Elsevier

Annals of Physics

Volume 429, June 2021, 168491
Annals of Physics

Embedded class-I solution of compact stars in f(R) gravity with Karmarkar condition

https://doi.org/10.1016/j.aop.2021.168491Get rights and content

Abstract

This paper’s main aim is to investigate the existence of a new classification of embedded class-I solutions of compact stars, by using Karmarkar condition in f(R) gravity background. To achieve that goal, we consider two different models of the f(R) theory of gravity for the static spherically symmetric spacetime by considering anisotropic matter distribution. Further, we employ Karmarkar condition to relate the two components of metric potentials grr and gtt. We assume a particular model for one metric potential and obtain the second one by Karmarkar condition. Moreover, we also calculate the values of constant parameters by using the observational data of these compact stars, namely, VelaX1, PSRJ16142230, 4U160852, CenX3 and 4U182030. We perform different physical tests like variational behavior of energy density and pressure components, stability and equilibrium conditions, energy constraints, mass function and adiabatic index to check the viability of f(R) gravity models. All these physical attributes indicate the consistent behavior of our models. Our investigation also suggests that f(R) theory of gravity appears as a suitable theory in describing the viability of a new classification of embedded class-I solutions of compact objects.

Introduction

Compact objects are usually designated as neutron stars, white dwarfs and black holes in astrophysics. Neutron stars and white dwarfs are born in the result of gravitational collapse which generally happen due to degeneracy pressure of the relativistic objects. These objects possess huge densities values but are volumetrically smaller. Although, we do not know the precise characteristics of these kinds of compact stars, but these objects are presumed to be heavy stars having tiny radius. Every kinds of stellar objects are mostly acknowledged as degenerate stars, except black holes. In general relativity (GR) the study of configured equilibrium of compact stars is valuable because of the massive nature and high densities of these objects. The study of compact objects in the framework of GR and modified gravitational theories has always been considered as a topic of great interest in astrophysics. To investigate celestial compact structure modeling, we need an exact solution to Einstein field equations (EFE). In 1916, Schwarzschild [1] obtained the EFE solution for the interior structure of compact objects. In this regard, various modified gravitational theories have been presented in order to obtain the complicated exact solutions of EFE. In the framework of observational data, Tolman [2] and Oppenheimer [3] investigated some realistic models of non-transversable stellar objects and claimed that the physical characteristics of these objects represent the relationship between the internal pressure and the gravitational force which eventually leads in a state of equilibrium structures. In the study of stars internal configuration, this phenomenon has considerable importance and provides realistic results in various occasions. Further, compact celestial structure was examined by Baade and Zwicky [4]. According to their study the supernova might transform into smaller compact objects after the observation of strongly magnetized spinning neutrons. Moreover, Ruderman [5] identifies for the first time that at the center of the stellar object the nuclear density exhibits anisotropic behavior.

A perfect fluid was assumed to be a source of celestial structures for the creation of stellar objects. These objects are commonly defined as ultra-dense, isotropic, celestial bodies that seem to be spherically symmetric. Whereas, isotropy is assumed as a desired attribute, still it does not have a general feature of compact stars. The concept of non-zero anisotropy in a stellar configuration was first presented by Bowers and Liang [6]. Moreover, Ruderman [5] investigated the possibility that stars might come with huge densities 1015g/cm3, when the nuclear matter is anisotropic in nature. In fact, anisotropic matter distribution plays a crucial role in narrating the inner representation and development phase of the relativistic stellar objects. In anisotropic fluid distribution background, a lot of literature is available [7], [8], [9], [10], [11]. In anisotropic distribution, the pressure component of the fluid sphere divides into two components, radial and transverse.

One of the most promising topics of modern research in cosmology is the accelerated expansion of the universe. Cosmologists argue that the accelerating expansion of the universe depends on dark energy and dark matter which retains negative pressure. As a substitute of GR, different gravitational theories have been presented to unfold the mystery behind dark energy issues. These gravitational theories are recognized as modified theories of gravity. Some of these modified theories are f(R),f(R,T),f(G),f(R,G). Among these valuable theories, f(R) is one of the most simplest and popular theory, obtained as an arbitrary function of Ricci scalar. This theory was proposed by Buchdahl [12]. Later on, Nojiri and Odintsov [13] demonstrated some models of f(R) theory of gravity by placing curvature as a function of Ricci scalar and the outcomes of their considered models are quite viable and stable. Further, Starobinsky [14] presented an interesting class of f(R) theory of gravity models that showed the physically acceptable results in laboratory testing of the solar structure. Some f(R) gravity models were proposed by Hu and Sawicki [15] by ignoring the cosmological constant and their study evident some interesting results in regard to accelerating expansion phenomena. The viability of physical attributes of compact stars by taking exponential type models of f(R) theory of gravity was discussed by Cognola et al. [16]. In modified theories of gravity beyond GR and its Hilbert-Einstein action, diffeomorphism invariance and the Bianchi identities violation have attracted a lot of interest. Hamity and Barraco [17] derived the generalized Bianchi identities for the non-linear f(R) gravity to throw some light on the issue that f(R) theory of gravity generates higher than second order equations of motion and violates Bianchi identities. Further, Wang et al. [18] confirmed the local energy–momentum conservation of Bianchi identities by establishing the equivalence relation between Palatini f(R) and the Brans–Dicke gravity. Moreover, Koivisto [19] explored a composition of f(R) gravity and the generalized Brans–Dicke gravity and claimed covariant conservation from both the metric tensors and the Palatini variational techniques.

For physically stable models, one could use an analytical approach of EFE and assume the family of a four dimensional manifold and transform it into Euclidean space. The embedding family of curved geometry into geometries of higher dimensions is considered to create various new exact solutions in astrophysical stellar systems. Schlai [20] designated the embedding issue on geometrically important spacetimes for the very first time. In this regard, Nash [21] presented the isometric embedding theorem. Several features of anisotropic compact objects by employing embedding class one approach have been discussed in literature [22], [23], [24], [25], [26], [27], [28], [29]. A class of non-static fluid distribution along with non-vanishing acceleration was studied by Gupta and Gupta [30]. Further, Gupta and Sharma [31] assumed plane symmetric metric to explore the embedding class-I solutions of non-static perfect fluid. The embedding class constraint yields a differential equation in static spherically symmetric geometry relating the two components of metric potentials, known as Karmarkar condition [32]. A lot of work [33], [34], [35], [36] has been done related to Karmarkar condition, defined as R1414R2323=R1212R3434+R1224R1334. The Karmarkar condition develops a connection between two parts of metric tensors grr and gtt for a spherical static symmetric fluid distribution. Maurya et al. [37] and Bhar et al. [38] studied the EFE by adopting the Karmarkar condition and composing numerous classes of embedded class-I solutions. They also observed that these outcomes exhibit stable nature and might be helpful in exploring the internal structure of the stellar objects. Later, in the background of f(G) gravity, Sharif and Saba [39] investigated the charged anisotropic solutions and their derived solutions are physically consistent and stable. In this regard, a class of embedded solutions by adopting Karmarkar condition is recently presented by Upreti et al. [40].

In the background of GR, Maurya et al. [41] probe an anisotropic compact star using embedding class-I approach. They studied different features of compact stars in the presence of anisotropic fluid distribution and claimed that obtained outcomes describe the internal core of stellar objects. Further, Bhar et al. [42] presented a new relativistic anisotropic compact star model which is physically acceptable for embedding spacetime and can be used to describe the interior solution of stellar objects. Moreover, the relativistic model for anisotropic compact stars in GR background was studied by Prasad et al. [43]. Their results indicate that by adopting embedding class-I condition the obtained relativistic stellar structure is physically reasonable. Recently, Mustafa et al. [44] introduced new exact solutions of EFE in the framework of Bardeen black hole spacetime by using the well known Karmarkar condition. Their chosen model demonstrated the well-behaved nature under the particular values of the parameters. By getting motivated from literature, in this particular paper, we extended the concept of Bhar et al. [42] in the background of f(R) theory of gravity and analyzed the physical attributes of the compact stars, namely, VelaX1, PSRJ16142230, 4U160852, CenX3 and 4U182030. To meet our goal, we assume two different f(R) theory of gravity models by employing the Karmarkar condition in the framework of anisotropic pressure. We consider a particular model for one of the metric potential grr and by adopting the Karmarkar condition, we construct the second metric potential gtt of spherically symmetric spacetime.

The outline of the current article is organized as follows: In the next segment, the f(R) field equations have been developed by using Karmarkar condition with an anisotropic matter source. Two of viable f(R) gravity models along with boundary constraints have been introduced in Section 3. In Section 4, we compute the constant values by using the matching conditions. In Section 5, we discuss the physical attributes of the compact stars in detail. In the last portion, we provide the final verdict and conclusion of our study.

Section snippets

Modified field equations

First, we are proceeding to develop the field equations in f(R) gravity context. For this purpose, we consider the action of f(R) gravity [45] defined as S=[f(R)2κ+Lm]gd4x.Here, f is a function of Ricci scalar and Lm is the Lagrangian matter. Varying the action identified in (1) regarding the metric potential gηζ, we obtain the succeeding f(R) gravity field equation FRηζ12f(R)gηζηζF+gηζF=κTηζ.Thus, F=df(R)dR. Although, η and indicate covariant derivative and D’Alembertian notation,

Viable f(R) gravity models

Here, we consider the two realistic and simple viable f(R) gravity models to analyze the modified field equations presented in Eqs. (4)–(6).

Matching conditions

For the stellar compact objects, the intrinsic boundary metric irrespective of the geometry (interior or exterior) will remain the same. This phenomena verifies that whatever the coordinates system covering the surface of the boundary, the components of the metric tensor will remain continuous. For stellar compact objects, the Schwarzschild solution is observed as the most suitable possibility to choose from the various choices of the matching conditions in the context of GR. Jebsen-Birkhoff’s

Physical characteristics of the compact stars models

Here, we discuss the graphical behavior of the considered compact stars in the context of f(R) gravity models. We studied the graphical illustration of energy density, pressure components, stability and equilibrium condition, energy conditions, mass function, adiabatic index, etc. For plotting the graphs, we consider the same values of constants mentioned in Table 1. Moreover, for Model-1, we fix the constant parameter α=0.1 and the variation of β, for different compact stars is given as: β=1.5

Conclusion

In order to explore a new family of embedded class-I solutions in the anisotropy background, we assume two realistic f(R) gravity models by using the Karmarkar condition. Since the Karmarkar condition reduced the solution-generating method of EFE to a single metric potential by providing a connection between grr and gtt. For this purpose, we assume the metric potentials eλ=1+a2r2(1+br2)4 and by employing Karmarkar condition, we obtain second metric potential given as eν=[AaB2b(1+br2)]2, where a

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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