scholarly journals Distinct classes of compact stars based on geometrically deduced equations of state

2021 ◽  
pp. 2150029
Author(s):  
A. C. Khunt ◽  
V. O. Thomas ◽  
P. C. Vinodkumar
Keyword(s):  
Neutron Stars ◽  
Soft Matter ◽  
Equations Of State ◽  
Compact Objects ◽  
Exotic Matter ◽  
Compact Stars ◽  
Spacetime Geometry ◽  
Superdense Stars ◽  
Einstein's Equation ◽  
Envelope Model

We have computed the properties of compact objects like neutron stars based on equation of state (EOS) deduced from a core–envelope model of superdense stars. Such superdense stars have been studied by solving Einstein’s equation based on pseudo-spheroidal and spherically symmetric spacetime geometry. The computed star properties are compared with those obtained based on nuclear matter EOSs. From the mass–radius ([Formula: see text]–[Formula: see text]) relationship obtained here, we are able to classify compact stars in three categories: (i) highly compact self-bound stars that represents exotic matter compositions with radius lying below 9[Formula: see text]km; (ii) normal neutron stars with radius between 9 to 12[Formula: see text]km and (iii) soft matter neutron stars having radius lying between 12 to 20[Formula: see text]km. Other properties such as Keplerian frequency, surface gravity and surface gravitational redshift are also computed for all the three types. This work would be useful for the study of highly compact neutron like stars having exotic matter compositions.

scholarly journals Charged anisotropic compact star core-envelope model with polytropic core and linear envelope

2021 ◽  
Vol 81 (10) ◽  
Author(s):  
S. A. Mardan ◽  
I. Noureen ◽  
A. Khalid
Keyword(s):  
Graphical Representation ◽  
Compact Star ◽  
Compact Object ◽  
Compact Objects ◽  
Compact Stars ◽  
Physical Parameters ◽  
Polytropic Equation ◽  
Envelope Model ◽  
Linear Envelope ◽  
The Stability

AbstractThis manuscript is related to the construction of relativistic core-envelope model for spherically symmetric charged anisotropic compact objects. The polytropic equation of state is considered for core, while it is linear in the case of envelope. We present that core, envelope and the Reissner Nordstr$$\ddot{o}$$ o ¨ m exterior regions of stars match smoothly. It has been verified that all physical parameters are well behaved in the core and envelope region for the compact stars SAX J1808.4-3658 and 4U1608-52. Various physical parameters inside star are discussed herein, non-singularity and continuity at the junction has been catered as well. Impact of charged compact object together with core-envelope model on the mass, radius and compactification factor is described by graphical representation in both core and envelop regions. The stability of the model is worked out with the help of Tolman–Oppenheimer–Volkoff equations and radial sound speed.

A relativistic two-fluid model of compact stars

2017 ◽  
Vol 32 (10) ◽  
pp. 1750055 ◽  
Author(s):  
Koushik Chakraborty ◽  
Farook Rahaman ◽  
Arkopriya Mallick
Keyword(s):  
Quark Matter ◽  
Fluid Model ◽  
Compact Star ◽  
Conformal Symmetry ◽  
Compact Objects ◽  
Compact Stars ◽  
Baryonic Matter ◽  
Spacetime Geometry ◽  
Two Fluid Model ◽  
The Masses

We propose a relativistic model of compact star admitting conformal symmetry. Quark matter and baryonic matter which are considered as two different fluids, constitute the star. We define interaction equations between the normal baryonic matter and the quark matter and study the physical situations for repulsive, attractive and zero interaction between the constituent matters. The measured value of the Bag constant is used to explore the spacetime geometry inside the star. From the observed values of the masses of some compact objects, we have obtained theoretical values of the radii. Theoretical values of the radii match well with the previous predictions for such compact objects.

A uniparametric perfect fluid solution to represent compact stars

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].

Black Hole, Neutron Star and Numerical Relativity

2021 ◽  
Vol 30 (6) ◽  
pp. 7-13
Author(s):  
Jinho KIM
Keyword(s):  
Black Holes ◽  
General Relativity ◽  
Black Hole ◽  
Neutron Star ◽  
Neutron Stars ◽  
Numerical Method ◽  
High Velocity ◽  
Compact Objects ◽  
Compact Stars ◽  
Speed Of Light

Compact stars, e.g., black holes and neutron stars, are the most energetic objects in astrophysics. These objects are accompanied by extremely strong gravity and a high velocity, which approaches the speed of light. Therefore, compact objects should be dealt with in Einstein’s relativity. This article will briefly introduce a numerical method that will allow us to obtain general solutions in general relativity. Several applications using numerical relativistic simulations will also be presented.

scholarly journals Tidal deformability and other global parameters of compact stars with strong phase transitions

2019 ◽  
Vol 622 ◽  
pp. A174 ◽  
Author(s):  
M. Sieniawska ◽  
W. Turczański ◽  
M. Bejger ◽  
J. L. Zdunik
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Neutron Star ◽  
Neutron Stars ◽  
Equations Of State ◽  
Dense Matter ◽  
Compact Stars ◽  
High Mass ◽  
Density Jump ◽  
Strong Phase

Context. Using parametric equations of state (relativistic polytropes and a simple quark bag model) to model dense-matter phase transitions, we study global, measurable astrophysical parameters of compact stars such as their allowed radii and tidal deformabilities. We also investigate the influence of stiffness of matter before the onset of the phase transitions on the parameters of the possible exotic dense phase. Aims. The aim of our study is to compare the parameter space of the dense matter equation of state permitting phase transitions to a sub-space compatible with current observational constraints such as the maximum observable mass, tidal deformabilities of neutron star mergers, radii of configurations before the onset of the phase transition, and to give predictions for future observations. Methods. We studied solutions of the Tolman-Oppenheimer-Volkoff equations for a flexible set of parametric equations of state, constructed using a realistic description of neutron-star crust (up to the nuclear saturation density), and relativistic polytropes connected by a density-jump phase transition to a simple bag model description of deconfined quark matter. Results. In order to be consistent with recent observations of massive neutron stars, a compact star with a strong high-mass phase transition cannot have a radius smaller than 12 km in the range of masses 1.2 − 1.6 M⊙. We also compare tidal deformabilities of stars with weak and strong phase transitions with the results of the GW170817 neutron star merger. Specifically, we study characteristic phase transition features in the Λ1 − Λ2 relation, and estimate the deviations of our results from the approximate formulæ for Λ∼ − R (M1) and Λ-compactness proposed in the literature. We find constraints on the hybrid equations of state to produce stable neutron stars on the twin branch. For the exemplary equations of state most of the high-mass twins occur for the minimum values of the density jump λ = 1.33 − 1.54; corresponding values of the square of the speed of sound are α = 0.7 − 0.37. We compare results with observations of gravitational waves and with the theoretical causal limit and find that the minimum radius of a twin branch is between 9.5 and 10.5 km, and depends on the phase transition baryon density. For these solutions the phase transition occurs below 0.56 fm−3.

SCALING PROPERTY IN COLD COMPACT STARS

2000 ◽  
Vol 15 (20) ◽  
pp. 1341-1346 ◽  
Author(s):  
R. SHARMA ◽  
S. MUKHERJEE ◽  
S. D. MAHARAJ
Keyword(s):  
Neutron Stars ◽  
Strange Quark ◽  
Scaling Behavior ◽  
Exotic Matter ◽  
Compact Stars ◽  
Bag Model ◽  
Model Calculations ◽  
Quark Stars ◽  
Scaling Property ◽  
Simple Scaling

We point out a simple scaling property in the mass–radius relationship in cold compact stars. This property my be considered as a generalization of the scaling observed by Witten for strange quark stars. A particular model which explicitly exhibits this scaling behavior has been discussed. The model is relevant for neutron stars as well as stars made up of exotic matter, in particular, quark stars and other composites based on Bag model calculations.

Anisotropic solution for compact star in 5D Einstein–Gauss–Bonnet gravity

2021 ◽  
Vol 36 (32) ◽  
Author(s):  
S. K. Maurya ◽  
Anirudh Pradhan ◽  
Ayan Banerjee ◽  
Francisco Tello-Ortiz ◽  
M. K. Jasim
Keyword(s):  
Compact Star ◽  
Compact Objects ◽  
Compact Stars ◽  
Stellar Structure ◽  
Energy Conditions ◽  
Field Equations ◽  
Anisotropic Solution ◽  
Bonnet Gravity ◽  
Spacetime Geometry ◽  
Equilibrium Configurations

In astronomy, the study of compact stellar remnants — white dwarfs, neutron stars, black holes — has attracted much attention for addressing fundamental principles of physics under extreme conditions in the core of compact objects. In a recent argument, Maurya et al. [Eur. Phys. J. C 77, 45 (2017)] have proposed an exact solution depending on a specific spacetime geometry. Here, we construct equilibrium configurations of compact stars for the same spacetime that make it interesting for modeling high density physical astronomical objects. All calculations are carried out within the framework of the five-dimensional Einstein–Gauss–Bonnet gravity. Our main interest is to explore the dependence of the physical properties of these compact stars depending on the Gauss–Bonnet coupling constant. The interior solutions have been matched to an exterior Boulware–Deser solution for [Formula: see text] spacetime. Our finding ensures that all energy conditions hold, and the speed of sound remains causal, everywhere inside the star. Moreover, we study the dynamical stability of stellar structure by taking into account the modified field equations using the theory of adiabatic radial oscillations developed by Chandrasekhar. Based on the observational data for radii and masses coming from different astronomical sources, we show that our model is compatible and physically relevant.

scholarly journals Supermassive neutron stars in axion F(R) gravity

2020 ◽  
Vol 493 (1) ◽  
pp. 78-86 ◽  
Author(s):  
Artyom V Astashenok ◽  
Sergey D Odintsov
Keyword(s):  
Equation Of State ◽  
Neutron Stars ◽  
Scalar Curvature ◽  
Equations Of State ◽  
Compact Stars ◽  
Axion Mass ◽  
Axion Field ◽  
Wide Range ◽  
Star Surface ◽  
Effective Contribution

ABSTRACT We investigated realistic neutron stars in axion R2 gravity. The coupling between curvature and axion field ϕ is assumed in the simple form ∼R2ϕ. For the axion mass in the range ma ∼ 10−11–10−10 eV the solitonic core within neutron star and corresponding halo with size ∼100 km can exist. Therefore the effective contribution of R2 term grows inside the star and it leads to change of star parameters (namely, mass, and radius). We obtained the increase of star mass independent from central density for wide range of masses. Therefore, maximal possible mass for given equation of state grows. At the same time, the star radius increases not so considerably in comparison with GR. Hence, our model may predict possible existence of supermassive compact stars with masses $M\sim 2.2\!-\!2.3\, \mathrm{M}_\odot$ and radii Rs ∼ 11 km for realistic equation of state (we considered APR equation of state). In general relativity one can obtain neutron stars with such characteristics only for unrealistic, extremely stiff equations of state. Note that this increase of mass occurs due to change of solution for scalar curvature outside the star. In GR curvature drops to zero on star surface where ρ = p = 0. In the model underconsideration the scalar curvature dumps more slowly in comparison with vacuum R2 gravity due to axion ‘galo’ around the star.

FROM CRUST TO CORE: A BRIEF REVIEW OF QUARK MATTER IN NEUTRON STARS

2010 ◽  
Vol 19 (08n10) ◽  
pp. 1427-1436 ◽  
Author(s):  
F. WEBER ◽  
O. HAMIL ◽  
K. MIMURA ◽  
R. NEGREIROS
Keyword(s):  
Neutron Stars ◽  
Electric Fields ◽  
Quark Matter ◽  
Strange Quark ◽  
Thermal Evolution ◽  
Reaction Rates ◽  
Strange Quark Matter ◽  
Compact Objects ◽  
Compact Stars ◽  
The Impact

This paper provides a short overview of the multifaceted, possible role of quark matter for compact stars (neutron stars and strange quark matter stars). We began with a variational investigation of the maximum possible energy densities in the cores of neutron stars. This is followed by a brief discussion of the possible existence of quark matter in the cores of neutron stars and how such matter could manifest itself in neutron star observables. The possible presence of color superconducting strange quark matter nuggets in the crusts of neutron stars is reviewed next, and their impact on the pycnonuclear reaction rates in the crusts of neutron stars is discussed. The second part of the paper discusses the impact of ultra-strong electric fields on the bulk properties of strange quark matter stars and presents results of a preliminary study that models the thermal evolution of radio-quiet, X-ray bright, central compact objects (CCOs).

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