Gravitomagnetic Effects on Collective Plasma Oscillations in Compact Stars

The effects of gravitomagnetic force on plasma oscillations are investigated using the kinetic theory of homogeneous electrically neutral plasma in the absence of external electric or magnetic field. The random phase assumption is employed neglecting the thermal motion of the electrons with respect to a fixed ion background. It is found that the gravitomagnetic force reduces the characteristic frequency of the plasma thus enhancing the refractive index of the medium. The estimates for the predicted effects are given for a typical white dwarf, pulsar, and neutron star.

Compact stars admitting Finch-Skea symmetry in the presence of various matter fields

In the present study,we investigate the anisotropic stellar solutions admitting Finch-Skea symmetry(viable and non-singular metric potentials)in the presence of some exotic matter fields,such as Bose-Einstein Condensate(BEC)dark matter,the Kalb-Ramond fully anisotropic rank-2 tensor field from the low-energy string theory effective action,and the gauge field imposing U(1)symmetry.Interior spacetime is matched with both Schwarzchild and Reissner-N?rdstrom vacuum spacetimes for BEC,KB,and gauge fields.In addition,we study the energy conditions,Equation of State(EoS),radial derivatives of energy density and anisotropic pressures,Tolman-OppenheimerVolkoff equilibrium condition,relativistic adiabatic index,sound speed,and surface redshift.Most of the aforementioned conditions are satisfied.Therefore,the solutions derived in the current study lie in the physically acceptable regime.

Physical aspects of anisotropic compact stars in f(T,T) gravity with off diagonal tetrad

This study addresses the formation of anisotropic compact star models in the background of f(T,T)gravity(where T and T represent the torsion and trace of the energy momentum tensor,respectively).f(T,T)gravity is an extension of the f(T)theory,and it allows a general non-minimal coupling between T and T.In this setup,we apply Krori and Barua's solution to the static spacetime with the components ξ=Br^(2)+c and ψ=Ar^(2).To develop viable solutions,we select a well-known model f(T,T)=αT^(m)+βT+Ф(where α and β are coupling parameters,and Ф indicates the cosmological constant).We adopt the conventional matching of interior and exterior space time to evaluate the unknowns,which are employed in the stellar configuration.We present a comprehensive discussion on the stellar properties to elaborate the anisotropic nature of compact stars corresponding to well-known models:PSRJ1416-2230,4U1608-52,CenX-3,EXO1785-248,and SMCX-1.Via physical analysis,it is observed that the solution of compact spheres satisfy the acceptability criteria,and its models behave optimally and depict stability and consistency,in accordance with f(T,T)gravity.

Anisotropic compact stars in Karmarkar spacetime

We present a new class of solutions to the Einstein field equations for an anisotropic matter distribution in which the interior space-time obeys the Karmarkar condition. The necessary and sufficient condition required for a spherically symmetric space-time to be of Class One reduces the gravitational behavior of the model to a single metric function. By assuming a physically viable form for the grr metric potential we obtain an exact solution of the Einstein field equations which is free from any singularities and satisfies all the physical criteria. We use this solution to predict the masses and radii of well-known compact objects such as Cen X-3, PSR J0348＋0432, PSR B0943＋10and XTE J1739-285.

Behavior of Anisotropic Compact Stars in f(R,∅) Gravity

The aim of this paper is to investigate modified f(R, ?) theory of gravity, where R and ? represent the Ricci scalar and scalar potential respectively. Specifically, we take the spherically symmetric spacetime to discuss the possible emergence of compact stars. We study the physical behavior of compact stars by considering 4 U 1820-30, SAX J1808-3658 and Her X1, which are three popular models of compact stars. The graphical analysis of energy density, radial pressure, tangential pressure, energy conditions as well as stability of compact stars has been shown. It is concluded that behavior of these three stars is usual for f(R, ?) gravity models with some specific choices of model parameters.

Exploring physical properties of compact stars in f(R,T)-gravity:An embedding approach

Solving field equations exactly in f(R,T)−gravity is a challenging task.To do so,many authors have adopted different methods such as assuming both the metric functions and an equation of state(EoS)and a metric function.However,such methods may not always lead to well-behaved solutions,and the solutions may even be rejected after complete calculations.Nevertheless,very recent studies on embedding class-one methods suggest that the chances of arriving at a well-behaved solution are very high,which is inspiring.In the class-one approach,one of the metric potentials is estimated and the other can be obtained using the Karmarkar condition.In this study,a new class-one solution is proposed that is well-behaved from all physical points of view.The nature of the solution is analyzed by tuning the f(R,T)−coupling parameterχ,and it is found that the solution leads to a stiffer EoS forχ=−1 than that forχ=1.This is because for small values ofχ,the velocity of sound is higher,leading to higher values of Mmax in the M−R curve and the EoS parameterω.The solution satisfies the causality condition and energy conditions and remains stable and static under radial perturbations(static stability criterion)and in equilibrium(modified TOV equation).The resulting M−R diagram is well-fitted with observed values from a few compact stars such as PSR J1614-2230,Vela X-1,Cen X-3,and SAX J1808.4-3658.Therefore,for different values ofχ,the corresponding radii and their respective moments of inertia have been predicted from the M−I curve.

Maximum Momentum,Minimal Length and Quantum Gravity Effects of Compact Star Cores

Based on the generalized uncertainty principle with maximum momentum arid minimal length, we discuss the equation of state of ideal ultra-relativistic Fermi gases at zero temperature. Maximum momentum avoids the problem that the Fermi degenerate pressure blows up since the increase of the Fermi energy is not limited. Applying this equation of state to the Tolman-Oppenheimer Volkoff （TOV） equation, the quantum gravitational effects on the cores of compact stars are discussed. In the center of compact stars, we obtain the singularity-free solution of the metric component, gtt ～-（1 ＋ 0.2185×r^2）. By numerically solving the TOV equation, we find that quantum gravity plays an important role in the region r～10^4α0（△x）min. Current observed masses of neutron stars indicate that the dimensionless parameter α0 cannot exceed 10^19.

Magnetic Field of a Compact Spherical Star under f(R,T) Gravity

We present the interior solutions of distributions of magnetized fluid inside a sphere in f（R, T） gravity. Tile magnetized sphere is embedded in an exterior Reissner NordstrOm metric. We assume that all physical quantities are in static equilibrium. The perfect fluid matter is studied under a particular form of the Lagrangian density f（R, T）. The magnetic field profile in modified gravity is calculated. Observational data of neutron stars are used to plot suitable models of magnetized compact objects. We reveal the effect of f（R, T） gravity on the magnetic field profile, with application to neutron stars, especially highly magnetized neutron stars found in x-ray pulsar systems. Finally, the effective potential Veff and innermost stable circular orbits, arising out of the motion of a test particle of negligible mass influenced by attraction or repulsion from the massive center, are discussed.

Physical properties and maximum allowable mass-radius relation of complexity-free compact stellar objects within modified f(R,T)gravity formalism

This paper investigates the physical properties and predicted radii of compact stars generated by the Tolman-IV complexity-free model within the background of modified gravity theory,particularly the f(R,T)-gravity theory,under complexity formalism for a spherically symmetric spacetime proposed by L.Herrera[Phys Rev D 97:044010,2018].By solving the resulting set of differential equations,we obtain the explicit forms of the energy-momentum(EM)tensor components,including the density,radial pressure,and tangential pressure.The influence of the parameterχon various physical properties of the star is thoroughly investigated.The model undergoes a series of rigorous tests to determine its physical relevance.The findings indicate that the model exhibits regularity,stability,and a surface with vanishing pressure.The boundary of this surface is determined by carefully selecting the parameter space.The complexity method employed in f(R,T)gravity offers an interesting approach for developing astrophysical models that are consistent with observable events as demonstrated by recent experiments.In this regard,we use observational data from the GW190814 event,detected by the LIGO and Virgo observatories,to investigate the validity of the Tolman-IV model in f(R,T)gravity.The analysis includes comparing the model's predictions with the observed characteristics of the compact object involved in the merger.In addition,data from two-millisecond pulsars,PSR J1614-2230 and PSR J0952-0607,are incorporated to further constrain the theoretical theories.However,we present a diagram depicting the relationship between the total mass and radius of the compact object candidates for different values ofχ.

Class of Charged Fluid Balls in General Relativity

In the present study, we have obtained a new analytical solution of combined Einstein-Maxwell field equations describing the interior field of a ball having static spherically symmetric isotropic charged fluid within it. The charge and electric field intensity are zero at the center and monotonically increasing towards the boundary of the fluid ball. Besides these, adiabatic index is also increasing towards the boundary and becomes infinite on it. All other physical quantities such as pressure, density, adiabatic speed of sound, charge density, adiabatic index are monotonically decreasing towards the surface. Causality condition is obeyed at the center of ball. In the limiting case of vanishingly small charge, the solution degenerates into Schwarzchild uniform density solution for electrically neutral fluid. The solution joins smoothly to the Reissner-Nordstrom solution over the boundary. We have constructed a neutron star model by assuming the surface density . The mass of the neutron star comes with radius 14.574 km.

Spherical Anisotropic Fluid Distribution in General Relativity

In the present investigation of a spherically symmetric electrically neutral anisotropic static fluid, we present a new solution of the Einstein’s general relativistic field equations. The solution shows positive finite central pressures, central density and central red shift. The causality condition is obeyed at the centre. The anisotropy parameter is zero at the center and monotonically increasing toward the surface. The adiabatic index is also increasing towards the surface. All the other physical quantities such as matter-energy density, radial pressure, tangential pressure, velocity of sound and red shift are monotonically decreasing towards the surface. Further by assuming the surface density , we have constructed a model of massive neutron star with mass 2.95 with radius 18 km with all degree of suitability.

A new class of viable and exact solutions of EFEs with Karmarkar conditions:an application to cold star modeling

In this work we present a theoretical framework within Einstein’s classical general relativity which models stellar compact objects such as PSR J1614-2230 and SAX J1808.4-3658.The Einstein field equations are solved by assuming that the interior of the compact object is described by a class I spacetime.The so-called Karmarkar condition arising from this requirement is integrated to reduce the gravitational behaviour to a single generating function.By appealing to physics we adopt a form for the gravitational potential which is sufficiently robust to accurately describe compact objects.Our model satisfies all the requirements for physically realistic stellar structures.

Detailed analysis of the relativistic configuration of Bardeen anisotropic spheres in modified f(G)gravity

The aim of this work is to investigate anisotropic compact objects within the framework of f(G)modified theory of gravity.For our present work,we utilize Krori-Barua metrics,i.e.,λ(r)=Xr^(2)+Y andβ(r)=Zr^(2).We use some matching conditions of spherically symmetric spacetime with Bardeen's model as an exterior geometry.Further,we establish some expressions of energy density and pressure components to analyze the stellar configuration of Bardeen compact stars by assuming viable f(G)models.We examine the energy conditions for different stellar structures to verify the viability of our considered models.Moreover,we also investigate some other physical features,such as equilibrium condition,equation of state parameters,adiabatic index,stability analysis,mass function,surface redshift,and compactness factor,respectively.It is worthwhile to mention here for the current study that our stellar structure in the background of Bardeen's model is more viable and stable.

Topology change and emergent scale symmetry in compact star matter via gravitational wave detection

Topological structure has been extensively studied and confirmed in highly correlated condensed matter physics. We explore the gravitational waves emitted from binary neutron star mergers using the pseudoconformal model for dense nuclear matter for compact stars. This model considers the topology change and the possible emergent scale symmetry and satisfies all the constraints from astrophysics. We find that the location of the topology change affects gravitational waves dramatically owing to its effect on the equation of state. In addition, the effect of this location on the waveforms of the gravitational waves is within the ability of the on-going and up-coming facilities for detecting gravitational waves, thus suggesting a possible way to measure the topology structure in nuclear physics.

Anisotropic strange quark star in Finch-Skea geometry and its maximum mass for non-zero strange quark mass(m_(s)≠0)

A class of relativistic astrophysical compact objects is analyzed in the modified Finch-Skea geometry described by the MIT bag model equation of state of interior matter,p=1/3(ρ-4B),where B is known as the bag constant.B plays an important role in determining the physical features and structure of strange stars.We consider the finite mass of the strange quark(m_(s)≠0) and study its effects on the stability of quark matter inside a star.We note that the inclusion of strange quark mass affects the gross properties of the stellar configuration,such as maximum mass,surface red-shift,and the radius of strange quark stars.To apply our model physically,we consider three compact objects,namely,(ⅰ) VELA X-1,(ⅱ) 4U 1820-30,and(ⅲ) PSR J 1903+327,which are thought to be strange stars.The range of B is restricted from 57.55 to Bstable(MeV/fm3),for which strange matter might be stable relative to iron(^(56)Fe).However,we also observe that metastable and unstable strange matter depend on B and ms.All energy conditions hold well in this approach.Stability in terms of the Lagrangian perturbation of radial pressure is studied in this paper.

Relativistic models of anisotropic superdense star in the regime of Karmarkar's condition

We obtained a new class of solutions for a relativistic anisotropic compact star by utilizing the Karmarkar embedding condition.To obtain the closed-form solution a suitable form of one of the gravitational potentials has been chosen to determine the other by analyzing the Karmarkar condition.The resulting solutions are found to be well-behaved and regular and could describe a compact stellar object.Considering the current estimated values of the mass and radius of the pulsar 4U1820-30 as input parameters,all the physically relevant parameters are shown to be well-behaved to a very good degree of accuracy.

Anisotropic stellar structures in the f(T)theory of gravity with quintessence via embedding approach

This work suggests a new model for anisotropic compact stars with quintessence in f(T)gravity by us-ing the off-diagonal tetrad and the power-law as f(T)=βT^(n),where T is the scalar torsion andβand n are real con-stants.The acquired field equations incorporating the anisotropic matter source along with the quintessence field,in f(T)gravity,are investigated by making use of the specific character of the scalar torsion T for the observed stars PSRJ1614-2230,4U1608-52.CenX-3,EXO1785-248,and SMCX-1.It is suggested that all the stellar struc-tures under examination are advantageously independent of any central singularity and are stable.Comprehensive graphical analysis shows that various physical features which are crucially important for the emergence of the stellar structures are conferred.

Recent progress on dense nuclear matter in skyrmion approaches

The Skyrme model provides a novel unified approach to nuclear physics. In this approach, single baryon, baryonic matter and medium-modified hadron properties are treated on the same footing. Intrinsic density dependence(IDD) reflecting the change of vacuum by compressed baryonic matter figures naturally in the approach. In this article, we review the recent progress on accessing dense nuclear matter by putting baryons treated as solitons, namely, skyrmions, on crystal lattice with accents on the implications in compact stars.

Strange quark star and the parameter space of the quasi-particle model

The properties of strange quark stars are studied within the quasi-particle model. Taking into account chemical equilibrium and charge neutrality, the equation of state(EOS) of(2+ 1)-flavor quark matter is obtained. We illustrate the parameter spaces with constraints from two aspects: one is based on the astronomical results of PSR J0740+ 6620 and GW 170 817,and the other is based on the constraints proposed from the theoretical study of a compact star that the EOS must ensure the tidal deformability Λ_(1.4)=190_(-120)^(+390) and support a maximum mass above 1.97M⊙. It is found that neither type of constraints can restrict the parameter space of the quasi-particle model in a reliable region and thus we conclude that the low mass compact star cannot be a strange quark star.

Neutron star cooling and GW170817 constraint within quark-meson coupling models

In the present work,we used five different versions of the quark-meson coupling(QMC)model to compute astrophysical quantities related to the GW170817 event and the neutron star cooling process.Two of the models are based on the original bag potential structure and three versions consider a harmonic oscillator potential to confine quarks.The bag-like models also incorporate the pasta phase used to describe the inner crust of neutron stars.With a simple method studied in the present work,we show that the pasta phase does not play a significant role.Moreover,the QMC model that satisfies the GW170817 constraints with the lowest slope of the symmetry energy exhibits a cooling profile compatible with observational data.