Surface Redshift Research Articles

Study of anisotropic compact stars with quintessence field and modified chaplygin gas in f(T) gravity

In this work, we get an idea of the existence of compact stars in the background of f(T) modified gravity where T is a scalar torsion. We acquire the equations of motion using anisotropic property within the spherically compact star with electromagnetic field, quintessence field and modified Chaplygin gas in the framework of modified f(T) gravity. Then by matching condition, we derive the unknown constants of our model to obtain many physical quantities to give a sketch of its nature and also study anisotropic behavior, energy conditions and stability. Finally, we estimate the numerical values of mass, surface redshift etc from our model to compare with the observational data for different types of compact stars.

Analysis of compact stars in logarithmic-corrected R2 gravity

We discuss the existence of compact stars in the context of [Formula: see text] gravity model, where additional logarithmic corrections are assumed. Here, [Formula: see text] is the Ricci scalar and [Formula: see text], [Formula: see text] are constant values. Further, the compact stars are considered to be anisotropic in nature, due to the spherical symmetry and high density. For this purpose, we derive the Einstein field equations by considering Krori–Barua spacetime. For our proposed model, the physical acceptability is verified by employing several physical tests like the energy conditions, Herrera cracking concept and stability condition. In addition to this, we also discuss some important properties such as mass–radius relation, surface redshift and the speed of sound are analyzed. Our results are compared with observational stellar mass data, namely, 4U 1820-30, Cen X-3, EXO 1785-248 and LMC X-4. The graphical representation of obtained solutions provide strong evidences for more realistic and viable stellar model.

Constraining values of bag constant for strange star candidates

We provide a strange star model under the framework of general relativity by using a general linear equation of state (EOS). The solution set thus obtained is employed on altogether 20 compact star candidates to constraint values of MIT bag model. No specific value of the bag constant ([Formula: see text]) a priori is assumed, rather possible range of values for bag constant is determined from observational data of the said set of compact stars. To do so, the Tolman–Oppenheimer–Volkoff (TOV) equation is solved by homotopy perturbation method (HPM) and hence we get a mass function for the stellar system. The solution to the Einstein field equations represents a nonsingular, causal and stable stellar structure which can be related to strange stars. Eventually, we get an interesting result on the range of the bag constant as [Formula: see text]. We have found the maximum surface redshift [Formula: see text] and shown that the central redshift ([Formula: see text]) cannot have value larger than [Formula: see text], where [Formula: see text]. Also, we provide a possible value of bag constant for neutron star with quark core using hadronic as well as quark EOS.

Analytic core envelope models for the Crab and the Vela pulsars

We study analytic core-envelope models obtained in Negi et al. (1989) under slow rotation. We have regarded in the present study, the lower bound on the estimate of moment of inertia of the Crab pulsar, $I_{\rm Crab,45} \geq 2$ (where $I_{45}=I/10^{45}\rm g{cm}^2$) obtained by Gunn and Ostriker (1969) as a round off value of the recently estimated value of $I_{\rm Crab,45} \geq 1.93$ (Bejger \& Haensel 2003) for the Crab pulsar. If this value of lower bound is combined with the other observational constraint obtained for the Crab pulsar (Crawford \& Demiansky 2003), $G_h = I_{\rm core}/I_{\rm total} \geq 0.7$ ( where $G_h$ is called the glitch healing parameter and represents the fractional moment of inertia of the core component in the starquake mechanism of glitch generation), the models yield the mass, $M$, and surface redshift, $z_a$, for the Crab pulsar in the range, $M = 1.79M_\odot - 1.88M_\odot$; $z_a = 0.374 - 0.393$ ($I_{45} = 2$) for an assigned value of the surface density, $E_a = 2\times 10^{14}\rm g{cm}^{-3}$ (like, Brecher and Caporaso 1976). This assigned value of surface density, in fact, is an outcome of the first observational constraint imposed on our models that further yields the mass $M = 1.96M_\odot $ and surface redshift $z_a=$ 0.414 ($I_{45}= 2$) for the values of $G_h \approx 0.12$, which actually belongs to the observed `central' weighted mean value for the Vela pulsar. These values of mass and surface redshift predict the energy of a gravitationally redshifted electron-positron annihilation line, $E (\rm {MeV}) = 0.511/(1+z_a)$ (Lindblom 1984) in the range about 0.396 - 401 MeV from the Crab and about 0.389 MeV from the Vela pulsar. The evidence of a line feature at about 0.40MeV from the Crab pulsar (Leventhal et al. 1977) agrees quite well with the finding of this study.

Neutron star under homotopy perturbation method

We obtain a mass function solving the Tolman–Oppenheimer–Volkoff (TOV) equation for isotropic and spherically symmetric system via homotopy perturbation method (HPM). Using the mass function we construct a stellar model which can be determined from the equation of state (EOS) parameter (ω) and a model parameter (n). With the help of Einstein field equations we develop three solutions which can describe different properties and the core–crust structure of neutron star (NS). Solution I is valid for NS having the inner and outer radius near the surface of the star. The star is physical up to the inner radius whereas negative density occurs and the energy conditions are violated in the upper region from the inner to outer radius. Solution II represents NS with high gravitational redshift as well as compactification factor. All the features of NS can be given by solution III which involves only the EOS parameter. Our model predicts maximum mass for a NS with the central density 5.5×1015 g/cm3 and surface redshift 0.69 is to be 2.01M⊙ for the EOS parameter ω=0.73. The predicted range for the surface redshift is 0.57<Zs<1.95 for the allowed ranges 8.4<n<10.9 and 1∕3<ω<1 in the presented NS model.

Analytical model of strange star in Durgapal spacetime

A new strange star model based on Durgapal IV metric (Durgapal 1982) is presented here.Here we have applied a specific method to study the inner physical properties of the compact objects 4U 1702-429, 2A 1822-371, PSR J1756-2251,PSR J1802-2124 and PSR J1713+0747.The main objective of our study is to determine central density ($\rho_{0}$), surface density ($\rho_{b}$), central pressure ($p_{0}$), surface redshift ($Z_{s}$), compactness and radius. Further we perform different tests to study the stability of our model and finally we are able to give an equation based on pressure and density i.e probable equation of state (EoS) which has an important significances in the field of Astrophysics.

Relativistic modeling of Vela X-1 using the Karmarkar condition

The objective of this work is to explore a new parametric class of exact solutions of the Einstein field equations coupled with the Karmarkar condition. Assuming a new metric potential [Formula: see text] with parameter (n), we find a parametric class of solutions which is physically well-behaved and represents compact stellar model of the neutron star in Vela X-1. A detailed study specifically shows that the model actually corresponds to the neutron star in Vela X-1 in terms of the mass and radius. In this connection, we investigate several physical properties like the variation of pressure, density, pressure–density ratio, adiabatic sound speeds, adiabatic index, energy conditions, stability, anisotropic nature and surface redshift through graphical plots and mathematical calculations. All the features from these studies are in excellent conformity with the already available evidences in theory. Further, we study the variation of physical properties of the neutron star in Vela X-1 with the parameter (n).

Gravastars with Kuchowicz metric potential

Abstract In the present article we are going to study different features of a Gravitationally Vacuum Condensate Star, i.e., Gravastar with the help of the Kuchowicz metric potential [B. Kuchowicz, Acta. Phys. Pol. 33, 541 (1968)] in ( 3 + 1 ) dimensional spacetime. The gravastar consists of three regions namely the interior, intermediate thin shell and the exterior. Our main objective is to study the non-singular solutions of the system considered here. Also we want to study different features of the shell region, composed of ultra relativistic plasma obeying Zel’dovich’s conjecture of stiff fluid, like the energy, proper length, entropy and surface redshift which will confirm the stability of our model. Using the Kuchowicz metric potential we have found another metric potential (i.e. e λ ) for the interior and shell region which is non-singular in its nature for both of the region. The exterior of the gravastar has been described as the Schwarzschild type. We have found the numerical values of different constants imposing the boundary conditions. This theoretical model of gravastar can completely overcome the singularity problem related to black holes and hence can be taken as a promising alternative to the classical black hole within the framework of Einstein’s General Relativity.

Compact star models in class I spacetime

In the present article, we have presented completely new exact, finite and regular class I solutions of Einstein’s field equations i.e. the solutions satisfy the Karmarkar condition. For this purpose needfully we have introduced a completely new suitable g_{rr} metric potential to generate the model. We have investigated the various physical aspects for our model such as energy density, pressure, anisotropy, energy conditions, equilibrium, stability, mass, surface and gravitational red-shifts, compactness parameter and their graphical representations. All these physical aspects have ensured that our proposed solutions are well-behaved and hence represent physically acceptable models for anisotropic fluid spheres. The models have satisfied causality and energy conditions. The presented models are also stable by satisfying Bondi condition and Abreu et al. condition, in equilibrium position and static by satisfying TOV equation, Harrison–Zeldovich–Novikov condition, respectively. For the parameters chosen in the paper are matching in modeling Vela X-1, Cen X-3, EXO 1785-248 and LMC X-4. The M–R graph generated from the solutions is matching the ranges of masses and radii for the considered compact stars. This work also estimated the approximate moment of inertia for the mentioned compact stars.

Stellar evolution of compact stars in curvature–matter-coupling gravity

This paper is devoted to the study of stellar evolution of compact objects whose energy density and pressure of the fluid are interlinked by means of MIT bag model and a realistic polytropic equation of state in the scenario of $f(R,T,Q)$ gravity, where $Q=R_{ab}T^{ab}$. We derive the field equations as well as the hydrostatic equilibrium equation and analyze their solutions numerically for $R+\delta Q$ functional form with $\delta$ being a coupling parameter. We discuss the dependence of various physical properties such as pressure, energy density, total mass and surface redshift on the chosen values of the model parameter. The physical acceptability of the proposed model is examined by checking the validity of energy conditions, causality condition, and adiabatic index. We also study the effects arising due to matter-curvature coupling on the compact stellar system. It is found that maximum mass point lies within the observational range which indicates that our model is appropriate to describe dense stellar objects.

Models of anisotropic compact stars in the Rastall theory of gravity

In the present work, we have studied the anisotropic compact stars in the regime of Rastall theory using Karori and Barua $(\mathit{KB})$ type metric. For this investigation, we have considered three different known compact stars with anisotropic configuration namely, 4U 1820-30, Her X-1 and SAX J 1808.4-3658(SSI) with radii 10 km, 7.7 km and 7.07 km respectively. Formulation of field equations in this modified theory have also been presented. The unknown constants in KB metric have been obtained by using the matching of interior solution with the exterior vacuum solution. We investigate the various physical aspects of the presented model such as variation of energy density, energy conditions, hydrostatic equilibrium, stability, compactness and surface redshift. Graphical analysis of the variables involved in the obtained solution is also presented to observe their behavior more clearly. We also demonstrate the numerical values of some physical parameters in the tabular form by using the observational values of mass and radius. We also compute the numerical values of $\frac{2M}{R}$ using the presented model to check the Buchdahl limit for our system. The detailed analysis of matter variables revealed the regularity and stability of our proposed model.

Generating solutions for charged geodesic anisotropic spherical collapse with shear and heat radiation

In this paper we give a generating function for solutions of the type of collapse mentioned in the title. It satisfies a simple Riccati equation, derived from a formula, which holds in the interior of the star. The generating function is unique, unlike in the neutral case. Every neutral solution has a charged companion with the same generating function. The charged solution has bigger radius, surface redshift, mass and compactness than the neutral one. Its rate of collapse is slower. A class of known exact neutral solutions, containing generalised travelling waves, is charged.

SYNTHESIS OF EOSIN Y-SENSITIZED Ag-TiO2 NANO-HYBRID FOR OPTIMIZED PHOTOCATALYTIC DEGRADATION OF AQUEOUS CAFFEINE

The visible light spectral response of titania nanoparticles synthesized by hybrid treatment of TiO2 with silver dopant and Eosin Y-sensitizer was evaluated using caffeine, an emerging pollutant. The photocatalyst was characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD), solid state UV/ Visible spectrometry and Fourier transform infrared spectrometry (FTIR). Upon sensitization and doping, downsizing, increase in surface area and redshift were observed without significant annihilation of the tetragonal anatase and rutile structures. Factors affecting the degradation of caffeine were studied using Taguchi orthogonal array model and optimized using a response surface methodology (RSM) based on faced-centered composite design (FCCD). Caffeine degradation under the conditions of the study was not affected significantly by temperature but pH, initial concentration and catalyst concentration. Evidence for electron injection and red shift with sensitization was corroborated using quantum calculations at DFT/B3LYP level of the theory.

The shapes of the rotation curves of star-forming galaxies over the last ≈10 Gyr

We analyse maps of the spatially-resolved nebular emission of $\approx$1500 star-forming galaxies at $z\approx0.6$-$2.2$ from deep KMOS and MUSE observations to measure the average shape of their rotation curves. We use these to test claims for declining rotation curves at large radii in galaxies at $z\approx1$-$2$ that have been interpreted as evidence for an absence of dark matter. We show that the shape of the average rotation curves, and the extent to which they decline beyond their peak velocities, depends upon the normalisation prescription used to construct the average curve. Normalising in size by the galaxy stellar disk-scale length after accounting for seeing effects ($R_{\rm{d}}^{\prime}$), we construct stacked position-velocity diagrams that trace the average galaxy rotation curve out to $6R_{\rm{d}}^{\prime}$ ($\approx$13 kpc, on average). Combining these curves with average HI rotation curves for local systems, we investigate how the shapes of galaxy rotation curves evolve over $\approx$10 Gyr. The average rotation curve for galaxies binned in stellar mass, stellar surface mass density and/or redshift is approximately flat, or continues to rise, out to at least $6R_{\rm{d}}^{\prime}$. We find a trend between the outer slopes of galaxies' rotation curves and their stellar mass surface densities, with the higher surface density systems exhibiting flatter rotation curves. Drawing comparisons with hydrodynamical simulations, we show that the average shapes of the rotation curves for our sample of massive, star-forming galaxies at $z\approx0$-$2.2$ are consistent with those expected from $\Lambda$CDM theory and imply dark matter fractions within $6R_{\rm{d}}$ of at least $\approx60$ percent.

Analytical model of compact star in low-mass X-ray binary with de Sitter spacetime

In this article, we suggest a Sitter model in favor of compact stars in low-mass X-ray binaries. Here, we have considered the presence of a cosmological constant (on a small scale) to investigate the stellar structure. We conclude that this approach is very suitable for the familiar physicalmodel of compact stars in low-mass X-ray binaries. We calculate the probable radius, compactness (u) and surface redshift (Zs) of six compact stars in low-mass X-ray binaries, namely Cyg X-2, V395 Carinae/2S 0921−630, XTE J2123−058, X1822−371 (V691 CrA), 4U 1820−30 and GR Mus (XB 1254−690). We also offer a possible equation of state (EOS) for the stellar objects.

Effect of electric charge on conformal compact stars

We investigate the effect of electric charge in anisotropic compact stars with conformal symmetry. We assume that the pressure and the density of the matter inside the stellar structure are large with strong gravitational fields. The strong electric field produces significant effects on the phenomenology of the stellar objects, in order of 10^{20}~text {V m}^{-1} in MKSA units. The conformal symmetry condition produces an integral relationship between the metric functions. We use this condition to find a new anisotropic solution to the Einstein–Maxwell field equations. This solution is relevant in modelling a relativistic compact star. Radii and masses are consistent with stellar objects such PSR J1614-2230, Vela X1, PSR J1903+327 and Cen X-3. The mass-radius ratio and the surface red shift are in agreement with realistic constraints. Also our model displays constraint on the maximum stellar mass, central density and radius for the upper bound redshift requirements.

Logistic Test Effort Function Based Software Reliability Growth Model with Multi up Gradation

We here investigate a relativistic model together with some new exact class of solutions for an anisotropic radiating star using Tewari and Charan [1] solution; as a seed solution. The interior metric obeyed all the relevant physical and thermodynamic conditions and matched with Vaidya exterior metric over the boundary. The model is physically and thermodynamically sound as it corresponds to the non-negative expressions for non homogeneous fluid density, both the radial and transverse pressures and radiation flux density throughout the fluid ball. Initially the interior solutions represent a static configuration of perfect fluid which then gradually starts evolving into radiating collapse. Consequently we have obtained the expressions of mass energy, physical radius, apparent luminosity, surface redshift and surface temperature of collapsing radiating star.

Effects of hadron–quark phase transition on properties of neutron stars

We investigate the possible scenario of deconfinement of hyperon rich hadronic matter to quark matter at high densities and the resulting hybrid star (HS) properties are analyzed. In the relativistic mean-field framework, we construct the equation of state (EoS) of hadronic matter using the effective chiral model while the pure quark matter is described using the MIT bag model. We revisit the hyperon puzzle and analyze the possibility of hadron–quark phase transition with proper choice of the bag constant. In static condition the maximum mass of the resultant HSs are in good agreement with the recent observational bounds on the same from high mass pulsars such as PSR J1614−2230 and PSR J0348+0432. On invoking the phenomenon of phase transition, the radius of canonical mass (R1.4) and value of R1.6 predicted by the model lie within the range prescribed from binary neutron star merger detected by the LIGO-Virgo collaboration in 2017. The surface redshift obtained for the HSs also satisfy the constraints from pulsars RX J0 720.4-3125 and 1E 1 207.4-5209. It is worth noting that unlike several other works, we add no modifications to the original form of the Bag model to satisfy these recent observational and empirical constraints on NS properties. We also discuss the rotational aspects of the HSs by calculating the properties like rotational mass, radius, energy density, moment of inertia at different angular velocities. The maximum bound on rotational frequency from the rapidly rotating pulsars like PSR B1937+21 and PSR J1748−2446ad are satisfied with the HS configuration. We also test the universality of our hybrid EoS in terms of normalized moment of inertia.

Role of σR2 + γRμνTμν model on anisotropic polytropes

This paper analyzes the anisotropic stellar evolution governed by polytropic equation-of-state in the framework of [Formula: see text] gravity, where [Formula: see text]. We construct the field equations, hydrostatic equilibrium equation and trace equation to obtain their solutions numerically under the influence of [Formula: see text] gravity model, where [Formula: see text] and [Formula: see text] are arbitrary constants. We examine the dependence of various physical characteristics such as radial/tangential pressure, energy density, anisotropic factor, total mass and surface redshift for specific values of the model parameters. The physical acceptability of the considered model is discussed by verifying the validity of energy conditions, causality condition and adiabatic index. We also study the effects arising due to strong nonminimal matter-curvature coupling on anisotropic polytropes. It is found that the polytropic stars are stable and their maximum mass point lies within the required observational Chandrasekhar limit.

Isotropic compact stars model in Rastall theory admitting conformal motion

We study a new solution for an isotropic compact star model admitting conformal motion in the background of Rastall theory. The conformal Killing vector (\(\mathit{CKV}\)) and the equation of state (EoS) \(p=\omega \rho \), where \(\omega \) satisfying \(0<\omega <1\) is the \(\mathit{EoS}\) parameter for normal matter distribution, are the main ingredients of our methodology. Several physical aspects of the model has been explored analytically to observe the behavior of compact stars such as \(\mathit{TOV}\) equation, energy conditions, Buchdahl condition, stability analysis, compactness and surface redshift. Graphical analysis of the physical parameters have also been presented to support our analytical investigation. We observed that all the physical requirements are fulfilled and the presented model is quite acceptable.