of unknown parameters, we use the matching conditions at the boundary by taking spherical space-time metric as interior and Schwarzschild space-time solution as exterior space-time metric. The viability of our proposed model is checked by exploring the nature of thermodynamical variables (ρ , p r , p t ), anisotropy, energy condition, Tolman–Oppenheimer–Volkov (TOV) forces, Abreu condition, equation of state parameters and adiabatic index taking the observational data of different compact stars like PSR J1416-2230, 4U 1608-52, Cen X-3, EXO 1785-248 and SMC X-1. Our stellar model is found. [ABSTRACT FROM AUTHOR]