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This study uses the Krori-Barua type metric to represent hybrid stars within the $f(Q)$ theory of gravity. We postulate that the hybrid star also contains strange quark matter in addition to regular baryonic matter. To investigate the physical viability of the hybrid star model, we present the graphical behavior of the energy density, radial pressure, and tangential pressure, equation of state parameters, anisotropy, and stability analysis, respectively, by choosing the compact star EXO 1785-248 with a mass of $1.3_{-0.2}^{+0.2}~M_{\odot}$ and radius $8.849_{-0.4}^{+0.4}$ km with five different values of the coupling constants as $a=2$, $a=4$, $a=6$, $a=8$, and $a=10$. The maximum allowed masses and corresponding radii have been calculated using the $M-R$ curve for three different coupling parameter '$a$' values which match the observational data of three distinct compact stars, namely LMC X-4, SMC X-1, and 4U 1538-52. So, the $f(Q)$ theory of gravity can provide results that are plausible for describing the macroscopical characteristics of hybrid star candidates., Comment: Physics of the Dark Universe published version

In our investigation, we pioneer the development of geometrically deformed strange stars within the framework of teleparallel gravity theory through gravitational decoupling via the complete geometric deformation (CGD) technique. The significant finding is the precise solution for deformed strange star (SS) models achieved through the vanishing complexity factor scenario. Further, we introduce the concept of space-time deformation caused by dark matter (DM) content in DM haloes, leading to perturbations in the metric potentials $g_{tt}$ and $g_{rr}$ components. Mathematically, this DM-induced deformation is achieved through the CGD method, where the decoupling parameter $\alpha$ governs the extent of DM influence. To validate our findings, we compare our model predictions with observational constraints, including GW190814 (with a mass range of $2.5-2.67 M_{\odot}$) and neutron stars (NSTRs) such as EXO 1785-248 [mass=$1.3_{-0.2}^{+0.2}~M_{\odot}$], 4U 1608-52 [mass=$1.74_{-0.14}^{+0.14}~M_{\odot}$], and PSR J0952-0607 [mass=$2.35_{-0.17}^{+0.17}~M_{\odot}$]. Our investigation delves into the stability of the model by considering causality conditions, Herrera's Cracking Method, the adiabatic index, and the Harrison-Zeldovich-Novikov criterion. We demonstrate that the developed model mimics a wide range of recently observed pulsars. To emphasize its compatibility, we highlight the predicted mass and radius in tabular form by varying both the parameters $\alpha$ and $\zeta_1$. Notably, our findings are consistent with the observation of gravitational waves from the first binary merger event. Furthermore, we compare our results with those obtained for a slow-rotating configuration. In addition to this, we discuss the moment of inertia using the Bejger-Haensel approach in this formulation., Comment: EPJC published version

In the conventional method of studying wormhole (WH) geometry, traversability requires the presence of exotic matter, which also provides negative gravity effects to keep the wormhole throat open. In dRGT massive gravity theory, we produce two types of WH solutions in our present paper. Selecting a static and spherically symmetric metric for the background geometry, we obtain the field equations for exact WH solutions. We derive the WH geometry completely for the two different choices of redshift functions. All the energy conditions including the NEC are violated by the obtained WH solutions. Various plots are used to illustrate the behavior of the wormhole for a suitable range of $m^2c_1$, where $m$ is the graviton mass. It is observed that the photon deflection angle becomes negative for all values of $m^2c_1$ as a result of the repulsive action of gravity. It is also studied that the repulsive impact of massive gravitons pushes the spacetime geometry so strongly that the asymptotic flatness is affected. The Volume Integral Quantifier (VIQ) has also been computed to determine the amounts of matter that violate the null energy condition. The complexity factor of the proposed model is also discussed.

The wormhole solution in dRGT massive gravity is examined in this paper in the background of non-commutative geometry. In order to derive the wormhole model, along with the zero tidal force, we assume that the matter distribution is given by the Gaussian and Lorentzian distributions. The shape function in both models involves the massive gravity parameters m2c1 and m2c2. But the spacetime loses its asymptotic flatness due to the action of the massive gravity parameter. It is noticed that the asymptotic flatness is affected by the repulsive effect induced in the massive gravitons that push the spacetime geometry very strongly. We observed that each model violates the null energy criteria, indicating the presence of exotic matter which is necessary to sustain the wormholes. The exotic matter is measured using the volume integral quantifier. Moreover, it is discovered that the model is stable under the hydrostatic equilibrium condition by utilizing the TOV equation. Finally, our research encompassed an exploration of the repulsive influence exerted by gravity. Our findings demonstrated that the presence of repulsive gravity results in a negative deflection angle for photons following null geodesics. Remarkably, we consistently observed negative values for the deflection angle across all values of r0 in the two scenarios examined. This consistent negativity unequivocally signifies the manifestation of the repulsive gravity effect., Comment: 16 pages, Accepted in Nuclear Physics B

We examine the behavior of non-commutative Schwarzschild black holes in the context of massive gravity. According to the investigation, corresponding to a minimal mass, the black hole can have two horizons, one horizon, or no horizon at all. Our results imply the existence of a stable black hole remnant, whose mass can be uniquely calculated in terms of the non-commutative parameter $\theta$ and gravity mass $m$. Thermodynamic features such as heat capacity and Hawking temperature are studied. We also examine a scalar linear perturbation on the black hole. Quasinormal frequencies are computed via Wentzel-Kramers-Brillouin(WKB) method with Pade improvement. All quasinormal frequencies considered in this work have a negative imaginary part. In the eikonal limit, we investigate the angular velocity and the Lyapunov exponent as a function of $M/\sqrt{\theta}$. Additionally, we explore the black hole's shadow across various model parameters. Our findings indicate that non-commutativity leads to a reduction in the black hole's shadow, with this effect exhibiting a nonlinear relationship. Furthermore, we observe that the inclusion of a massive graviton in the theory results in an increase in the black hole's shadow radius, particularly at greater observer distances., Comment: 19 pages, 17 figures, 1 table

The physical properties of a (3+1)-D gravastar in the context of massive gravity are discussed in this work. In present investigation, the field equations have been solved for a static, uncharged sphere in order to achieve the gravastar model as proposed by Mazur and Mottola [Mazur and Mottola in Report No. LA-UR-01-5067,(2001); Mazur and Mottola, {\em Proc Natl Acad Sci} USA 101:9545, (2004)]. We address length of thin shell, energy, and entropy for the thin shell containing an ultra-relativistic stiff fluid. Israel matching criteria are used to ensure that the inner and outside geometries join smoothly. It turns out that the behavior of the gravastar is entirely altered by the existence of the graviton mass. Particularly, when $m\rightarrow0$, our findings precisely matched the outcomes of general relativity., Comment: Submitted to journal

In this work, a new static, non-singular, spherically symmetric fluid model has been obtained in the background of $f(R,\,T)$ gravity. Here we consider the isotropic metric potentials of Durgapal-IV [M.C. Durgapal, J. Phys. A {\bf 15} 2637 (1982)] solution as input to handle the Einstein field equations in $f(R,\,T)$ environment. For different coupling parameter values of $\chi$, graphical representations of the physical parameters have been demonstrated to describe the analytical results more clearly. It should be highlighted that the results of General Relativity (GR) are given by $\chi=0$. With the use of both analytical discussion and graphical illustrations, a thorough comparison of our results with the GR outcomes is also covered. The numerical values of the various physical attributes have been given for various coupling parameter $\chi$ values in order to discuss the impact of this parameter. Here we apply our solution by considering the compact star candidate LMC X-4 [M.L. Rawls et al., Astrophys. J. {\bf 730} 25 (2011)] with mass$=(1.04 \pm 0.09)M_{\odot}$ and radius $= 8.301_{-0.2}^{+0.2}$ km. respectively, to analyze both analytically and graphically. To confirm the physical acceptance of our model, we discuss certain physical properties of our obtained solution such as energy conditions, causality, hydrostatic equilibrium through a modified Tolman-Oppenheimer-Volkoff (TOV) conservation equation, pressure-density ratio, etc. Also, our solution is well-behaved and free from any singularity at the center. From our present study, it is observed that all of our obtained results fall within the physically admissible regime, indicating the viability of our model., Comment: 17 pages, 9 figures, 2 tables. arXiv admin note: text overlap with arXiv:2212.07810

In this study, we explore several new characteristics of a static anisotropic hybrid star with strange quark matter (SQM) and ordinary baryonic matter (OBM) distribution. Here, we use the MIT bag model equation of state to connect the density and pressure of SQM inside stars, whereas the linear equation of state $p_r =\alpha \rho-\beta$ connects the radial pressure and matter density caused by baryonic matter. The stellar model was developed under a background of $f(Q)$ gravity using the quadratic form of $f(Q)$. We utilized the Tolman-Kuchowicz ansatz to find the solutions to the field equations under modified gravity. We have matched the interior solution to the external Schwarzschild spacetime in order to acquire the numerical values of the model parameters. We have selected the star Her X-1 to develop various profiles of the model parameters. Several significant physical characteristics have been examined analytically and graphically, including matter densities, tangential and radial pressures, energy conditions, anisotropy factor, redshirt, compactness, etc. The main finding is that there is no core singularity present in the formations of the star under investigation. The nature of mass and the bag constant $B_g$ have been studied in details through equi-mass and equi-$B_g$ contour. The maximum allowable mass and the corresponding radius have been obtained via $M-R$ plots., Comment: EPJ C published version

Employing $f(T)$ gravity, where $T$ is the torson, we have developed a new model of an anisotropic compact star in this work. Tolman-Kuchowicz (TK) metric potential has been used to solve the set of field equations. Furthermore, the matching conditions for interior and exterior geometry have been discussed. We have considered observation data of the compact star LMC X-4 and analyzed thermodynamical properties (density, pressure, equation of state parameter, square speed of sound, and equilibrium condition) analytically and graphically to test the validity of the solution. The compact star is found to meet the energy conditions. Through the causality condition and Herrera's cracking concept, the stability analysis of the present model has been presented and it confirms the physical acceptability of the solution. It has been shown that the obtained interior solutions for compact stars are consistent with all necessary physical criterions and therefore relevant as well as physically acceptable., Comment: 14 pages, Accepted for publication in Chinese Journal of Physics

The aim of the research is to look into a new solution for isotropic compact stars in the context of the $f(R,\,T)$ theory of gravity. We used the Buchdahl [H.A. Buchdahl, Phys. Rev. {\bf 116} (1959) 1027] metric potentials as input to deal with the field equations in the $f(R,\,T)$ framework. For different values of the coupling parameter $\chi$, graphical representation of the model parameters have been shown to canvass the analytical results more clearly. Interestingly, we have proven that for $\chi=0$, the standard General Relativity (GR) results can be recovered. A comparison of our obtained solutions with the GR results is also discussed. To study the effect of the coupling parameter $\chi$, the numerical values of the different physical variables have been tabulated for the values of the coupling parameter $\chi=0,\,0.25,\,0.5,\,0.75,1,\,1.25$. We used the compact stars candidate LMC X-4 with mass$=(1.04 \pm 0.09)M_{\odot}$; Radius $= 8.301_{-0.2}^{+0.2}$ km. respectively, for graphical analysis. To determine the physical acceptability of the model, we looked into the necessary physical properties such as energy conditions, causality, hydrostatic equilibrium, and pressure-density ratio etc. and found that our system satisfies all of these criteria, indicating that the model is physically reasonable., Comment: 15 Pages, 9 Figures ; Accepted for publication in New Astronomy on 14.12.2022 (Manuscript Number: NEWAST-D-22-00280R1). arXiv admin note: text overlap with arXiv:2112.07581

In the present work we have investigated a new anisotropic solution for polytropic star in the framework of $4D$ Einstein-Gauss-Bonnet (EGB) gravity. The possibility of determining the masses and radii of compact stars which puts some limitations on equation of state (EoS) above the nuclear saturation density. For this purpose, the $4D$ EGB field equations are solved by taking a generalized polytropic equation of state (EoS) with Finch-Skea ansatz. The generalized solution for anisotropic model has been tested for different values of Gauss-Bonnet constant $\alpha$ which satisfies all the physical criteria including causality with static stability via mass vs central mass density ($M-\rho_c$), Bondi and Abreu criterion. The adiabatic index shows a minor influence of the GB coupling constant whereas the central and surface redshifts in the EGB gravity always remain lower than the GR. We present the possibility of fitting the mass and radius for some known compact star via $M-R$ curve which satisfies the recent gravitational wave observations from GW 170817 event., Comment: 11 Pages, 10 Figures, 1 Table (to be appeared in EPJC)

Dark energy is the component in the present Universe with the greatest abundance, and it is responsible for the accelerating expansion of the Universe. As a result, dark energy is likely to interact with any compact astrophysical object [Muhammad F.A.R. Sakti and Anto Sulaksono, {\it Phys. Rev. D} {\bf 103}, 084042 (2021)]. In present paper, we propose a model for a dark energy star made up of dark and ordinary matter in which the density of dark energy is proportional to the density of isotropic perfect fluid matter. In the context of general relativity, the model is derived in the curved Tolman-Kuchowicz spacetime geometry [Tolman, Phys Rev 55:364, (1939); Kuchowicz, Acta Phys Pol 33:541, (1968)]. Here, we look at how dark energy affects stellar mass, compactness, and equilibrium etc. The physical parameters of the model e.g., pressure, density, mass function, surface redshift etc. are investigated, and the stability of stellar configuration is studied in detail. The model has interesting properties because it meets all energy criteria and is free from central singularities. The maximum allowable mass has been obtained from our model with the help of $M-R$ diagram. We analyse many physical properties of the model and checked that it meets all regularity constraints, is stable, and therefore physically realistic., Comment: 15 Pages, 10 Figures

The purpose of our present 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 $p_q = \frac{1}{3}(\rho_q - 4B_g)$ 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 $p_r = \alpha \rho - \beta$. In order to obtain the solution of the Einstein field equations, we have used the Tolman-Kuchowicz {\em ansatz} [Tolman, Phys Rev 55:364, 1939; Kuchowicz, Acta Phys Pol 33:541, 1968] 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 $X-1$ [Mass=$(0.98 \pm 0.12)M_{\odot}$; Radius= $8.1_{-0.41}^{+0.41}$ 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 etc. 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)., Comment: 29 pages, 11 figures, 4 tables ; Accepted in the International Journal of Geometric Methods in Modern Physics (IJGMMP) on 22.02.2022

In this article, we studied the behavior of relativistic spherical objects considering Tolman IV spacetime in modified $f(R,\,T)$ gravity for the uncharged perfect fluid matter. We have chosen the matter Lagrangian as $\mathcal{L}_m=-p$ to develop our present model. In particular, for this investigation we have reported for the compact object LMC $X-4$ [Mass=$(1.04 \pm 0.09)M_{\odot}$; Radius= $8.301_{-0.2}^{+0.2}$ Km] in our paper. The effect of the coupling parameter $\beta$ on the local matter distribution of compact stars has been investigated in this paper. It can be seen that with greater values of $\beta$, the sound speed and adiabatic index are higher. On contrary, the mass function takes lower value for higher values of $\beta$. Our obtained solution does not admit singularities in the matter density, pressure and metric functions. According to our graphical analysis, this new stellar model satisfies all physical requirements anticipated in a realistic star., Comment: 13 pages, 8 figures; Accepted in Chinese Journal of Physics on 19 November 2021; Available online 5 December 2021

In this article, we have discussed Morris and Thorne (MT) wormhole solutions in a modified theory of gravity that admits conformal motion. Here we explore the wormhole solutions in $f(R,\,T)$ gravity, which is a function of the Ricci scalar ($R$) and the trace of the stress-energy tensor ($T$). To study wormhole geometries, we make assumption of spherical symmetric static spacetime and the existence of conformal Killing symmetry to get more acceptable astrophysical outcomes. To do this, we choose the expression of $f(R,\,T)$ as $f(R,T)= R+2 \gamma T$. Here we employ the phantom energy EoS relating to radial pressure and density given by $p_r=\omega \rho$ with $\omega<-1$ to constrain our model. Following a discussion of wormhole geometry and behavior of shape function, the study moves on to the computation of proper radial distance, active mass function, the nature of total gravitational energy and a discussion on the violation of energy conditions. We have shown that the wormhole solutions exist for positive as well as negative values of the coupling constant $\gamma$. From our analysis we see that no wormhole solution exists for $\gamma =-4\pi,\,-\pi(3+\omega)$. All the physical parameters have been drawn by employing the values of $\gamma$ as $\gamma=-0.3,\,-0.2,\,-0.1,\,0,\,0.1$ and $0.2$, where $\gamma=0$ corresponds to general relativity (GR) case. It is found that for our proposed model, a realistic wormhole solutions satisfying all the properties can be obtained., Comment: Accepted in International Journal of Modern Physics D on 23 November 2021; 15 Pages, 8 Figures

In this work, we discuss the configuration of a gravastar (gravitational vacuum stars) in the context of $f(R, \,T )$ gravity by employing the Mazur-Mottola conjecture [P. Mazur and E. Mottola, Report No. LA-UR-01-5067; P. Mazur and E. Mottola, Proc. Natl. Acad. Sci. USA $101$, $9545$ ($2004$)]. Gravastar is conceptually a substitute for a black hole theory as available in literature and it has three regions with different equation of states. By assuming that the gravastar geometry admits conformal killing vector, the Einstein-Maxwell field equations have been solved in different regions of gravastar by taking a specific equation of state as proposed by Mazur and Mottola. We match our interior spacetime to the exterior spherical region which is completely vacuum and described by Reissner-Nordstr\"{o}m geometry. For a particular choice of $f(R,\,T)$ as $f(R, \,T )=R+2\gamma T$, here we analyze various physical properties of the thin shell and also presented our results graphically for these properties. The stability analysis of our present model is also studied by introducing a new parameter $\eta$ and we explored the stability regions. Our proposed gravastar model in presence of charge might be treated as a successful stable alternative of the charged black hole in the context of this gravity., Comment: 23 Pages, 10 Figures, Accepted for publication in European Physical Journal C on 12.08.2021

In this present work, we have obtained a singularity-free spherically symmetric stellar model with anisotropic pressure in the background of Einstein's general theory of relativity. The Einstein's field equations have been solved by exploiting Tolman {\em ansatz} [Richard C Tolman, Phys. Rev. 55:364, 1939] in $(3+1)$-dimensional space-time. Using observed values of mass and radius of the compact star PSR J1903+327, we have calculated the numerical values of all the constants from the boundary conditions. All the physical characteristics of the proposed model have been discussed both analytically and graphically. The new exact solution satisfies all the physical criteria for a realistic compact star. The matter variables are regular and well behaved throughout the stellar structure. Constraints on model parameters have been obtained. All the energy conditions are verified with the help of graphical representation. The stability condition of the present model has been described through different testings., Comment: 18 Pages, 8 Figures, Accepted in European Physical Journal C on 12.06.2021

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., Comment: arXiv admin note: text overlap with arXiv:2105.12572

In present paper a well-behaved new model of anisotropic compact star in (3+1)-dimensional spacetime has been investigated in the background of Einstein's general theory of relativity. The model has been developed by choosing $g_rr$ component as Krori-Barua (KB) ansatz [Krori and Barua in J. Phys. A, Math. Gen. 8:508, 1975]. The field equations have been solved by a proper choice of the anisotropy factor which is physically reasonable and well behaved inside the stellar interior. Interior spacetime has been matched smoothly to the exterior Schwarzschild vacuum solution and it has also been depicted graphically. Model is free from all types of singularities and is in static equilibrium under different forces acting on the system. The stability of the model has been tested with the help of various conditions available in literature. The solution is compatible with observed masses and radii of a few compact stars., Comment: Accepted for publication in International Journal of Geometric Methods in Modern Physics

A new model of anisotropic compact star is obtained in our present paper by assuming the pressure anisotropy. The proposed model is singularity free. The model is obtained by considering a physically reasonable choice for the metric potential $g_{rr}$ which depends on a dimensionless parameter `n'. The effect of $n$ is discussed numerically, analytically and through plotting. We have concentrated a wide range for n ($10\leq n \leq 1000$) for drawing the profiles of different physical parameters. The maximum allowable mass for different values of $n$ have been obtained by M-R plot. We have checked that the stability of the model is increased for larger value of $n$. For the viability of the model we have considered two compact stars PSR J1614-2230 and EXO 1785-248. We have shown that the expressions for the anisotropy factor and the metric component may serve as generating functions for uncharged stellar models in the context of the general theory of relativity., Comment: 17 pages, 12 figures, Accepted in Journal of Astrophysics and Astronomy on 03.01.2021

In this current study, our main focus is to model a specific charged compact star SAX J 1808.4-3658 (M = 0.88 $M_{\odot}$,\, R = 8.9 km) within the realm of $f(R,\,T)$ modified gravity theory using the metric potentials proposed by Tolman-Kuchowicz (Tolman, Phys Rev 55:364, 1939; Kuchowicz Acta Phys Pol 33:541, 1968) and the interior spacetime is matched to the exterior Reissner-Nordstr\"{o}m line element at the surface of the star. Tolman-Kuchowicz metric potentials provide a singularity-free solution which satisfies the stability criteria. Here we have used the simplified phenomenological MIT bag model equation of state (EoS) to solve Einstein-Maxwell field equations where the density profile ($\rho$) is related to the radial pressure ($p_r$) as $p_r(r) = (\rho - 4B_g)/3$. Further, to derive the values of unknown constants $a,\, b,\, B,\, C$ and the bag constant $B_g$, we match our interior space-time to the exterior Reissner-Nordstr\"{o}m line element at the surface of stellar system. In addition to this, to check the physical validity and stability of our suggested model, we evaluate some important properties such as effective energy density, effective pressures, radial and transverse sound velocities, relativistic adiabatic index, all energy conditions, compactness factor and surface redshift. It is depicted from our current study that all our derived results lie within the physically accepted regime which provides the viability of our present model in the context of $f(R,\,T)$ modified gravity., Comment: 15 Pages

The present work discusses about the existence of compact star model in the context of $f(R,T)$ gravity with $R$ as the Ricci scalar and $T$ as the trace of energy-momentum tensor $T_{\mu \nu}$. The model has been developed by considering the spherically symmetric spacetime consisting of isotropic fluid with $f(R,T)=R+2 \beta T$ with $\beta$ be the coupling parameter. The corresponding field equations are solved by choosing well known Finch-Skea {\em ansatz} [Finch, M.R., Skea, J.E.F.:{\it Class. Quantum Gravity} {\bf 6}, 467 (1989)]. For spacetime continuity we elaborate the boundary conditions by considering the exterior region as Schwarzschild metric. The unknown constants appearing in the solution are evaluated for the compact star PSR~J 1614-2230 for different values of coupling constant. The physical properties of the model, e.g., matter density, pressure, stability etc. have been discussed both analytically and graphically. This analysis showed that the geometry and matter are compatible with each other as well as the model is in stable equilibrium in the context of $f(R,\,T)$ modified gravity., Comment: Accepted in International Journal of Geometric Methods in Modern Physics on 25.05.2021

Our present study involves the strange stars model in the framework of $f(R,T)$ theory of gravitation. We have taken a linear function of the Ricci scalar $R$ and the trace $T$ of the stress-energy tensor $T_{\mu \nu}$ for the expression of $f(R,T)$, i.e., $f(R,T)=R+ 2 \gamma T $ to obtain the proposed model, where $\gamma$ is a coupling constant. Moreover, to solve the hydrostatic equilibrium equations, we consider a linear equation of state between the radial pressure $p_r$ and matter density $\rho$ as $p_r=\alpha \rho-\beta$, where $\alpha$ and $\beta$ are some positive constants, Both $\alpha,\,\beta$ depend on coupling constant $\gamma$ which have been also depicted in this paper. By employing the Krori-Barua {\em ansatz} already reported in the literature [J. Phys. A, Math. Gen. 8:508, 1975] we have found the solutions of the field equations in $f (R, T )$ gravity. The effect of coupling constant $\gamma$ have been studied on the model parameters like density, pressures, anisotropic factor, compactness, surface redshift, etc. both numerically and graphically. A suitable range for $\gamma$ is also obtained. The physical acceptability and stability of the stellar system have been tested by different physical tests, e.g., the causality condition, Herrera cracking concept, relativistic adiabatic index, energy conditions, etc. One can regain the solutions in Einstein gravity when $\gamma\rightarrow 0$, Comment: 26 Pages, 10 Figures

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 {\em ansatz} for one of the metric potentials $g_{rr}$ 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. have 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 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 (M-R) 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 allow us to estimate central density for a given radius (or mass) of a compact star., Comment: 20 pages, 14 figures, Submitted in Eur. Phys. J. C

The present article is devoted to the study of local anisotropies effects on the Durgapal's fourth model in the context of gravitational decoupling via the Minimal Geometric Deformation approach. To do it, the most general equation of state relating the components of the $\theta$--sector is imposed to obtain the decoupler function $f(r)$. In addition, certain properties of the obtained solution are investigated, such as the behavior of the salient material content threading the stellar interior, causality and energy conditions, hydrostatic balance through modified Tolman--Oppenheimer--Volkoff conservation equation and stability mechanism against local anisotropies by means of adiabatic index, sound velocity of the pressure waves, convection factor and Harrison--Zeldovich--Novikov procedure, in order to check if the model is physically admissible or not. Regarding the stability analysis, it is found that the model presents unstable regions when the sound speed of the pressure waves and convection factor are used in distinction with what happens in the adiabatic index and Harrison--Zeldovich--Novikov case. To produce a more realistic picture the numerical data for some known compact objects was placed and different values of the parameter $\alpha$ were considered to compare with the GR case \i.e, $\alpha=0$., Comment: To be published in Chinese Physics C

The present work is devoted to the study of anisotropic compact matter distributions within the framework of 5-dimensional Einstein-Gauss-Bonnet gravity. To solve the field equations, we have considered that the inner geometry is described by Tolman-Kuchowicz spacetime. The Gauss-Bonnet Lagrangian is coupled to Einstein-Hilbert action through a coupling constant. When this coupling tends to zero general relativity results are recovered. We analyze the effect of this parameter on the principal salient features of the model, such as energy density, radial and tangential pressure and anisotropy factor.Additionally, the behaviour of the subliminal sound speed of the pressure waves in the principal direction of the configuration and the conduct of the energy-momentum tensor throughout the star are analyzed employing causality condition and energy conditions, respectively. All these subjects are supported by mean of physical, mathematical and graphical survey

Present paper provides a model of static charged anisotropic fluid sphere in the EinsteinMaxwell- GaussBonnet (EMGB) theory of gravitation. We select KB anstz as the metric co-efficient along with electric field intensity. To develop our model we assume a linear EoS between the radial pressure and the matter density. The model obtained here is found to satisfy the elementary physical requirements like non-negativity nature of pressure and density, the energy conditions, continuity of the metric co-efficients, subliminal velocity of sound and also pressure free hypersurface etc. It may also noted that the solution is free from any kind of singularities.

In the present paper we have constructed a new relativistic anisotropic compact star model having a spherically symmetric metric of embedding class one. Here we have assumed an arbitrary form of metric function and solved the Einstein relativistic field equations with the help of Karmarkar condition for an anisotropic matter distri- bution. The physical properties of our model such as pressure, density, mass function, surface red-shift, gravitational red-shift are investigated and the stability of the stellar configuration is discussed in details. Our model is free from central singularities and satisfies all energy conditions. The model we present here satisfy the static stability criterion., Comment: 21 pages, 17 figures, 3 tables, Accepted for publication in International Journal of Modern Physics D

In this paper, model of charged gravastar under $f(T)$ modified gravity is obtained. The model has been explored by taking the diagonal tetrad field of static spacetime together with electric charge. To solve the Einstein-Maxwell field equations, along with $f(T)$ gravity, we assume the existence of a conformal Killing vector which relates between geometry and matter through the Einstein-Maxwell field equations by an inheritance symmetry. We study several cases of interest to explore physically valid features of the solutions. Some physical properties of the model are discussed and we match our interior spacetime to the exterior Reissner-Nordstr\"{o}m spacetime in presence of thin shell., Comment: 15 pages

In the present article, we discover a new well-behaved charged anisotropic solution of Einstein-Maxwell's field equations. We ansatz the metric potential $g_{00}$ of the form given by Maurya el al. (arXiv:1607.05582v1) with $n=2$. In their article it is mentioned that for $n=2$ the solution is not well-behaved for neutral configuration as the speed of sound is non-decreasing radially outward. However, the solution can represent a physically possible configuration with the inclusion of some net electric charged i.e. the solution can become a well-behaved solution with decreasing sound speed radially outward for a charged configuration. Due to the inclusion of electric charged the solution leads to a very stiff equation of state (EoS) with the velocity of sound at the center $v_{r0}^2=0.819, ~v_{t0}^2=0.923$ and the compactness parameter $u=0.823$ is closed to the Buchdahl limit 0.889. This stiff EoS support a compact star configuration of mass $5.418M_\odot$ and radius of $10.1 km$., Comment: Published in Int. J. Mod. Phys. D 0, 1750078 (2017)

A new model of charged compact star is reported by solving the Einstein-Maxwell field equations by choosing a suitable form of radial pressure. The model parameters $\rho$, $p_r$, $p_{\perp}$ and $E^{2}$ are in closed form and all are well behaved inside the stellar interior. A comparative study of charged and uncharged model is done with the help of graphical analysis., Comment: 10 figures, 2 tables

In this article, we study the possibility of sustaining a static and spherically symmetric traversable wormhole geometries admitting conformal motion in Einstein gravity, which presents a more systematic approach to search a relation between matter and geometry. In wormhole physics, the presence of exotic matter is a fundamental ingredient and we show that this exotic source can be dark energy type which support the existence of wormhole spacetimes. In this work we model a wormhole supported by dark energy which admits conformal motion. We also discuss the possibility of detection of wormholes in the outer regions of galactic halos by means of gravitational lensing. The studies of the total gravitational energy for the exotic matter inside a static wormhole configuration are also done., Comment: 9 pages, 2 tables and 4 figures, Accepted for Publication in European Physical Journal C

In present paper a spherically symmetric stellar configuration has been analyzed by assuming the matter distribution of the stellar configuration is anisotropic in nature and compared with the realistic objects, namely, the low mass X-ray binaries (LMXBs) and X-ray pulsars. The analytic solution has been obtained by utilizing the dark energy equation of state for the interior solution corresponding to the Schwarzschild exterior vacuum solution at the junction interface. Several physical properties like energy conditions, stability, mass-radius ratio, and surface redshift are described through mathematical calculations as well as graphical plots. It is found that obtained mass-radius ration of the compact stars candidates like 4U 1820-30, PSR J 1614-2230, Vela X-1 and Cen X-3 are very much consistent with the observed data by Gangopadhyay et al. (Mon. Not. R. Astron. Soc. 431, 3216 (2013)). So our proposed model would be useful in the investigation of the possible clustering of dark energy., Comment: 10 pages & 7 figures, Accepted for publication in Can.J.Phys. It matches with the published version

In this paper we utilise the Krori-Barua ansatz to model compact stars within the framework of Einstein- Gauss-Bonnet (EGB) gravity. The thrust of our investigation is to carry out a comparative analysis of the physical properties of our models in EGB and classical general relativity theory.

In this work we provide a framework for modeling compact stars in which the interior matter distribution obeys a generalised Chaplygin equation of state. The interior geometry of the stellar object is described by a spherically symmetric line element which is simultaneously comoving and isotropic with the exterior spacetime being vacuum. We are able to integrate the Einstein field equations and present closed form solutions which adequately describe compact strange star candidates like Her X-1, RX J 1856-37, PSRJ 1614-2230 and SAX J1808.4-3658., Comment: 13 pages,10 figures and 2 tables

In the present paper we are willing to model anisotropic star by choosing a new grr metric potential. All the physical parameters like the matter density, radial and transverse pressure and are regular inside the anisotropic star, with the speed of sound less than the speed of light. So the new solution obtained by us gives satisfactory description of realistic astrophysical compact stars. The model of the present paper is compatible with observational data of compact objects like RX J1856-37, Her X-1, Vela X-12 and Cen X-3. A particular model of Her X-1 (Mass 0.98 times solar mass and radius=6.7 km.) is studied in detail and found that it satisfies all the condition needed for physically acceptable model. Our model is described analytically as well as with the help of graphical representation., Comment: 14 pages, 9 figures

In the present article, we have constructed static anisotropic compact star models of Einstein field equations for the spherical symmetric metric of embedding class one. By assuming the particular form of metric function $\nu$, We have solved the Einstein field equations for anisotropic matter distribution. The anisotropic models are representing the realistic compact objects such as SAX J 1808.4-3658 (SS1), Her X - 1, Vela X-12, PSR J1614-2230 and Cen X - 3. We have reported our results in details for compact star Her X-1 on the ground of physical properties such as pressure, density, velocity of sound, energy conditions, TOV equation and red-shift etc. Along with these, we have also discussed about stability of the compact star models. Finally we made the comparison between our anisotropic stars with the realistic objects on the key aspects as central density, central pressure, compactness and surface red-shift., Comment: 16 pages, 15 figures and 2 tables, Eur. Phys. J. A (2016) 52: 312

Present paper provides a new model for perfect fluid sphere filled with charge in higher dimensional spacetime admitting conformal symmetry. We consider a linear equation of state with coefficients fixed by the matching conditions at the boundary of the source corresponding to the exterior Reissner-Nordstr\"{o}m higher dimensional spacetime. Several physical features for different dimensions, starting from four up to eleven, are briefly discussed. It is shown that all the features as obtained from the present model are physically desirable and valid as far as the observed data set for the compact star $SAX~J1808.4-3658~(SS2)$ is concerned., Comment: 18 pages, 8 figures

A new singularity free model of anisotroipic compact star is proposed. The Einstein field equations are solved in closed form by utilizing Matese \& Whitman mass function. The model parameters $\rho$, $p_r$ and $p_t$ all are well behaved inside the stellar interior and our model satisfies all the required conditions to be physically acceptable. The model given in the present work is compatible with observational data of compact objects like SAX J 1808.4-3658 (SS1), SAX J 1808.4-3658 (SS2) and 4U 1820-30. A particular model of 4U 1820-30 is studied in detail and found that it satisfies all the condition needed for physically acceptable model. The present work is the generalization of Sharma and Ratanpal\cite{sharma13} model for compact stars admitting quadratic equation of state., Comment: 14 pages, 2 tables and 12 figures, communicated in IJMPD

In this paper we have discussed geodesics and the motion of test particle in the gravitational field of noncommutative charged black hole spacetime. The motion of massive and massless particle have been discussed seperately. A comparative study of noncommutative charged black hole and usual Reissner-Nordstrom black hole has been done. The study of effective potential has also been included. Finally, we have examined the scattering of scalar waves in noncommutative charged black hole spacetime., Comment: 12 pages, 8 figures, Accepted in Commun. Theor. Phys

In this paper, we construct thin-shell wormholes in (2+1)-dimensions from noncommutative BTZ black hole by applying the cut-and-paste procedure implemented by Visser. We calculate the surface stresses localized at the wormhole throat by using the Darmois-Israel formalism, and we find that the wormholes are supported by matter violating the energy conditions. In order to explore the dynamical analysis of the wormhole throat, we consider that the matter at the shell is supported by dark energy equation of state P = w\rho with w < 0. The stability analysis is carried out of these wormholes to linearized spherically symmetric perturbations around static solutions. Preserving the symmetry we also consider the linearized radial perturbation around static solution to investigate the stability of wormholes which explored by the parameter \beta (speed of sound)., Comment: 13 pages, 16 figures, accepted in Int.J.Mod.Phys. D. arXiv admin note: text overlap with arXiv:0806.4459, arXiv:0909.1071 by other authors