scholarly journals Neutron Stars in f(R)-Gravity and Its Extension with a Scalar Axion Field

Particles ◽  
2020 ◽  
Vol 3 (3) ◽  
pp. 532-542 ◽  
Author(s):  
Artyom Astashenok ◽  
Sergey Odintsov
Keyword(s):  
General Relativity ◽  
Neutron Stars ◽  
Equations Of State ◽  
Einstein Equations ◽  
Central Density ◽  
Axion Field ◽  
Mass Profile

We present a brief review of general results about non-rotating neutron stars in simple R 2 gravity and its extension with a scalar axion field. Modified Einstein equations are presented for metrics in isotropical coordinates. The mass–radius relation, mass profile and dependence of mass from central density on various equations of state are given in comparison to general relativity.

scholarly journals Neutron stars in Palatini $$R+\alpha R^2$$ and $$R+\alpha R^2+\beta Q$$ theories

2021 ◽  
Vol 81 (10) ◽  
Author(s):  
Georg Herzog ◽  
Hèlios Sanchis-Alepuz
Keyword(s):  
General Relativity ◽  
Equation Of State ◽  
Neutron Stars ◽  
Equations Of State ◽  
Stellar Structure ◽  
Modified Theories Of Gravity ◽  
State Uncertainty ◽  
Structure Equations ◽  
Theories Of Gravity

AbstractWe study solutions of the stellar structure equations for spherically symmetric objects in modified theories of gravity, where the Einstein-Hilbert Lagrangian is replaced by $$f(R)=R+\alpha R^2$$ f ( R ) = R + α R 2 and $$f(R,Q)=R+\alpha R^2+\beta Q$$ f ( R , Q ) = R + α R 2 + β Q , with R being the Ricci scalar curvature, $$Q=R_{\mu \nu }R^{\mu \nu }$$ Q = R μ ν R μ ν and $$R_{\mu \nu }$$ R μ ν the Ricci tensor. We work in the Palatini formalism, where the metric and the connection are assumed to be independent dynamical variables. We focus on stellar solutions in the mass-radius region associated to neutron stars. We illustrate the potential impact of the $$R^2$$ R 2 and Q terms by studying a range of viable values of $$\alpha $$ α and $$\beta $$ β . Similarly, we use different equations of state (SLy, FPS, HS(DD2) and HS(TMA)) as a simple way to account for the equation of state uncertainty. Our results show that for certain combinations of the $$\alpha $$ α and $$\beta $$ β parameters and equation of state, the effect of modifications of general relativity on the properties of stars is sizeable. Therefore, with increasing accuracy in the determination of the equation of state for neutron stars, astrophysical observations may serve as discriminators of modifications of General Relativity.

scholarly journals ON THE MAXIMUM MASS OF GENERAL RELATIVISTIC UNIFORMLY ROTATING WHITE DWARFS

2011 ◽  
Vol 20 (supp01) ◽  
pp. 136-140 ◽  
Author(s):  
KUANTAY BOSHKAYEV ◽  
JORGE RUEDA ◽  
REMO RUFFINI
Keyword(s):  
General Relativity ◽  
Quadrupole Moment ◽  
White Dwarfs ◽  
Rotation Period ◽  
Moment Of Inertia ◽  
Einstein Equations ◽  
Central Density ◽  
Maximum Mass ◽  
General Relativistic ◽  
The Moment

The properties of uniformly rotating white dwarfs are analyzed within the framework of general relativity. Hartle's formalism is applied to construct self-consistently the internal and external solutions to the Einstein equations. The mass, the radius, the moment of inertia and quadrupole moment of rotating white dwarfs have been calculated as a function of both the central density and rotation period of the star. The maximum mass of rotating white dwarfs for stable configurations has been obtained.

scholarly journals Neutron stars in frames of R2-gravity and gravitational waves

2019 ◽  
Vol 16 (01) ◽  
pp. 1950004 ◽  
Author(s):  
Artyom V. Astashenok ◽  
Alexey S. Baigashov ◽  
Sergey A. Lapin
Keyword(s):  
General Relativity ◽  
Neutron Stars ◽  
Gravitational Waves ◽  
Simple Formula ◽  
Equations Of State ◽  
Dense Matter ◽  
Dilaton Field ◽  
Spatial Infinity ◽  
Schwarzschild Solution ◽  
Distant Observer

The realistic models of neutron stars are considered for simple [Formula: see text] gravity and equivalent Brance–Dicke theory with dilaton field in Einsein frame. For negative values of [Formula: see text] we have no acceptable results from astrophysical viewpoint: the resulting solution for spherical stars doesn’t coincide with Schwarzschild solution on spatial infinity. The mass of star from viewpoint of distant observer tends to very large values. For [Formula: see text] it is possible to obtain solutions with required asymptotics and well-defined star mass. The mass confined by stellar surface decreases with increasing of [Formula: see text] but we have some contribution to mass from gravitational sphere appearing outside the star. The resulting effect is increasing of gravitational mass from viewpoint of distant observer. But another interpretation take place in a case of equivalent Brance–Dicke theory with massless dilaton field in Einstein frame. The mass of star increases due to contribution of dilaton field inside the star. We also considered the possible constraints on [Formula: see text] gravity from GW 170817 data. According to results of Bauswein et al. the lower limit on threshold mass is [Formula: see text][Formula: see text][Formula: see text]. This allows to exclude some equations of state (EoS) for dense matter. But in [Formula: see text] gravity the threshold mass increases for given EoS with increasing of [Formula: see text]. In principle it can helps in future discriminate between General Relativity and square gravity (of course one need to know EoS with more accuracy rather than now).

scholarly journals Rotating neutron stars in F(R) gravity with axions

2020 ◽  
Vol 498 (3) ◽  
pp. 3616-3623
Author(s):  
Artyom V Astashenok ◽  
Sergey D Odintsov
Keyword(s):  
General Relativity ◽  
Neutron Stars ◽  
Compact Stars ◽  
Static Case ◽  
Rotating Stars ◽  
New Class ◽  
Equilibrium Configurations ◽  
Axion Field ◽  
Central Energy ◽  
Fast Rotating

ABSTRACT We investigate equilibrium configurations of uniformly rotating neutron stars in R2 gravity with axion scalar field for GM1 equation of state (EoS) for nuclear matter. The mass–radius diagram, mass–central energy density are presented for some frequencies in comparison with static stars. We also compute equatorial and polar radii and moment of inertia for stars. For axion field ϕ, the coupling in the form ∼R2ϕ is assumed. Several interesting results follow from our consideration. Maximal possible star mass with given EoS increases due to the contribution of coupling term. We discovered the possibility to increase maximal frequency of the rotation in comparison with General Relativity. As a consequence, the lower bound on mass of the fast rotating stars decreases. For frequency f = 700 Hz, neutron stars with masses ∼M⊙ can exist for some choice of parameters (in General Relativity for same EoS, this limit is around 1.2 M⊙). Another feature of our solutions is relatively small increase of stars' radii for high frequencies in comparison with static case. Thus, eventually, the new class of neutron stars in R2 gravity with axions is discovered namely fast rotating compact stars with intermediate masses.

Rapidly rotating neutron stars in general relativity: Realistic equations of state

1994 ◽  
Vol 424 ◽  
pp. 823 ◽  
Author(s):  
Gregory B. Cook ◽  
Stuart L. Shapiro ◽  
Saul A. Teukolsky
Keyword(s):  
General Relativity ◽  
Neutron Stars ◽  
Equations Of State

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.

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