Anisotropic Stars Research Articles

Spherically symmetric anisotropic charged neutron stars in f(Q, T) gravity

In this work, we have investigated a new solution for charged anisotropic compact stellar system in the framework of an extended theory of symmetric teleparallel gravity known as f(Q, T) gravity with the non-metricity term Q and the trace T for energy–momentum tensor. We have constructed a complete set of the gravitational field equations for a non-linear function f(Q,T)=αQ+β(1+Q2)+λT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(Q,T)=\\alpha Q + \\beta (1+Q^{2}) + \\lambda T$$\\end{document} with α,β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha , \\beta $$\\end{document} and λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda $$\\end{document} are dimensionless constant parameters in case of static spherically symmetric space-time. To evaluate the expression of relevant unknown constants interior space-time has been matched with exterior Reissner–Nordstro¨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ddot{\ extrm{o}}$$\\end{document}m metric. We have performed a graphical discussion in detail to test the behavior of physical parameters in the interior region of a stellar system like energy density, radial and tangential pressure, anisotropies of matter portion, electric field intensity etc. Also, to check the physical validity of our solutions, we have performed various tests viz., energy conditions, stability, mass–radius relation, surface redshift etc. The hydrostatic equilibrium position of our stellar system has been analysed through the TOV equation. Finally, we have determined that our stellar structure solutions satisfy all required physical conditions for viability and acceptability in the context of some pulsar like neutron stars. So our model can be used to characterise the neutron stars in f(Q, T) modified gravity.

Anisotropy Induced by Electric Charge: A Computational Analytical Approach

This paper presents a novel class of interior solutions for anisotropic stars under the imposition of a self-similar symmetry. This means proposing exact solutions to the Einstein field equations to describe charged matter distribution with radiation flow. The Einstein–Maxwell system by employing specific choices of mass function is formulated to describe the gravitational collapse of charged, anisotropic, spherically symmetric distributions using the Schwarzschild metric. Two ordinary differential equations governing the dynamics are derived by matching a straightforward solution of the symmetry equations to the charged exterior (Reissner–Nordström–Vaidya). Models with satisfactory physical behavior are constructed by extensively exploring self-similar solutions for a set of parameters and initial conditions. Finally, the paper presents the evolution of physical variables and the collapsing radius, demonstrating the inevitable collapse of the matter distribution.

Investigating the equation-of-state, stability and mass–radius relationship of anisotropic and massive neutron stars embedded in f(R,T) modified gravity

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 [Formula: see text], where [Formula: see text] is the Ricci scalar and [Formula: see text], the trace of energy–momentum tensor with [Formula: see text] 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 [Formula: see text]–[Formula: see text] curve can predict the maximum achievable mass is about [Formula: see text] with lowest possible radius of around [Formula: see text] 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 [Formula: see text] modified gravity.

Finslerian extension of an anisotropic strange star in the domain of modified gravity

In this article, we apply the Finsler spacetime to develop the Einstein field equations in the extension of modified geometry. Following Finsler geometry, which is focused on the tangent bundle with a scalar function, a scalar equation should be the field equation that defines this structure. This spacetime maintains the required causality properties on the generalized Lorentzian metric manifold. The matter field is coupled with the Finsler geometry to produce the complete action. The developed Einstein field equations are employed on the strange stellar system to improve the study. The interior of the system is composed of a strange quark matter, maintained by the MIT bag equation of state. In addition, the modified Tolman–Oppenheimer–Volkov (TOV) equation is formulated. In particular, the anisotropic stress attains the maximum at the surface. The mass-central density variation confirms the stability of the system.

Non-radial oscillations in anisotropic dark energy stars

We study the non-radial f-mode oscillations of both isotropic and anisotropic dark energy stars by using the modified Chaplygin prescription of dark energy to model the stellar matter. The anisotropic pressure in the system is modeled with Bowers–Liang prescription. By solving the stellar structure equations in presence of anisotropy, we study the global properties of the dark energy star and compare the mass-radius profiles with data from GW events and milli-second pulsars. The stellar radial and anisotropic pressure profiles have also been investigated. We proceed to determine the prominent non-radial l=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$l=2$$\\end{document}f-mode frequencies of the anisotropic dark energy star by employing the Cowling approximation and analyse and quantify the spectra by varying the anisotropic parameter. We report that f-mode spectra of dark energy star have distinctly different behaviour compared to neutron star and quark star, and this may possibly help in its future identification. Further, the tidal deformability factors of the anisotropic dark energy stars have also been analyzed.

Compactness bound of Buchdahl–Vaidya–Tikekar anisotropic star in $$D\ge 4$$ dimensional spacetime

Compactness bound of Buchdahl–Vaidya–Tikekar anisotropic star in $$D\ge 4$$ dimensional spacetime

The impact of anisotropy on neutron star properties: insights from 𝖨–𝖿–𝖢 universal relations

Anisotropy in pressure within a star emerges from exotic internal processes. In this study, we incorporate pressure anisotropy using the Quasi-Local model. Macroscopic properties, including mass (M), radius (R), compactness (C), dimensionless tidal deformability (Λ), the moment of inertia (I), and oscillation frequency (f), are explored for the anisotropic neutron star. Magnitudes of these properties are notably influenced by anisotropy degree. Universal I–f–C relations for anisotropic stars are explored in this study. The analysis encompasses various EOS types, spanning from relativistic to non-relativistic regimes. Results show the relation becomes robust for positive anisotropy, weakening with negative anisotropy. The distribution of f-mode across M–R parameter space as obtained with the help of C–f relation was analyzed for different anisotropic cases. Using tidal deformability data from GW170817 and GW190814 events, a theoretical limit for canonical f-mode frequency is established for isotropic and anisotropic neutron stars. For isotropic case, canonical f-mode frequency for GW170817 event is f 1.4 = 2.606+0.457 -0.484kHz; for GW190814 event, it is f 1.4 = 2.097+0.124 -0.149kHz. These relationships can serve as reliable tools for constraining nuclear matter EOS when relevant observables are measured.

New class of anisotropic charged strange quark star in Durgapal $IV$ metric and its maximum mass

New class of anisotropic charged strange quark star in Durgapal $IV$ metric and its maximum mass

Anisotropic strange stars in extended [formula omitted] gravity with electromagnetic field

In the present article, we have studied the modified field equations for static, spherically symmetric spacetime in a more extended f(T,B,T) gravity in the framework of teleparallel geometries. In order to apply this gravity, we have explored anisotropic strange star configurations with the electromagnetic field. For this purpose, we have chosen off-diagonal tetrad fields to represent gravitational interaction. Also, a linear relationship between radial pressure and energy density through Bag constant C, defined as MIT Bag equation of state, has been employed to describe the strange quark matter distribution. A lot of tests have been done in the context of our solutions to confirm the stability and physical acceptability of our obtained stellar structure. In this connection, we have demonstrated a lot of graphs for two selected known compact stars VelaX−1 and SAXJ1808.4−3658, with respect to our calculations. Again, the significant role of T in our stellar configuration has been displayed by plotting all graphs for five selected values of β, the coefficient parameter of T in the functional form f(T,B,T). Finally, we have observed that stability criteria and physically well-behaved nature for thermodynamical parameters holds good to represent a strange star configuration in this model, corresponding to our considered constant parameters explicit expressions. Hence this proposed model becomes viable and acceptable to study strange star configuration in f(T,B,T) gravity.

Conformal motion for higher-dimensional compact objects

In this work, we present a new framework for five-dimensional spherical symmetry anisotropic stars that admits conformal motion. The behaviour of model characteristic pressure, stress, density profile and surface tension is investigated with the inclusion of a particular density profile for the higher dimensional Einstein’s field equations. All the physical parameters are well-behaved for the presented solution in higher dimensions. The analysis predict the possible existence of compact stars in five dimensions, more likely strange quark star.

Impact of spacetime curvature on the physical behaviour of Vaidya and Tikekar (VT) type anisotropic compact objects

The Vaidya and Tikekar (1982) ansatz has so far provided numerous stellar solutions which are useful for the description of compact stars like neutron stars and quark stars. We re-examine the impacts of the spacetime curvature parameter of the Vaidya–Tikekar ansatz on the physical properties of the relativistic anisotropic star. For a spherically symmetric, static and anisotropic stellar configuration, in the Vaidya–Tikekar background spacetime, we develop new exact solutions by assuming a polytropic type equation of state pr=kρ1+1n−β. Recent data from observed pulsars are taken as input parameters to validate the developed model. We examine the impact of the spheroidal curvature parameter of the VT spacetime on the stellar behaviour and mass–radius (M–R) relationship. It is observed that with the increase in the departure from spherical geometry, values of physical quantities, such as density and pressure, increase.

Stability analysis of anisotropic stars in f(R, T) gravity through cracking technique

In this article, cracking technique is developed for spherically symmetric compact sources in the framework of f(R, T) gravity, where R denotes Ricci scalar and T stands for trace of energy momentum tensor. The characteristics of a star with anisotropic pressure stresses are investigated by utilizing the Tolman–Kuchowicz spacetime solutions. Modified field equations are developed for a particular model i.e., f(R,T)=R+2γT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(R,T)=R+2\\gamma T$$\\end{document}, where γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document} is constant, that are further used to develop expressions for matter density, radial and tangential pressures. A generalized form of the Tolman Oppenheimer Volkoff (TOV) equation is developed for the modified field equations. The consequence of the local density perturbation scheme, as presented by Biswas et al. (Eur Phys J C 80:175, 2020) is considered. The mathematical framework for cracking has been tested on five realistic stars namely, Vela X-1, Cen X-3, SMC X-1, PSR J1614-2230 and PSR J1903+327. The graph of forces distribution of these stars have been observed to check the stability regions. The results of cracking/overturning for various values of the parameters involved in this model are observed by checking the instability regions in the form intervals.

NEW SPHERICALLY SYMMETRIC ANISOTROPIC NEUTRON STAR MODEL USING FIELD EQUATIONS

In this paper, we obtain a new static spherically symmetric anisotropic fluid model of a neutron star in curvature coordinates. We consider , where , by taking the value of parameter in Durgapal solutions (Durgapal, 1982). The energy density, radial pressure, tangential pressure, and redshift are positive finite, and decreasing with respect to the increasing radius in this model. The pressure and density also decrease from the inner core to the crust of the object. We obtain this model by the specific choice of the constants and the anisotropy factor . The central value of the anisotropy factor is zero due to the perfect fluid nature at the center of the anisotropic model and it is increasing with the increasing radius. In our analysis we take and The model well behaves for the anisotropic neutron stars, which is shown analytically and graphically. The solution is free from any central singularity. We take realistic objects such as EXO 1785-248, SMC X-1, Her X-1, 4U 1538-52, LMC X-4, RX J1856-37, Cen X-3, PSR J1903-327, and Vela X-1 to represent our solutions graphically and numerically. We obtain this solution for for the first time. Therefore, the solution is quite new and derived for the first time.

Anisotropic strange quintessence stars in modified f(R,ϕ) theory of gravity

This paper investigated the anisotropic quintessence stars in the [Formula: see text] theory of gravity, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the scalar potential. We investigated the field equations of the [Formula: see text] 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.

Anisotropic strange stars and its maximum mass in Finch-Skea geometry in dimensions D ≥ 4

In this article, we have demonstrated a stellar model for compact stars in the presence of strange matter embedded in D ≥ 4 dimensional space-time defined by the Finch-Skea metric. To study the relevant physical properties of the interior matter, we consider the equation of state (henceforth EOS) as proposed in the MIT bag model, where B is termed the bag constant. The mass-radius relationship in four and higher dimensions is determined using the range of values of surface density through the relation ρ s = 4B, for which bulk strange matter may be a viable issue for compact objects. Here, we choose the range of B so that stable strange matter may exist relative to a neutron at zero external pressure. We note that a maximum value of the stellar radius exists when B is fixed at a given allowed value for which metric functions considered to be real here. This is the maximum allowed radius in this model, which depends on the surface density of a strange star. In four dimensions, the compactness of a star is found to be greater than 0.33. In the case of higher dimensions (D > 4), we observed different values of compactness. Causality conditions are satisfied interior to the star up to the maximum allowed radius , for which the metric function is real. The validity of the energy conditions, surface redshift and other parameters of the stellar configuration are studied, and new results are found. The stability of the model is also studied.

Relativistic model of anisotropic star with Bose–Einstein density depiction

In this article, we present a new model for anisotropic compact stars confined to physical dark matter (DM) based on the Bose–Einstein DM density profile and a bag model type equation of state (EoS). The obtained solutions are physically well-behaved and represent the physical and stable matter configuration by satisfying the energy conditions, causality conditions, and essential conditions on the stability factor and adiabatic index. The solutions supporting the matter sphere are in an equilibrium state by satisfying the generalized TOV equation. We also find the surface redshift, compactness parameter at the surface, maximum mass, and interestingly, all these values are under the desired range that makes our solution more physically viable. Here, the radially symmetric profiles of energy density, radial and transverse pressures are demonstrated.

Minimally deformed anisotropic stars in dark matter halos under EGB-action

In this paper, we introduce an anisotropic model using a dark matter (DM) density profile in Einstein–Gauss–Bonnet (EGB) gravity using a gravitational decoupling method introduced by Ovalle (Phys Rev D 95:104019, 2017), which has provided an innovative approach for obtaining solutions to the EGB field equations for the spherically symmetric structure of stellar bodies. The Tolman and Finch–Skea (TFS) solutions of two metric potentials, g_{tt} and g_{rr}, have been used to construct the seed solution. Additionally, the presence of DM in DM halos distorts spacetime, causing perturbations in the g_{rr} metric potential, where the quantity of DM is determined by the decoupling parameter beta . The physical validity of the solution, along with stability and equilibrium analysis, has also been performed. Along with stability and equilibrium analysis, the solution’s physical validity has also been examined. Additionally, we have shown how both constants affect the physical characteristics of the solution. Using a M{-}R diagram, it has been described how the DM component and the GB constant affect the maximum permissible masses and their corresponding radii for various compact objects. Our model predicts the masses beyond the 2~M_{odot } and maximum radii 11.92^{+0.02}_{-0.01} and 12.83^{+0.01}_{-0.02} for larger value of alpha under density order 10^{15}~text {g}/text {cm}^3 and 10^{14}~text {g}/text {cm}^3, respectively, while the radii become 11.96^{+0.01}_{-0.01} and 12.81^{+0.01}_{-0.02} for larger value of beta .

Physically viable strange quark star models in modified teleparallel gravity

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

Charged anisotropic tolman IV solution in matter-geometry coupled theory

This paper discusses the interior distribution of several anisotropic star models coupled with an electromagnetic field in the context of gravity, where . In this regard, a standard model of this modified gravity is taken as , where ν 3 symbolizes an arbitrary coupling constant. We assume a charged spherically symmetric metric that represents the interior geometry of compact quark stars and develop the corresponding modified field equations. These equations are then solved with the help of metric potentials of Tolman IV spacetime and a linear bag model equation of state. We consider the experimental data (i.e. radii and masses) of different quark models such as SMC X-4, SAX J 1808.4-3658, Her X-I and 4U 1820-30 to analyze how the charge and modified corrections affect their physical characteristics. The viability and stability of the resulting model is also checked for the considered star candidates with two different values of ν 3. We conclude that only two models, Her X-I and 4U 1820-30 show stable behavior in this modified framework for both values of the coupling constant.

Core-envelope anisotropic star model admitting Karmarkar condition

We generate a neutral core-envelope stellar model with anisotropic fluid distribution which admits Karmarkar condition. The core and envelope layers are assumed to compose quark matter and neutron fluid consistent with linear and quadratic equations of state, respectively. The results obtained indicate that the present model conforms with physical and stability tests. In the present model we have generated radii and masses of astrophysical objects consistent with observations such as HerX-1, 4U 1538-52 and SAX J1808.4-3658. It is interesting to note that the study of multilayered stars in Karmarkar condition is missing in the stellar models generated in the past.