An anisotropic model for stars

A stellar model with anisotropic pressure is constructed and analyzed, the metric components that describe the geometry and the source of matter satisfy Einstein’s equations and both are finite inside the star. In addition, density and pressure are decreasing monotone functions of the radial distance. The speed of sound is positive and less than the speed of light, furthermore the model is potentially stable. The model allows describing compact objects with compactness of [Formula: see text] and as a result of the anisotropic value there is a range of values of the central density, in particular for the maximum value of compactness a star with [Formula: see text] and a value of anisotropic parameter [Formula: see text] we get a stellar radius of [Formula: see text] and a central density [Formula: see text]. The above makes the solution a physically realistic model that can be used to describe dense objects such as neutron stars whose characteristic density is of the order of nuclear density.

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An anisotropic charged fluids with Chaplygin equation of state

Modern Physics Letters A ◽

10.1142/s0217732321501534 ◽

2021 ◽

Vol 36 (21) ◽

pp. 2150153

Author(s):

Joaquin Estevez-Delgado ◽

Noel Enrique Rodríguez Maya ◽

José Martínez Peña ◽

Arthur Cleary-Balderas ◽

Jorge Mauricio Paulin-Fuentes

Keyword(s):

Speed Of Sound ◽

Radial Pressure ◽

State Equation ◽

Stellar Model ◽

Compact Objects ◽

Anisotropic Fluid ◽

The Stability ◽

Electrically Charged ◽

Chaplygin Equation ◽

Graphic Description

A stellar model with an electrically charged anisotropic fluid as a source of matter is presented. The radial pressure is described by a Chaplygin state equation, [Formula: see text], while the anisotropy [Formula: see text] is annulled in the center of the star [Formula: see text] is regular and [Formula: see text], the electric field, is also annulled in the center. The density pressures and the tangential speed of sound are regular, while the radial speed of sound is monotonically increasing. The model is physically acceptable and meets the stability criteria of Harrison–Zeldovich–Novikov and in respect of the cracking concept the solution is unstable in the region of the center and potentially stable near the surface. A graphic description is presented for the case of an object with a compactness rate [Formula: see text], mass [Formula: see text] and radius [Formula: see text] km that matches the star Vela X-1. Also, the interval of the central density [Formula: see text], which is consistent with the expected magnitudes for this type of stars, which shows that the behavior is accurate for describing compact objects.

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A charged perfect fluid solution

Modern Physics Letters A ◽

10.1142/s0217732320501205 ◽

2020 ◽

Vol 35 (15) ◽

pp. 2050120

Author(s):

Joaquin Estevez-Delgado ◽

Jose Vega Cabrera ◽

Jorge Mauricio Paulin-Fuentes ◽

Joel Arturo Rodriguez Ceballos ◽

Modesto Pineda Duran

Keyword(s):

Speed Of Sound ◽

Maxwell Equations ◽

Radial Distance ◽

Stellar Model ◽

Speed Of Light ◽

Perfect Fluid Solution ◽

Charged Fluid ◽

Regular Functions ◽

Quark Stars ◽

Increasing Function

A static and spherically symmetric stellar model is described by a perfect charged fluid. Its construction is done using the solution of the Einstein–Maxwell equations for which we specify the temporal metric and the electric field which is a monotonic increasing function null in the center. The density, pressure and speed of sound turn out to be regular functions, positive and monotonic decreasing as function of the radial distance. Also, the speed of sound is lower than the speed of light, that is to say, it does not violate the condition of causality. The value of compactness [Formula: see text], so the model is useful to represent neutron stars of quark stars. In a complementary manner, we report the physical values when describing a star of mass [Formula: see text] and radius [Formula: see text], in such case [Formula: see text], and given the presence of the charge, the interval for the central density [Formula: see text].

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An analytical anisotropic compact stellar model of embedding class I

Modern Physics Letters A ◽

10.1142/s0217732321500280 ◽

2021 ◽

Vol 36 (05) ◽

pp. 2150028

Author(s):

Lipi Baskey ◽

Shyam Das ◽

Farook Rahaman

Keyword(s):

Radial Pressure ◽

Stellar Model ◽

Compact Objects ◽

Compact Stars ◽

Model Parameters ◽

Field Equations ◽

Principal Stresses ◽

Schwarzschild Solution ◽

Spherical Fluid ◽

Physical Profile

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.

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Observations of Boundary Layer Wind and Turbulence of a Landfalling Tropical Cyclone

10.21203/rs.3.rs-812123/v1 ◽

2021 ◽

Author(s):

Zhongkuo Zhao ◽

Ruiquan Gao ◽

Jun A. Zhang ◽

Yong Zhu ◽

Chunxia Liu ◽

...

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Radial Distance ◽

Flux Measurement ◽

Eddy Diffusivity ◽

Multiple Level ◽

A Value ◽

Momentum Fluxes ◽

Speed Range ◽

Wind Profiles

Abstract This study analyzed the atmospheric boundary layer characteristics based on the multiple level observations by a 350-m height tower during the landfall of Super Typhoon Mangkhut (1822). Mean wind profiles showed logarithmic wind profiles at different wind speed ranges suggesting nearly constant flux layers. The height of the constant layer increased with the wind speed and deceased with the radial distance from the storm centre. This behaviour was supported by flux observations. Momentum fluxes and turbulent kinetic energy increased with the wind speed at all flux measurement levels. The drag coefficient (surface roughness) estimated was nearly a constant with a value of 8´10-3 (0.09 m). Both the estimated eddy diffusivity and mixing length varied with height. The eddy diffusivity also varied with the wind speed. Our results supported that the eddy diffusivity is larger over land than over ocean in a same wind speed range.

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Inflation near a metastable de Sitter vacuum from moduli stabilisation

The European Physical Journal C ◽

10.1140/epjc/s10052-020-08581-9 ◽

2020 ◽

Vol 80 (11) ◽

Author(s):

Ignatios Antoniadis ◽

Osmin Lacombe ◽

George K. Leontaris

Keyword(s):

Realistic Model ◽

Intermediate Energy ◽

De Sitter ◽

World Sheet ◽

A Value ◽

String Loop ◽

Hybrid Inflation ◽

Perturbative Quantum ◽

De Sitter Vacuum ◽

Energy Physics

AbstractWe study the cosmological properties of a metastable de Sitter vacuum obtained recently in the framework of type IIB flux compactifications in the presence of three D7-brane stacks, based on perturbative quantum corrections at both world-sheet and string loop level that are dominant at large volume and weak coupling. In the simplest case, the model has one effective parameter controlling the shape of the potential of the inflaton which is identified with the volume modulus. The model provides a phenomenological successful small-field inflation for a value of the parameter that makes the minimum very shallow and near the maximum. The horizon exit is close to the inflection point while most of the required e-folds of the Universe expansion are generated near the minimum, with a prediction for the ratio of tensor-to-scalar primordial fluctuations $$r\simeq 4\times 10^{-4}$$ r ≃ 4 × 10 - 4 . Despite its shallowness, the minimum turns out to be practically stable. We show that it can decay only through the Hawking–Moss instanton leading to an extremely long decay rate. Obviously, in order to end inflation and obtain a realistic model, new low-energy physics is needed around the minimum, at intermediate energy scales of order $$10^{12}$$ 10 12 GeV. An attractive possibility is by introducing a “waterfall” field within the framework of hybrid inflation.

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Compact stars

Modern Physics Letters A ◽

10.1142/s0217732318500815 ◽

2018 ◽

Vol 33 (15) ◽

pp. 1850081 ◽

Cited By ~ 13

Author(s):

Gabino Estevez-Delgado ◽

Joaquin Estevez-Delgado

Keyword(s):

Speed Of Sound ◽

Anisotropy Parameter ◽

Compact Object ◽

Stellar Model ◽

Compact Stars ◽

Central Density ◽

Speeds Of Sound ◽

Light Speed ◽

Maximum Value ◽

Two Parameters

An analysis and construction is presented for a stellar model characterized by two parameters (w, n) associated with the compactness ratio and anisotropy, respectively. The reliability range for the parameter w [Formula: see text] 1.97981225149 corresponds with a compactness ratio u [Formula: see text] 0.2644959374, the density and pressures are positive, regular and monotonic decrescent functions, the radial and tangential speed of sound are lower than the light speed, moreover, than the plausible stability. The behavior of the speeds of sound are determinate for the anisotropy parameter n, admitting a subinterval where the speeds are monotonic crescent functions and other where we have monotonic decrescent functions for the same speeds, both cases describing a compact object that is also potentially stable. In the bigger value for the observational mass M = 2.05 M[Formula: see text] and radii R = 12.957 Km for the star PSR J0348+0432, the model indicates that the maximum central density [Formula: see text] = 1.283820319 × 10[Formula: see text] Kg/m3 corresponds to the maximum value of the anisotropy parameter and the radial and tangential speed of the sound are monotonic decrescent functions.

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Study on Anisotropic Strange Stars in f(T,T) Gravity

10.20944/preprints202008.0723.v1 ◽

2020 ◽

Author(s):

Ines G. Salako ◽

M. Khlopov ◽

Saibal Ray ◽

M.Z. Arouko ◽

Pameli Saha ◽

...

Keyword(s):

Energy Momentum Tensor ◽

Torsion Tensor ◽

Equations Of Motion ◽

Momentum Tensor ◽

Compact Objects ◽

Strange Star ◽

Energy Conditions ◽

Anisotropic Pressure ◽

Spacetime Geometry ◽

Physical Features

In this work, we study the existence of strange star in the background of f(T,T) gravity in the Einstein spacetime geometry, where T is the torsion tensor and T is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified TOV equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in f(T,T) gravity as well as their observational signatures.

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A uniparametric perfect fluid solution to represent compact stars

Modern Physics Letters A ◽

10.1142/s0217732321500681 ◽

2021 ◽

Vol 36 (10) ◽

pp. 2150068

Author(s):

Joaquin Estevez-Delgado ◽

Noel Enrique Rodríguez Maya ◽

José Martínez Peña ◽

David Rivera Rangel ◽

Nancy Cambron Muñoz

Keyword(s):

Neutron Stars ◽

Perfect Fluid ◽

Stellar Model ◽

Compact Objects ◽

Compact Stars ◽

Index Formula ◽

Perfect Fluid Solution ◽

Gravitational Theories ◽

State Formula ◽

The Stability

In the description of neutron stars, it is very important to consider gravitational theories as general relativity, due to the determining influence on the behavior of the different types of stars, since some objects show densities even bigger than nuclear density. This paper starts with Einstein’s equations for a perfect fluid and then we present a uniparametric stellar model which allows to describe compact objects like neutron stars with compactness ratio [Formula: see text]. The pressure and density are monotone decreasing regular functions, the speed of sound satisfies the causality condition, while the value for its adiabatic index [Formula: see text] guarantees the stability. In addition, the graph of [Formula: see text] versus [Formula: see text] shows a quasi-linear relationship for the equation of state [Formula: see text], which is similar to the so-called MIT Bag equation when we have the interaction between quarks. In our case it is due to the interaction of the different components found inside the star, such as electrons and neutrons. As an application of the model, we describe the star PSR J1614-2230 with a observed mass of [Formula: see text] and a radius [Formula: see text], the model shows that the maximum central density occurs for a maximal compactness value [Formula: see text].

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Propagation of Large Amplitude Pulsar Waves

Symposium - International Astronomical Union ◽

10.1017/s0074180900092688 ◽

1981 ◽

Vol 95 ◽

pp. 53-56

Author(s):

E. Asséo ◽

X. Llobet ◽

R. Pellat

Keyword(s):

Large Amplitude ◽

Low Frequency ◽

Spherical Geometry ◽

Wave Model ◽

Realistic Model ◽

Relativistic Plasma ◽

A Value ◽

Plasma Effects ◽

Magnetic Dipole Field ◽

Wave Emission

Large amplitude waves (hereafter l.a.w.) have been studied mainly in connection with pulsars. A rotating neutron star, with an intense non-aligned magnetic dipole field, surrounded by a vacuum, radiates beyond the light cylinder distance, large amplitude electromagnetic vacuum waves of very low frequency (Ostriker and Gunn, 1969). In such a rotating configuration electromagnetic effects completely dominate and imply the existence of a relativistic plasma in the pulsar magnetosphere (Goldreich and Julian, 1969). Because the oblique vacuum model predicts a value of 3 for the braking index of the pulsar whereas the observed or computed values are different, a realistic model has to include both the relativistic plasma outflow and the electromagnetic wave emission. The vacuum wave model has been changed to include self-consistent plasma effects (Asséo et al., 1975; Asséo et al., 1978) in plane geometry and inhomogeneities linked to spherical geometry (Asséo et al., 1981). This results in very restrictive conditions for the possibility of propagation of the l.a.w.

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Quark Stars in Massive Brans–Dicke Gravity with Tolman–Kuchowicz Spacetime

Universe ◽

10.3390/universe6080124 ◽

2020 ◽

Vol 6 (8) ◽

pp. 124

Author(s):

Amal Majid ◽

M. Sharif

Keyword(s):

Scalar Field ◽

Coupling Parameter ◽

Realistic Model ◽

Stellar Model ◽

Junction Conditions ◽

Anisotropic Model ◽

Field Equations ◽

Quark Stars ◽

Mit Bag Model ◽

Bag Constant

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the stellar model to evaluate the unknown constants in terms of mass and radius of the star. The radius of the strange star candidate PSR J1614-2230 is predicted by assuming maximum anisotropy at the surface of the star for different values of the coupling parameter, mass of the scalar field and bag constant. We examine various properties as well as the viability and stability of the anisotropic sphere. We conclude that the astrophysical model agrees with the essential criteria of a physically realistic model for higher values of the coupling parameter as well as mass of the scalar field.

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