Pramana – J. Phys. (2020) 94:90 © Indian Academy of Sciences https://doi.org/10.1007/s12043-020-01960-7 Singularity-free non-exotic compact star in f (R, T)gravity ANIL KUMAR YADAV1, MONIMALA MONDAL2 and FAROOK RAHAMAN2,∗ 1Department of Physics, United College of Engineering and Research, Greater Noida 201 310, India 2Department of Mathematics, Jadavpur University, Kolkata 700 032, India ∗Corresponding author. E-mail: [email protected] MS received 19 January 2020; revised 14 March 2020; accepted 9 April 2020 Abstract. In the present work, we have searched for the existence of anisotropic and non-singular compact star in the f (R, T ) gravity by taking into account the non-exotic equation of state (EoS). In order to obtain the solutions of the matter content of the compact object, we assume the well-known barotropic form of EoS that yields the linear relation between pressures and energy density. We propose the existence of non-exotic compact star which shows the validation of energy conditions and stability within the perspective of f (R, T ) extended theory of gravity. The linear material correction in the extended theory and matter content of compact star can remarkably satisfy energy condition. We discuss various physical features of the compact star and show that the proposed model of the stellar object satisfies all regularity conditions and is stable as well as singularity-free. Keywords. Compact star; f (R, T ) gravity; singularity. PACS Nos 04.40.Nr; 04.20.Jb; 04.20.Dw 1. Introduction numerous researchers in different physical contexts [15– 17]. In 2006, the most significant compact star has been Harko et al [1] have proposed the extended theory of observed by Rosat Surveys due to their X-ray emis- gravity, the so-called f (R, T ) gravity, by changing the sion [18]. This means that the gravitational energy of geometrical part of the Einstein field equations instead the compact star is radiated through X-rays. Long ago, of changing the source side by taking a generalised Hewish et al [19] investigated some rapidly pulsating functional form of the argument to address galactic, radio source which is in general the beam of electro- extra-galactic and cosmic dynamics. In this theory, the magnetic radiation. This discovery inspires physicists gravitational terms of total action is defined by the to think about modelling compact star-like neutron star functional form of f (R) and f (T ).Themainaimof and quark star in the framework of general relativity and this theory is to address some observational phenomena its extended form [20–23]. It is common understand- such as dark energy [2], dark matter [3] and massive ing that one cannot analyse the structure and properties pulsars [4] that were hardly explained by general relativ- of compact star by taking into account the equation of ity (GR). Among all the extended/modified theories of state which relate the pressure and energy density in gravity, the f (R, T ) theory attracts more attention due proportion. In refs [24–26], it has been found that the to its unique feature of non-minimal coupling of matter pressure of the compact star is anisotropic in nature. and geometry [5]. In the recent past, several applica- Recently, Momeni et al [27] have constructed a model tions of f (R, T ) gravity [6–14] have been reported in of compact star in Horndeski theory of gravity and the literature. analysed it in modified theory of gravity. However, In this paper, we focus ourselves on investigating a our model deals with the singularity-free compact star non-exotic compact star within f (R, T ) = R + 2ζ T composed of non-exotic matter in f (R, T ) theory of formalism where ζ is an arbitrary constant. The com- gravity and its functional form f (R, T ) = R + 2ζ T .In pact star is a hypothetical dense body that may be a refs [6,28], some applications of f (R, T ) theory with black hole or a degenerate star and the pressure inside respect to steller objects are reported. Some other rel- it is not isotropic. In astrophysics, the structure and evant investigations on different functional forms of properties of the compact star had been studied by extended f (R, T ) theory of gravitation can be observed 0123456789().: V,-vol 90 Page 2 of 9 Pramana – J. Phys. (2020) 94:90 in [29,30] under different physical contexts. In 2014, 1 Rij f (R, T ) − f (R, T )gij Rahaman et al [31] studied the static wormhole in f (R) 2 gravity with Lorentzian distribution which generates i + gij i −i j f (R, T ) two models – one is derived from the power law form ˙ ˙ and the other one is based on the assumption of partic- = 8πTij − f (R, T )θij − f (R, T )Tij. (2) ular shape function which allows the reconstruction of ˙ the f (R) theory. Zubair et al [32] investigated numer- Here, f (R, T ) = ∂ f /∂ R, f (R, T ) = ∂ f /∂T and ical solutions for different wormhole matter content in θij reads as the realm of f (R, T ) gravity. Moraes [9] constructed ∂Tij the model of static wornhole by applying f (R, T ) for- θ = gij . (3) ij ∂ ij malism. g In the present paper, we are concerned with the In this paper, we take the more generic form of matter singularity-free non-exotic model of compact star with Lagrangian as Lm =−ρ [6]. Hence eq. (3) leads to the realm of functional form of f (R, T ) = R + 2ζ T .It is worth mentioning that our model is derived from the θij =−2Tij − ρgij. (4) well-known barotropic equation of state (EoS) in Krori and Barua (KB) space–time [33] that yield singularity- Following the proposition of ref. [1], we assume the f (R, T ) = f (R) + ζ T ζ free solution. Das et al [23] have investigated a model of functional form of 2 where is stellar object in the static spherically symmetric space– a constant. In the literature, this functional form is com- f (R, T ) time which is probably singular and generates a set monly used to obtain cosmological solution in of solutions describing the interior of a compact star theory of gravitation [7–9]. under f (R, T ) theory of gravity which admits confor- Equations (2)and(4) lead to mal motion whereas the present investigation is one with Gij = (8π + 2ζ)Tij + ζ(2ρ + T )gij, (5) singularity-free solution. However, a common feature of both the investigations is the non-exotic matter config- where Gij is the Einstein’s tensor. uration in f (R, T ) gravity. The paper is structured as follows: The basics of f (R, T ) = f (R) + 2ζ T formalism are presented in 3. The KB metric and field equations §2. Section 3 deals with the KB metric, solution of field equations and physical behaviour of the model. The Krori and Barua space–time [33,34] reads as In §4, we provide the boundary conditions, which are ν( ) λ( ) essential for finding the values of constants. In §5, ds2 =−e r dt2 + e r dr 2 + r 2(dθ 2 + sin2 θ dφ2) we demonstrate the validity of energy conditions, sta- (6) bility and mass–radius relation to show the physical 2 2 acceptance of the model. In §6, we match the model with λ(r) = Ar and ν(r) = Br + C where A, B and parameters with observation data sets. In §6, we give C are constants. our results and discuss the future perspectives of the In this paper, we take an anisotropic fluid satisfying study. the matter content of the stellar object as Tij = diag(−ρ, pr , pt , pt ), (7) where ρ, pr and pt are the energy density, radial pressure 2. The f (R, T) = f (R) + 2ζ T formalism and tangential pressure respectively. Thus, the trace of energy momentum tensor may be expressed as T = ( , ) The total action for the f R T theory of gravitation −ρ + pr + 2pt . [1] reads as Metric (6) and field equation (5) along with eq. (7) give the following equations: 1 √ √ S = d4xf(R, T ) −g + d4xL −g, (1) π m −λ λ 1 1 4 e − + r r 2 r 2 where R is the Ricci scalar, T is the trace of energy– = ( π + ζ)ρ − ζ( + ) 8 pr 2pt (8) momentum tensor T i , g is the metric determinant and j −λ ν 1 1 L e + − m is the matter Lagrangian density. r r 2 r 2 By varying the total action S with respect to metric = ζρ + (8π + 3ζ)pr + 2ζ pt (9) gij, we obtain Pramana – J. Phys. (2020) 94:90 Page 3 of 9 90 −λ ν 2 − λ ν ν − λ 0.35 e + + ν 2 2 r 0.3 = ζρ + ζ pr + (8π + 4ζ)pt . (10) 0.25 0.2 ) r ( 3.1 Solution of field equations and physical λ 0.15 parameters 0.1 To solve the above set of equations for the matter content 0.05 of the compact star, it is useful to invoke the equation of 0 state (EoS) which gives the relation between energy den- 0 0.5 1 1.5 2 2.5 3 3.5 sity and pressure. The most common barotropic forms r of EoS [35]are Figure 1. Plot of λ vs. r. pr = αρ, (11) = pt = βρ, (12) We also note that at r 0, the second derivative of energy density as well as radial pressure are negative where α and β are constants having values in the range which shows that the energy density and radial pressure (0,1).