arXiv subjects

Piyali Bhar

Publications and source records attributed to Piyali Bhar.

At least 19 recordsLinked to original sources

arXiv2024

Modeling and Analyzing Stability of Hybrid Stars within $f(Q)$ Gravity

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.

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arXiv2024

The Stability of Anisotropic Compact Stars Influenced by Dark Matter under Teleparallel Gravity: An Extended Gravitational Deformation Approach

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.

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arXiv2024

Properties of wormhole model in de Rham-Gabadadze-Tolley like massive gravity with specific matter density

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.

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arXiv2024

Noncommutative wormhole in de Rham-Gabadadze-Tolley like massive gravity

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.

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arXiv2024

Noncommutative black hole in de Rham-Gabadadze-Tolley like massive gravity

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 $θ$ 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 $M/\sqrtθ$. 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.

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arXiv2024

(3+1)-D gravastar in de Rham-Gabadadze-Tolley-like massive gravity

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.

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arXiv2023

Relativistic isotropic stellar model in $f(R,\,T)$ gravity with Durgapal- IV Metric

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 $χ$, 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 $χ=0$. 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 $χ$ 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.

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arXiv2023

Physical Characteristics and Maximum Allowable Mass of Hybrid Star in the Context of $f(Q)$ Gravity

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 $p_r =αρ-β$ 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.

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arXiv2023

Compact star in $f(T)$ gravity with Tolman-Kuchowicz metric potential

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.

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arXiv2022

Isotropic Buchdahl's relativistic fluid sphere within $f(R,\,T)$ gravity

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 $χ$, graphical representation of the model parameters have been shown to canvass the analytical results more clearly. Interestingly, we have proven that for $χ=0$, 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 $χ$, the numerical values of the different physical variables have been tabulated for the values of the coupling parameter $χ=0,\,0.25,\,0.5,\,0.75,1,\,1.25$. 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.

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arXiv2022

Anisotropic solution for polytropic stars in 4D Einstein-Gauss-Bonnet gravity

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 $α$ which satisfies all the physical criteria including causality with static stability via mass vs central mass density ($M-ρ_c$), 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.

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arXiv2022

Dark Energy Stars in Tolman-Kuchowicz spacetime in the context of Einstein Gravity

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.

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arXiv2022

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

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 $p_q = \frac{1}{3}(ρ_q - 4B_g)$ 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 $p_r = αρ- β$. 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).

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arXiv2021

Tolman IV fluid sphere in f(R, T) gravity

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 $β$ on the local matter distribution of compact stars has been investigated in this paper. It can be seen that with greater values of $β$, the sound speed and adiabatic index are higher. On contrary, the mass function takes lower value for higher values of $β$. 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.

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arXiv2021

Phantom energy supported wormhole model in $f(R,\,T)$ gravity assuming conformal motion

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 $f(R,T)= R+2 γT$. Here we employ the phantom energy EoS relating to radial pressure and density given by $p_r=ωρ$ with $ω<-1$ 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 $γ$. From our analysis we see that no wormhole solution exists for $γ=-4π,\,-π(3+ω)$. All the physical parameters have been drawn by employing the values of $γ$ as $γ=-0.3,\,-0.2,\,-0.1,\,0,\,0.1$ and $0.2$, where $γ=0$ 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.

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arXiv2021

Stable and self consistent charged gravastar model within the framework of $f(R,\,T)$ gravity

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-Nordström geometry. For a particular choice of $f(R,\,T)$ as $f(R, \,T )=R+2γT$, 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 $η$ 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.

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arXiv2021

Relativistic compact stars in Tolman spacetime via an anisotropic approach

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.

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arXiv2021

Charged gravastar model in $f(T)$ gravity admitting conformal motion

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-Nordström spacetime in presence of thin shell.

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