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In this study, the geometrically deformed compact objects in the f(Q, T) gravity theory under an electric field through gravitational decoupling via minimal geometric deformation (MGD) technique are developed for the first time. The decoupled field equations are solved via two different mimic approaches Θ00=ρ${\Theta}_{0}^{0}=\rho $ and Θ11=pr${\Theta}_{1}^{1}={p}_{r}$ through the Karmarkar condition. Physical viability tests are conducted on our models and examine how decoupling parameters affect the physical qualities of objects. The obtained models are compared with the observational constraints for neutron stars PSR J1810+174, PSR J1959+2048, and PSR J2215+5135, including GW190814. Particularly, by modifying parameters α and n, the occurrence of a “mass gap” component is accomplished. The resulting models exhibit stable, well‐behaved mass profiles, regular behavior and no gravitational collapse, as verified by the Buchdahl–Andréasson's limit. Furthermore, a thorough physical analysis that is based on two parameters: n (f(Q, T)–coupling parameter) and α (decoupling parameter) is provided. This work extends our current understanding of compact star configurations and sheds light on the behavior of compact objects in the f(Q, T) gravity. [ABSTRACT FROM AUTHOR]

We study the higher dimensional scenario of an anisotropic compact star using the Buchdahl–Vaidya–Tikekar metric ansatz. In our formalism, the anisotropy is assumed in such a way that, in the absence of it, the solution reduces to Schwarzschild's interior solution in D ≥ 4 dimensions. The model is so developed that it correlates anisotropy to the curvature parameter K which characterizes a departure from spherical geometry of the t = constant hypersurface of the associated spacetime when embedded in a 4 dimensional Euclidean space. Due to the particular choice of anisotropy, the pressure balancing equation for hydrostatic equilibrium continues to have the same form in higher dimensions. Consequently, our approach permits extending a four-dimensional solution to a higher dimensional spacetime without deforming the sphericity of the configuration. Making use of the model, we propose a higher dimensional anisotropic analogue of the Buchdahl bound on compactness. We show that additional dimension as well as anisotropy reduce the compactification limit. Our technique helps to regain the original Buchdahl limit in D = 4 dimensions and also, in the absence of anisotropy, the compactification limit in higher dimensions obtained earlier by Leon and Cruz (Gen Relativ Gravit 32:1207–1216, 2000. https://doi.org/10.1023/A:1001982402392). It turns out that the maximum achievable dimension remains model dependent through the causality condition and the compactification limit. We scrutinize the model under all the requisite physical conditions for a relativistic anisotropic fluid sphere which might serve as the internal structure of a compact star in higher dimensions. We analyze the consequences of the departure from homogeneous spherical distribution and dimensionality on the physical behaviour of the star. The EOS becomes stiffer in higher dimensions and comparatively lower anisotropic stress. Our calculation shows that the central density reduces as we move towards higher dimensions and inclusion of anisotropy increases the rate of fall of the density profile. We also note that the two pressures get reduced considerably in higher dimensions. We show that, for a given curvature parameter specifying the sphericity, an extra dimension is analogous to moving towards a homogeneous distribution of an anisotropic star. [ABSTRACT FROM AUTHOR]

In this paper, the possibility of existing a novel class of compact charged spheres based on a charged perfect fluid within the realm of f(Q)$f(Q)$ gravity theory is explored. The authors started by proposing physically meaningful explicit formulas for the potential, denoted λ(r)$\lambda (r)$, and the electric field to find a close‐form solution. More precisely, the change of the dependent variable approach by exploiting the transformation Y(r)=eζ(r)/2$Y(r)=e^{\zeta (r)/2}$ is applied. Successively, the field equations analytically are solved and generate the most general solution, which leads us to examine various significant aspects of the stellar system. These aspects comprise the regularity of gravitational potentials, energy density and pressure, electric charge, the mass‐radius relationship, subluminal sound velocities in the radial direction, and the adiabatic index for charged compact stars. For a more in‐depth system study, mass measurements using contour diagrams are carried out. This mainly involves varying the variable parameters β1$\beta _1$ and E0$E_0$ to distinguish their effect on the mass distribution within the stellar structure. What is more, the electric charge controls the stability of the stellar system is shown, which yields that a stable system can possess a maximum charge of order 1020C$10^{20}\nobreakspace \text{C}$. The results strongly argue that charged stars could conceivably exist in nature and that such a deviation from traditional theories may be seen in future astrophysical observations. In this paper, the possibility of existing a novel class of compact charged spheres based on a charged perfect fluid within the realm of f(Q)$f\!(\mathcal{Q})$ gravity theory is explored. The authors started by proposing physically meaningful explicit formulas for the potential, denoted λ$\lambda$(r), and the electric field to find a close‐form solution. More precisely, the change of the dependent variable approach by exploiting the transformation Y(r) = eζ(r)∕2 is applied. Successively, the field equations analytically are solved and generate the most general solution, which leads us to examine various significant aspects of the stellar system. These aspects comprise the regularity of gravitational potentials, energy density and pressure, electric charge, the mass‐radius relationship, subluminal sound velocities in the radial direction, and the adiabatic index for charged compact stars. For a more in‐depth system study, mass measurements using contour diagrams are carried out. This mainly involves varying the variable parameters β1 and E0 to distinguish their effect on the mass distribution within the stellar structure. What is more, the electric charge controls the stability of the stellar system is shown, which yields that a stable system can possess a maximum charge of order 1020 C. The results strongly argue that charged stars could conceivably exist in nature and that such a deviation from traditional theories may be seen in future astrophysical observations. [ABSTRACT FROM AUTHOR]

In this paper, we investigated anisotropic solution for a self-bound compact star model. The solution is obtained via Vaidya–Tikekar metric function 1 / W (r) = K + C r 2 K (1 + C r 2) and the vanishing complexity factor condition. The physical analysis of the solution has been discussed in detail which includes mass-radius ratio, and stability analysis via adiabatic index for a secondary component of the GW190814 event for different values K. Furthermore, we predicted the radii and obtained the compactness factor of different self-bound compact objects for different K. Moreover, the hydrostatic equilibrium condition of the solution is also checked. The physical result shows that the solution so obtained is well behaved and may be useful for modeling the self-bound compact objects as far as observational data is concerned. [ABSTRACT FROM AUTHOR]

The aim of this paper is to obtain new physically admissible exact anisotropic stellar models via gravitational decoupling technique. This will be done by taking Korkina and Orlyanskii model solution as a seed solution and extend it to an anisotropic domain via gravitational decoupling through the minimal geometric deformation (MGD) approach. We analyze the physical properties of four compact star models Her X − 1 (Mass M = 0. 8 5 M Θ , radius = 8. 1 km), LMC X − 4 (M = 1. 0 4 M Θ , radius = 8. 3 0 1 km), PSR J 1 9 0 3 + 3 2 7 (M = 1. 6 6 7 M Θ , radius = 9. 4 3 8 km) and PSR J 1 6 1 4 − 2 2 3 0 (M = 1. 9 7 M Θ , radius = 9. 6 9 km). We demonstrate the stability conditions of the obtained solution by plotting energy conditions, velocity of sound, adiabatic index and all the thermodynamic parameters and equilibrium conditions via TOV equation, etc. We found that the results are physically well behaved and also fulfill the requirement of stability criteria inside the anisotropic fluid sphere. We study the physical features of the solutions in detail, analytically and graphically for the four compact stars Her X − 1 , LMC X − 4 , PSR J 1 9 0 3 + 3 2 7 and PSR J 1 6 1 4 − 2 2 3 0. Based on the results we realized that the extended gravitational decoupling technique is very effective for the analysis of the interior of the compact stellar structures. [ABSTRACT FROM AUTHOR]

Quantum effects in pycnonuclear reactions in compact stars at zero temperatures are studied with high precision. The reaction 16O + 16O was analyzed using the method of multiple internal reflections. The study of such reactions requires full consideration of quantum fluxes in the internal nuclear region. This reduces the rate and number of pycnonuclear reactions up to 1.8 times. This leads to the appearance of new states (which we call quasibound states) where the compound nuclear system is formed with maximal probability. As shown, the minimal energy of such a state is slightly higher than the energy of zero-mode oscillations in the lattice nodes in the pycnonuclear reaction, however, the probability of the formation of a compound system in a quasibound state is significantly greater than the corresponding probability in a state of zero-mode oscillations. It is reasonable to say that the frequency of reactions in quasi-bound states is more likely than in states of zero-mode oscillations. This can lead to significant changes in estimates of reaction rates in stars.

ABSTRACTOur world is wonderful because of the normal but negligibly small baryonic part (i.e. atoms) although unknown dark matter and dark energy dominate the Universe. A stable atomic nucleus could be simply termed as ‘strong matter’ since its nature is dominated by the fundamental strong interaction. Is there any other form of strong matter? Although nuclei are composed of 2-flavoured (i.e. up and down flavours of valence quarks) nucleons, it is conjectured that bulk strong matter could be 3-flavoured (with additional strange quarks) if the baryon number exceeds the critical value, Ac in which case quarks could be either free (so-called strange quark matter) or localized (in strangeons, coined by combining ‘strange nucleon’). Bulk strong matter could be manifested in the form of compact stars, cosmic rays, and even dark matter. This trinity will be explained in this brief review that may impact dramatically on today’s physics, particularly in the era of multi-messenger astronomy after the discovery of gravitational wave.

Tolman VII solution (Tolman in Phys Rev 55(4):364, 1939) is an exact analytic solution to the Einstein field equations describing the space-time of a static spherically symmetric distribution of matter. The solution has been shown to be capable of describing the interior of compact objects like neutron stars. Generalized (Raghoonundun and Hobill in Phys Rev D 92(12):124005, 2015) and modified (Jiang and Yagi in Phys Rev D 99(12):124029, 2019) versions of the solution are also available in the literature, which have been subsequently developed to accommodate a wide range of neutron star EOS. The stability of the modified Tolman VII solution has recently been analyzed (Posada et al in Phys Rev D 103(10):104067, 2021), which provides a critical value of the adiabatic index above which the stellar configuration becomes unstable against radial oscillations. In this paper, making use of the generalized version of the Tolman VII solution, we prescribe an upper bound on the compactness (M/R) beyond which the star becomes unstable against radial oscillations (Chandrasekhar in Phys Rev Lett 12(4):114, 1964). Our study brings to attention the role of model parameters in the generalized Tolman VII solution. The analysis also provides new insight into the role of inhomogeneity of the matter distribution vis-a-vis equation of state (EOS) on the compactness of a relativistic star. [ABSTRACT FROM AUTHOR]

(1) Purpose: Conditions of formation of compound nuclear systems needed for synthesis of heavy nuclei in pycnonuclear reactions in compact stars are studied on a quantum mechanical basis. (2) Methods: The method of multiple internal reflections is applied for pycnonuclear reactions in compact stars with new calculations of quasibound spectra and spectra of zero-point vibrations. (3) Results: Peculiarities of the method are analyzed for reaction with isotopes of Carbon. The developed method takes into account continuity and conservation of quantum flux (describing pycnonuclear reaction) inside the full spacial region of reaction, including the nuclear region. This gives the appearance of new states (called quasibound states) in which compound nuclear systems of Magnesium are formed with the largest probability. These states have not been studied yet in synthesis of elements in stars. Energy spectra of zero-point vibrations and spectra of quasibound states are estimated with high precision for reactions with isotopes of Carbon. For the first time, the influence of plasma screening on quasibound states and states of zero-point vibrations in pycnonuclear reactions has been studied. (4) Conclusions: The probability of formation of a compound nucleus in quasibound states in pycnonuclear reaction is essentially larger than the probability of formation of this system in states of zero-point vibrations studied by Zel'dovich and followers. Therefore, synthesis of Magnesium from isotopes of Carbon is more probable through the quasibound states than through the states of zero-point vibrations in compact stars. Energy spectra of zero-point vibrations are changed essentially after taking plasma screening into account. Analysis shows that from all studied isotopes of Magnesium, only 24Mg is stable after synthesis at an energy of relative motion of 4.881 MeV of the incident nuclei 12C. [ABSTRACT FROM AUTHOR]

Making use of a particular solution for a class of relativistic compact stars, we develop a core–envelope model where the core region is assumed to be composed of strange matter admitting an equation of state p = 1 3 (ρ - 4 B) . In our model, we assume the bag parameter B of the MIT bag model to be density-dependent. For an anisotropic stellar configuration with two separate regions—the quark core-enveloped by baryonic matter—we analyse the effects of density and anisotropy on the bag value. Moreover, the effects of the background geometry of the configuration are studied. The physical features of the model are investigated. [ABSTRACT FROM AUTHOR]

In the present paper, we focused on exploring the possibility of providing a new class of exact solutions for viable anisotropic stellar systems by means of the massive Brans–Dicke (BD) theory of gravity. In this respect, we used the decoupling of gravitational sources by minimal geometric deformation (MGD) ( e − η = Ψ + β h ) for compact stellar objects in the realm of embedding class-one space-time to study anisotropic solutions for matter sources through the modified Einstein field equations. For this purpose, we used the ansatz for Ψ relating to the prominent, well-known and well-behaved Finch–Skea model via Karmarkar condition, and the determination scheme for deformation function h (r) was proposed via mimic requirement on radial pressure component: θ 1 1 (r) = p r (r) and matter density: θ 0 0 (r) = ρ (r) for the anisotropic sector. Moreover, we analyzed the main physical highlights of the anisotropic celestial object by executing several physical tests for the case θ 1 1 (r) = p r (r) . We have clearly shown how the parameters α , β and ω B D introduced by massive BD gravity via the MGD approach incorporating the anisotropic profile of the matter distribution have an immense effect on many physical parameters of compact bodies such as LMC X-4, LMC X-4, Her X-1, 4U 1820-30, 4U 1608-52, SAX J1808.4–658 and many others that can be fitted. [ABSTRACT FROM AUTHOR]

Quantum effects in pycnonuclear reactions in compact stars at zero temperatures are studied with high precision. The reaction 16O + 16O was analyzed using the method of multiple internal reflections. The study of such reactions requires full consideration of quantum fluxes in the internal nuclear region. This reduces the rate and number of pycnonuclear reactions up to 1.8 times. This leads to the appearance of new states (which we call quasibound states) where the compound nuclear system is formed with maximal probability. As shown, the minimal energy of such a state is slightly higher than the energy of zero-mode oscillations in the lattice nodes in the pycnonuclear reaction, however, the probability of the formation of a compound system in a quasibound state is significantly greater than the corresponding probability in a state of zero-mode oscillations. It is reasonable to say that the frequency of reactions in quasi-bound states is more likely than in states of zero-mode oscillations. This can lead to significant changes in estimates of reaction rates in stars. [ABSTRACT FROM AUTHOR]

In this work, we consider static quark star (QS) within the framework of Rastall gravity. Rastall gravity purports to be the nonconservative theory of gravity and an unusual nonminimal coupling between matter and geometry. In our study, we consider a Quantum chromodynamics (QCD)-motivated Equation of State (EoS) to determine the properties of QSs in Rastall gravity. Depending on the values of parameters, we seek to determine the mass–radius relations for QSs in Rastall gravity, identifying the deviation from standard general relativity (GR) counterparts. Interestingly, we find the value of the maximum gravitational mass to be more than 2 M ⊙ for the given equation of state (EoS). We present the essential features regarding the stability of QSs. [ABSTRACT FROM AUTHOR]

In our previous study, (Eur Phys J Plus 135:362, 2020 & Eur Phys J Plus 135:637, 2020), we have discussed the possible existence of the dark-matter-admixed pulsars, located in dwarf as well as in massive spiral galaxies (based on Singular Isothermal Sphere dark-matter density profile) and in the Milky Way galaxy (based on Universal Rotational Curve dark-matter density profile). In this article, we use the Navarro–Frenk–White (NFW) dark-matter density profile to get analogous results for the pulsars in the disk region of the Milky Way galaxy. These findings may be treated as valuable complements to the previous findings. We conclude from our findings that there is a unique possibility of the presence of dark-matter-admixed pulsars in all the regions of the galaxies. [ABSTRACT FROM AUTHOR]

(1) Purpose: Conditions of formation of compound nuclear systems needed for synthesis of heavy nuclei in pycnonuclear reactions in compact stars are studied on a quantum mechanical basis. (2) Methods: The method of multiple internal reflections is applied for pycnonuclear reactions in compact stars with new calculations of quasibound spectra and spectra of zero-point vibrations. (3) Results: Peculiarities of the method are analyzed for reaction with isotopes of Carbon. The developed method takes into account continuity and conservation of quantum flux (describing pycnonuclear reaction) inside the full spacial region of reaction, including the nuclear region. This gives the appearance of new states (called quasibound states) in which compound nuclear systems of Magnesium are formed with the largest probability. These states have not been studied yet in synthesis of elements in stars. Energy spectra of zero-point vibrations and spectra of quasibound states are estimated with high precision for reactions with isotopes of Carbon. For the first time, the influence of plasma screening on quasibound states and states of zero-point vibrations in pycnonuclear reactions has been studied. (4) Conclusions: The probability of formation of a compound nucleus in quasibound states in pycnonuclear reaction is essentially larger than the probability of formation of this system in states of zero-point vibrations studied by Zel’dovich and followers. Therefore, synthesis of Magnesium from isotopes of Carbon is more probable through the quasibound states than through the states of zero-point vibrations in compact stars. Energy spectra of zero-point vibrations are changed essentially after taking plasma screening into account. Analysis shows that from all studied isotopes of Magnesium, only 24Mg is stable after synthesis at an energy of relative motion of 4.881 MeV of the incident nuclei 12C.

In recent years, a class of compact objects called gravastars have drawn immense interest as regular solutions to end state stellar collapse. Since the energy density involved in collapse process is expected to be high, it is a natural choice to study such compact objects in context of modified gravity theories which reduce to General Relativity (GR) in the low energy regime. We have already framed a model of gravastar in such a modified gravity framework involving extra dimensional Randall–Sundrum (RS) single brane gravity [R. Sengupta, S. Ghosh, S. Ray, B. Mishra and S. K. Tripathy, Phys. Rev. D 102, 024037 (2020)]. As a sequel in this paper, we substantially improve our previous model by choosing the Kuchowicz function as one of the metric potentials, which leads to many new interesting results and physical features from our analysis as discussed in this paper. Also, we provide essential additional stability checks on our gravastar model to investigate the possibility of any instability creeping in due to the higher-dimensional framework. Our present improved gravastar model is found to clear all the stability checks successfully. Very interestingly, the static spherically symmetric matter distributions are found to accommodate both classes of solutions obeying and violating the modified energy conditions on the RS brane as we find in this work. We can conclude from our analysis that the Kuchowicz metric potential is very effective for describing regular solutions to compact objects at substantially high energies on the three-brane. [ABSTRACT FROM AUTHOR]

For a static, spherically symmetric, anisotropic and charged distribution of matter, we present a new class of exact solutions to the Einstein-Maxwell system. By assuming specific forms of the electric field and anisotropy, we transform the master equation of the Einstein-Maxwell system to Bessel and modified Bessel differential equations. The subsequent solutions turn out to be generalizations of the isotropic and uncharged stellar model of Finch and Skea (1989), the isotropic and charged stellar model of Hansraj and Maharaj (2006) and the anisotropic and charged stellar model of Maharaj et al. (2017). We analyze the physical viability of the solutions and utilize one particular class of solutions to examine the effects of charge and anisotropy on the mass-radius relationship of compact stars. [ABSTRACT FROM AUTHOR]

In the present paper, we focused on exploring the possibility of providing a new class of exact solutions for viable anisotropic stellar systems by means of the massive Brans–Dicke (BD) theory of gravity. In this respect, we used the decoupling of gravitational sources by minimal geometric deformation (MGD) (e−η=Ψ+βh) for compact stellar objects in the realm of embedding class-one space-time to study anisotropic solutions for matter sources through the modified Einstein field equations. For this purpose, we used the ansatz for Ψ relating to the prominent, well-known and well-behaved Finch–Skea model via Karmarkar condition, and the determination scheme for deformation function h(r) was proposed via mimic requirement on radial pressure component: θ11(r)=pr(r) and matter density: θ00(r)=ρ(r) for the anisotropic sector. Moreover, we analyzed the main physical highlights of the anisotropic celestial object by executing several physical tests for the case θ11(r)=pr(r). We have clearly shown how the parameters α, β and ωBD introduced by massive BD gravity via the MGD approach incorporating the anisotropic profile of the matter distribution have an immense effect on many physical parameters of compact bodies such as LMC X-4, LMC X-4, Her X-1, 4U 1820-30, 4U 1608-52, SAX J1808.4–658 and many others that can be fitted.

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 ansatz for one of the metric potentials g r r 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., has 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 by 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 (MR) 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 allows us to estimate central density for a given radius (or mass) of a compact star. We have explored the tidal deformabilities by calculating the Love numbers and showed the variation of tidal Love numbers with the central pressure of a star. [ABSTRACT FROM AUTHOR]

This paper explores a new embedding anisotropic charged version of a solution to Einstein–Maxwell field equations in four-dimensional spacetime through the Karmarkar conditions and the gravitational decoupling via minimal geometric decoupling (MGD) technique by choosing Pant's interior solution [Astrophys. Space Sci. 331, 633 (2011)] as a seed solution to coupled system. Later, we integrate the coupled system within the MGD and explore a family of solutions to represent the realistic structure of nonrotating compact objects. Through the matching of the interior solutions so obtained to the exterior Reissner–Nordström metric, we tune the arbitrary constants for feasible models. After that, we subject our model to a rigorous test for a chosen parameter space to verify the physical viability of the solution for the neutron stars in EXO 1785-248 for a range of values of the decoupling constant σ. Further, we prove that the constant σ is inherently connected to critical physical properties such as the gravitational and surface redshifts, compactification factor, mass/radius relation, etc., of the same compact star candidate EXO 1785-248. The solutions thus obtained exhibit physically viable features which are thoroughly demonstrated through graphical plots. [ABSTRACT FROM AUTHOR]

In this paper, we study novel classes of solutions characterizing the role of complexity on static and spherically symmetric self‐gravitating systems proposed by L. Herrera (Phys Rev D 97: 044010, 2018) in the gravitational decoupling background. We start by considering the minimal geometric deformation approach as a ground‐breaking tool for generating new physically viable models for anisotropic matter distributions by exploiting the Buchdahl and Tolman models. In both models, all solutions show similar results with a slight change in their magnitude for a non‐vanishing complexity factor i.e., 0≤β<1$0\le \beta < 1$, where β is a decoupling constant. However, under vanishing complexity factor i.e., β=1$\beta = 1$, the minimally deformed anisotropic Buchdahl model yielded a constant density isotropic fluid distribution and anisotropic matter distribution becomes an isotropic fluid matter distribution without invoking any isotropy requirement. On the other hand, Tolman model possesses an increasing pressure when the complexity factor vanishes. Furthermore, we also extend our findings by calculating the mass‐complexity factor relationship for both presented models, revealing that the mass is larger for small values of the decoupling constant β and the complexity factor YTF$Y_{TF}$. In this paper, novel classes of solutions will be studied characterizing the role of complexity on static and spherically symmetric self‐gravitating systems proposed by L. Herrera ([10]) in the gravitational decoupling background. The authors start by considering the minimal geometric deformation approach as a ground‐breaking tool for generating new physically viable models for anisotropic matter distributions by exploiting the Buchdahl and Tolman models. In both models, all solutions show similar results with a slight change in their magnitude for a non‐vanishing complexity factor i.e., 0 ≤ β < 1. However, under vanishing complexity factor i.e., β = 1, the minimally deformed anisotropic Buchdahl model yielded a constant density isotropic fluid distribution and anistropic matter distribution becomes an isotropic fluid matter distribution without invoking any isotropy requirement. On the other hand, Tolman model possesses an increasing pressure when complexity factor vanishes. Furthermore, the findings are extended by calculating the mass‐complexity factor relationship for both presented models, revealing that the mass is larger for small values of the decoupling constant β and the complexity factor YTF. [ABSTRACT FROM AUTHOR]

In this paper, we study the curved Krori–Barua spacetime geometry to describe compact stars with two components: dark and ordinary matter, using a two-fluid model approach. We choose the equation of state derived from the rotational curves of galaxies for dark matter and the polytropic equation of state for ordinary matter. The physical requirements for a realistic compact star are satisfied using specific parameters. For the polytropic index η = 2 , we find the values of observed masses, reported in our previous investigation (Mafa et al., 2020). • Modeling Compact Stars with a Dual-Component Approach. • Component 1: Ordinary Matter with a Polytropic Equation of State. • Component 2: Dark Matter with an Equation of State Derived from Rotational Curves. • Comprehensive Analysis for Compact Object Modeling. [ABSTRACT FROM AUTHOR]

The Vaidya and Tikekar (1982) ansatz has so far provided numerous stellar solutions which are useful for the description of compact stars like neutron stars and quark stars. We re-examine the impacts of the spacetime curvature parameter of the Vaidya–Tikekar ansatz on the physical properties of the relativistic anisotropic star. For a spherically symmetric, static and anisotropic stellar configuration, in the Vaidya–Tikekar background spacetime, we develop new exact solutions by assuming a polytropic type equation of state p r = k ρ 1 + 1 n − β. Recent data from observed pulsars are taken as input parameters to validate the developed model. We examine the impact of the spheroidal curvature parameter of the VT spacetime on the stellar behaviour and mass–radius (M–R) relationship. It is observed that with the increase in the departure from spherical geometry, values of physical quantities, such as density and pressure, increase. • We have found a stable model of compact stars and fulfils all the regularity conditions and physical requirements. • An interesting feature of the model is that it provides a free parameter K, which makes it suitable for the description of a wide range of compact stars. [ABSTRACT FROM AUTHOR]

In this paper, we review how the "cusp" predicted in the nuclear symmetry energy generated by a topology change at density n 1 / 2 ≳ 2 n 0 can have a surprising consequence, so far unrecognized in nuclear physics and astrophysics communities, on the structure of dense compact-star matter. The topology change translated into nuclear EFT with "effective" QCD degrees of freedom encoded in hidden local and scale symmetries predicts an EoS that is soft below and stiff above n ≳ n 1 / 2 , and yields the properties of neutron stars with no tension with all the astrophysical observations available up to date. Furthermore it describes the interior core of the massive stars populated by fractionally charged quasi-fermions that are neither baryonic nor quarkonic. It is argued that the cusp "buried" in the symmetry energy resulting from strong correlations with hidden heavy degrees of freedom leads, at n ≳ n 1 / 2 , to a "pseudo-conformal" sound speed, v pcs 2 / c 2 ≈ 1 / 3 , converged from below at n 1 / 2 . It is not conformal since the trace of energy–momentum tensor is not zero even in the chiral limit. It reflects an emergent scale symmetry. This observation with the topology change implies that the quantities accurately measured at ∼ n 0 cannot give a qualitatively stringent constraint for what takes place at the core density of compact stars ∼ (3 – 7) n 0 . This is because there intervenes a change of degrees of freedom in the effective field theory. We discuss the implication of this on the recent PREX-II "dilemma" in the measured skin thickness of 2 0 8 Pb. [ABSTRACT FROM AUTHOR]

In this paper we present two new classes of solutions describing compact objects within the framework of five‐dimensional Einstein‐Gauss‐Bonnet (EGB) gravity. We employ the Complete Geometric Deformation (CGD) formalism which extends the Minimal Geometric Deformation (MGD) technique adopted in earlier investigations to generate anisotropic models from known isotropic solutions. The two solutions presented arise from mimicking the constraint for the pressure and density respectively which generate independent deformation functions. Rigorous physical tests show that contributions from CDG suppress the effective pressure but enhances the effective density and mass of the compact object, with the suppression/enhancement being modified by the EGB coupling constant. One of the highlights in our findings is that the deformation function along the radial component in CDG is nonzero at the boundary when we mimic both the pressure and density while in MGD we observe a vanishing of this deformation function at the boundary of the fluid configuration only for the pressure constraint. The difference in behavior of the deformation function at the surface predicts different stellar characteristics such as mass‐to‐radius and surface redshifts. In this paper two new classes of solutions are presented describing compact objects within the framework of five‐dimensional Einstein‐Gauss‐Bonnet (EGB) gravity. The Complete Geometric Deformation (CGD) formalism will be employed which extends the Minimal Geometric Deformation (MGD) technique adopted in earlier investigations to generate anisotropic models from known isotropic solutions. The two solutions presented arise from mimicking the constraint for the pressure and density respectively which generate independent deformation functions. Rigorous physical tests show that contributions from CDG suppress the effective pressure but enhances the effective density and mass of the compact object, with the suppression/enhancement being modified by the EGB coupling constant. One of the highlights in the findings is that the deformation function along the radial component in CDG is nonzero at the boundary when one mimics both the pressure and density while in MGD one observes a vanishing of this deformation function at the boundary of the fluid configuration only for the pressure constraint. The difference in behavior of the deformation function at the surface predicts different stellar characteristics such as mass‐to‐radius and surface redshifts. [ABSTRACT FROM AUTHOR]

In this work we model compact stars with an equation of state of dark matter with admixed matter. Physical features of an exact solution of the Einstein system are explored by incorporating the effects of linear and nonlinear terms in the equation of state. We present the upper and lower bound masses for our model, and perform a graphical analysis of certain matter variables. We show the influence of the nonlinear term in the equation of state on physical proprieties of the matter distribution within the compact object. In particular the maximum stellar mass of PSRJ0030+0451 is found for the admixed matter distribution. [ABSTRACT FROM AUTHOR]

In this paper we obtain relativistic solutions of compact stars in higher dimensions assuming a pseudo-spheroidal geometry comprising of anisotropic fluid in hydrostatic equilibrium. In the Vaidya-Tikekar model the ansatz for one metric potential is parametrized by two parameter namely the spheroidal parameter (λ) and curvature parameter (R ). In this model we obtained a toy model solution for anisotropic fluid satisfying the acoustic condition. For a suitable combination of parameters λ , R and anisotropic parameter α , we note that the equation of state of interior matter content of anisotropic fluid is resemble with the MIT bag model equation of state of the form p = 1 3 (ρ − 4 B) for quark matter with the value of B lying in the range necessary bulk strange matter relative to neutron. Here B is known as Bag constant. In this model the maximum radius of a star is depend on the spheroidal parameter (λ) , dimension (D) and anisotropic parameter (α) collectively. We also note that the limiting values of λ for stellar toy model in isotropic case get modified in presence of anisotropy. In case of higher dimensional anisotropic star, we note the limiting values of λ as λ < λ 1 and λ 2 < λ < λ 1 , where λ 1 = D 2 − 6 D + 1 + 2 α D 3 + (1 − 7 α) D 2 + (2 + 15 α) D + (1 − 9 α) D 2 − 8 D − 1 + 12 α − 4 D α and λ 2 = D − 1 + 2 α + 2 α 2 + (D − 1) α + 1 D − 3 . The non-negativity of energy density (ρ) also puts another bound on spheroidal parameter (λ) with a lower limit. We also evaluated the maximum mass, surface red-shift and checked the causality and stability conditions. We also note that a large number of space-time dimensions allowed in brane-world are not permitted in the context of compact star. [ABSTRACT FROM AUTHOR]

In this paper, we present a new exact anisotropic charged version of Pant's interior solution [Astrophys. Space Sci. 331, 633 (2011)] by employing the embedding class I condition and the gravitational decoupling via minimal geometric decoupling (MGD) approach. We successfully integrate the coupled Einstein–Maxwell field equations within the MGD and generate a family of new charged anisotropic solutions describing compact stellar objects. The matching of the interior to the exterior Reissner–Nordstrom solution help fix the arbitrary constants. For a chosen parameter space we subject our model to rigorous tests in order to validate its physical viability as a realistic self-gravitating, compact object. We show that the deformation parameter σ is intrinsically connected to key features such as compactness, mass-to-radius ratio and surface redshift for compact star candidates such as Her X-1, EXO 1785-248, PSR J1614-2230 and SAX J1808.4-3658. [ABSTRACT FROM AUTHOR]

In our previous study, (Eur Phys J Plus 135:362, 2020 & Eur Phys J Plus 135:637, 2020), we have discussed the possible existence of the dark-matter-admixed pulsars, located in dwarf as well as in massive spiral galaxies (based on Singular Isothermal Sphere dark-matter density profile) and in the Milky Way galaxy (based on Universal Rotational Curve dark-matter density profile). In this article, we use the Navarro–Frenk–White (NFW) dark-matter density profile to get analogous results for the pulsars in the disk region of the Milky Way galaxy. These findings may be treated as valuable complements to the previous findings. We conclude from our findings that there is a unique possibility of the presence of dark-matter-admixed pulsars in all the regions of the galaxies.

For the first time, we present Einstein's cluster model in embedding class one spacetime. This paper shows that for any neutral configurations there is only one Einstein cluster solution in embedding class one. In fact, one can find two solutions where the first solution i.e. g r r = e λ = 1 and g t t = e ν = C is an unphysical one as it has zero density profile as well as violates the Pandey–Sharma condition (i.e. not a class one solution). However, the second solution can describe matter distribution representing Einstein's cluster which is in static and equilibrium as it satisfies the static stability criterion and TOV-equation. The second solution not only satisfies the above conditions, but also satisfies the energy conditions. The equation of state parameter ω t is less than unity signifying that it can represent physical matters. Further, we have also shown that the Einstein's clusters may also exhibit the properties of compact stars. [ABSTRACT FROM AUTHOR]

We present a precise definition of a conserved quantity from an arbitrary covariantly conserved current available in a general curved space–time with Killing vectors. This definition enables us to define energy and momentum for matter by the volume integral. As a result we can compute charges of Schwarzschild and BTZ black holes by the volume integration of a delta function singularity. Employing the definition we also compute the total energy of a static compact star. It contains both the gravitational mass known as the Misner–Sharp mass in the Oppenheimer–Volkoff equation and the gravitational binding energy. We show that the gravitational binding energy has the negative contribution at maximum by 68% of the gravitational mass in the case of a constant density. We finally comment on a definition of generators associated with a vector field on a general curved manifold. [ABSTRACT FROM AUTHOR]

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 μ ν for the expression of f (R , T) , i.e., f (R , T) = R + 2 γ T to obtain the proposed model, where γ 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 ρ as p r = α ρ − β , where α and β are some positive constants, Both α , β depend on coupling constant γ which have been also depicted in this paper. By employing the Krori-Barua 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 γ have been studied on the model parameters like density, pressures, anisotropic factor, compactness, surface redshift, etc. both numerically and graphically. A suitable range for γ 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 γ → 0 . [ABSTRACT FROM AUTHOR]

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 (Durgapal, 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 (Rawls et al., 2011) with mass = (1. 04 ± 0. 09) M ⊙ and radius = 8. 30 1 − 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. • The aim is to investigate new static, non-singular, spherically symmetric fluid model in f(R, T) gravity. • We consider the isotropic Durgapal-IV metric potentials to obtain our model. • We have chosen the compact star candidate LMC X-4 for our investigation. • We use both analytical discussion and graphical illustrations by varying coupling parameter values. • We find that our proposed model is physically viable and well behaved. [ABSTRACT FROM AUTHOR]

In this work, we examine a spherically symmetric compact body with isotropic pressure profile. In this context we obtain a new class of exact solutions of Einstein-Maxwell field equation for compact stars with uniform charged distributions on the basis of Pseudo-spheroidal space-time with a particular form of electric field intensity and the metric potential g 11 . Taking these two parameters into account further examination has been done to decide unknown constants and to depict several compact strange star candidates. By the isotropic Tolman-Oppenhimer-Volkoff(TOV) equation, we explore the equilibrium among hydrostatic, gravitational and electric forces. Then, we analyze the stability of the model through adiabatic index(γ ) and velocity of sound (0 < d p c 2 d ρ < 1 ). We additionally talk about other physical features of this model e.g. pressure, redshift, density, energy conditions and mass-radius ratio of the stars in detail and demonstrated that our results satisfied all the basic prerequisites of a physically legitimate stellar model, showing density, pressure, pressure-density ratio, redshift and speed of sound are monotonically decreasing. The outcomes acquired are valuable in exploring the strength of other compact objects like white dwarfs, gravastars and neutron stars. Finally, we have shown that the obtained solutions are compatible with observational data for compact objects. [ABSTRACT FROM AUTHOR]

A class of solutions of Einstein field equations satisfying Karmarkar embedding condition is presented which could describe static, spherical fluid configurations, and could serve as models for compact stars. The fluid under consideration has unequal principal stresses i.e. fluid is locally anisotropic. A certain physically motivated geometry of metric potential has been chosen and codependency of the metric potentials outlines the formation of the model. The exterior spacetime is assumed as described by the exterior Schwarzschild solution. The smooth matching of the interior to the exterior Schwarzschild spacetime metric across the boundary and the condition that radial pressure is zero across the boundary lead us to determine the model parameters. Physical requirements and stability analysis of the model demanded for a physically realistic star are satisfied. The developed model has been investigated graphically by exploring data from some of the known compact objects. The mass-radius (M-R) relationship that shows the maximum mass admissible for observed pulsars for a given surface density has also been investigated. Moreover, the physical profile of the moment of inertia (I) thus obtained from the solutions is confirmed by the Bejger–Haensel concept. [ABSTRACT FROM AUTHOR]

In the present paper, a new model for perfect fluid sphere filled with charge, in higher-dimensional spacetime, admitting conformal symmetry has been investigated. We have considered a linear equation of state, with coefficients fixed by the matching conditions, at the boundary of the source corresponding to the exterior Reissner–Nordström higher-dimensional spacetime. Several physical features for different dimensions, starting from four up to eleven, have been briefly discussed. It has been shown that all the features as obtained from the present model are physically desirable. [ABSTRACT FROM AUTHOR]

The effect of chemical potential on boson star rotation is studied under weak-field approximations after incorporating the chemical potential in the potential field of boson star. With the effect of chemical potential, the rotation amplitude is enhanced from the earlier work. It indicates that in chemically rich system, the rotation of boson star with the perturbation of the chemical potential amplifies the rotation amplitudes of boson star. [ABSTRACT FROM AUTHOR]

Recently, Hod showed that neutral stationary scalar field clouds can exist outside neutral reflecting compact stars. In the present paper, we extend the discussion by considering charged stationary scalar fields in the background of charged reflecting compact stars. If stationary scalar clouds exist, we analytically show that the field frequency belongs to an interval. For certain discrete field frequency values in the interval, we obtain numerical solutions of scalar field clouds. We also study effects of model parameters on discrete frequency. [ABSTRACT FROM AUTHOR]

The present article is an attempt to arrive at a new solution for an isotropic charged compact star model admitting Karmarkar's condition in the context of general relativity. For this we chose a new metric potential e λ and then determine another gravitational potential e ν by using embedding class one condition. Different physical viewpoints have been investigated logically to watch the behavior of compact stars such as TOV condition, vitality conditions, Buchdahl condition, stability investigation, compactness and surface redshift by considering a particularized metric potential and the electric field. Graphical examination of these physical parameters involved within the gotten arrangement is additionally displayed to watch their behavior more clearly. The main physical and geometrical properties of the present charged model are quite consistent with compact stellar observational objects like SAXJ1808.4-3658, LMC X-4, CenX-3, PSRJ1614-2230, and PSR J1903+327. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We study the possibility of forming anisotropic compact stars in the framework of f(R)-modified gravity in a static spherically symmetric space-time. We find the unknown coefficients involved in the metric using masses and radii of the compact stars 4U 1820-30, Cen X-3, EXO 1785-248, and LMC X-4. We obtain the hydrostatic equilibrium equation for different forces and use the generalized Tolman-Oppenheimer-Volkoff equation to analyze the behavior of stars. Moreover, we verify the regularity conditions, anisotropic behavior, energy conditions, and stability of the compact stars. We use the effective energy-momentum tensor in f(R) gravity for the analysis. We show that in the framework of f(R) gravity theory, these compact stars have physically acceptable patterns. Our results here also agree with those in general relativity, which is a special case of f(R) gravity. [ABSTRACT FROM AUTHOR]

In the present article, the Eiesland condition has been used to obtain a new solution for compact star model by considering a non-singular well behaved gravitational potential of the form e λ (r) = 1 + a r 2 [ 1 + tanh (b r 2 + c) ] n in the framework of anisotropic matter distribution. The solution so obtained is physically acceptable, which is exploited to compare the predicted masses and radii of known compact object as EXO 1785-248 (M = 1.3 M ⊙ ) for n = 3 to 15. Moreover, the obtained solution satisfies the causality condition, Herrera cracking criterion, Tolman-Oppenheimer-Volkoff (TOV) equation, and adiabatic index Γ including all energy conditions. It is noted that the velocity of sound is increasing at n = 3 and start decreasing when n ≥ 6 which shows that the parameter n plays an important role to describe a well-behaved solution for anisotropic compact object. The moment of inertia (I) is also obtained by Bejger-Haensel formula for n = 3 to 15. In addition to that, the maximum mass for the compact star has been discovered via. M − R curve for different values of n . [ABSTRACT FROM AUTHOR]

Abstract We obtain a new anisotropic solution for spherically symmetric spacetimes by analyzing the Karmarkar embedding condition. For this purpose we construct a suitable form of one of the gravitational potentials to obtain a closed form solution. This form of the remaining gravitational potential allows us to solve the embedding equation and integrate the field equations. The resulting new anisotropic solution is well behaved, which can be utilized to construct realistic static fluid spheres. Also we estimated the masses and radii of fluid spheres for LMC X-4, EXO 1785-248, PSR J1903+327 and 4U 1820-30 by using observational data set values. The masses and radii obtained show that our anisotropic solution can represent fluid spheres to a very good degree of accuracy. The physical validity of the solution depends on the parameter values of a, b and c. The solution is well behaved for the wide range of parameters values $$0.00393\le a \le 0.0055$$ 0.00393 ≤ a ≤ 0.0055 , $$0.0002 \le b \le 0.0025$$ 0.0002 ≤ b ≤ 0.0025 and $$0.0107 \le c \le 0.0155$$ 0.0107 ≤ c ≤ 0.0155 . The range of corresponding physical parameters for the different compact stars are $$0.3266\le v_{r0} \le 0.3708$$ 0.3266 ≤ v r 0 ≤ 0.3708 , $$0.1583\le v_{t0} \le 0.2558$$ 0.1583 ≤ v t 0 ≤ 0.2558 , $$0.3256\le z_{s} \le 0.4450$$ 0.3256 ≤ z s ≤ 0.4450 and $$4.3587\le \Gamma _{0} \le 5.6462$$ 4.3587 ≤ Γ 0 ≤ 5.6462 .

In literature, there are three simplest methods of solving Einstein's field equations, namely, (a) assuming conformally flat spacetime, (b) using conformal killing vector and (c) using Karmarkar conditions. In all these approaches the two metric functions gtt and grr are link via a bridge. However, the first two approaches are facing a critical failure while determining central red-shift while the last method always yields well-behaved solution. Therefore, we are adopting the last method and discover a generalized class one solution. It is found that the maximum mass and radius of the compact star describe by the solution strongly depends on the parameter n. As n increases the maximum mass and radius also increases. For n=3.3, Mmax=1.459M⊙ and Rmax=9.52 km, and for n=4.8 have Mmax=1.766M⊙ with Rmax=10.31 km. For n=4.8 the equation of state is behaving linearly as the speed of sound is almost constant at 0.333. In overall the presented solution is well-behaved in all respects.