Neutron Stars In f(R,T)f(R,T)f(R,T) Theory: Slow Rotation Approximation (original) (raw)

Slowly rotating neutron and strange stars in R 2 gravity

Journal of Cosmology and Astroparticle Physics, 2014

In the present paper we investigate self-consistently slowly rotating neutron and strange stars in Rsquared gravity. For this purpose we first derive the equations describing the structure of the slowly rotating compact stars in f (R)-gravity and then simultaneously solve the exterior and the interior problem. The structure of the slowly rotating neutron stars is studied for two different hadronic equations of state and a strange matter equation of state. The moment of inertia and its dependence on the stellar mass and the R-squared gravity parameter a is also examined in details. We find that the neutron star moment of inertia for large values of the parameter a can be up to 30% larger compared to the corresponding general relativistic models. This is much higher than the change in the maximum mass induced by R-squared gravity and is beyond the EOS uncertainty. In this way the future observations of the moment of inertia of compact stars could allow us to distinguish between general relativity and f (R) gravity, and more generally to test the strong field regime of gravity.

Rapidly rotating neutron stars in scalar-tensor theories of gravity

Physical Review D, 2013

We present the field equations governing the equilibrium of rapidly rotating neutron stars in scalar-tensor theories of gravity, as well as representative numerical solutions. The conditions for the presence of a nontrivial scalar field and the deviations from the general relativistic solutions are studied. Two examples of scalar-tensor theories are examinedone case that is equivalent to the Brans-Dicke theory and a second case, that is perturbatively equivalent to Einstein's General Relativity in the weak field regime, but can differ significantly for strong fields. Our numerical results show that rapidly rotating neutron star models with a nontrivial scalar field exist in both cases and that the effect of the scalar field is stronger for rapid rotation. If we consider values of the coupling parameters in accordance with current observations, only the second example of scalar-tensor theories has significant influence on the neutron star structure. We show that scalarized, rapidly rotating neutron stars exist for a larger range of the parameters than in the static case, since a nontrivial scalar field is present even for values of the coupling constant β > −4.35, and that these solutions are energetically more favorable than the general relativistic ones. In addition, the deviations of the rapidly rotating scalar-tensor neutron stars from the generalrelativistic solutions is significantly larger than in the static case.

Fast Rotating Neutron Stars with Realistic Nuclear Matter Equation of State

We construct equilibrium configurations of uniformly rotating neutron stars for selected relativis-tic mean-field nuclear matter equations of state (EOS). We compute in particular the gravitational mass (M), equatorial (Req) and polar (R pol) radii, eccentricity, angular momentum (J), moment of inertia (I) and quadrupole moment (M2) of neutron stars stable against mass-shedding and secular axisymmetric instability. By constructing the constant frequency sequence f = 716 Hz of the fastest observed pulsar, PSR J1748–2446ad, and constraining it to be within the stability region, we obtain a lower mass bound for the pulsar, Mmin = [1.2–1.4] M⊙, for the EOS employed. Moreover we give a fitting formula relating the baryonic mass (M b) and gravitational mass of non-rotating neutron stars, M b /M⊙ = M/M⊙ + (13/200)(M/M⊙) 2 [or M/M⊙ = M b /M⊙ − (1/20)(M b /M⊙) 2 ], which is independent on the EOS. We also obtain a fitting formula, although not EOS independent, relating the gravitational mass and the angular momentum of neutron stars along the secular axisymmetric instability line for each EOS. We compute the maximum value of the dimensionless angular momentum , a/M ≡ cJ/(GM 2) (or " Kerr parameter "), (a/M)max ≈ 0.7, found to be also independent on the EOS. We compare and contrast then the quadrupole moment of rotating neutron stars with the one predicted by the Kerr exterior solution for the same values of mass and angular momentum. Finally we show that, although the mass quadrupole moment of realistic neutron stars never reaches the Kerr value, the latter is closely approached from above at the maximum mass value, as physically expected from the no-hair theorem. In particular the stiffer the EOS is, the closer the Kerr solution is approached.

Fast Rotation of Neutron Stars

The Astrophysical Journal, 1996

We show that for realistic equations of state of dense matter, the universal proportionality factor relating the maximum rotation rate of neutron stars due to mass-shedding limit to the mass and radius of maximum allowable mass configuration of non-rotating models results from a universal proportionality between masses and radii of static maximum-mass neutron stars and those of maximally rotating configurations. These empirical relations cannot be obtained in the slow rotation approximation.

Further stable neutron star models from f ( R ) gravity

Journal of Cosmology and Astroparticle Physics, 2013

Neutron star models in perturbative f (R) gravity are considered with realistic equations of state. In particular, we consider the FPS, SLy and other equations of state and a case of piecewise equation of state for stars with quark cores. The mass-radius relations for f (R) = R + R(e −R/R 0 − 1) model and for R 2 models with logarithmic and cubic corrections are obtained. In the case of R 2 gravity with cubic corrections, we obtain that at high central densities (ρ > 10ρns, where ρns = 2.7 × 10 14 g/cm 3 is the nuclear saturation density), stable star configurations exist. The minimal radius of such stars is close to 9 km with maximal mass ∼ 1.9M⊙ (SLy equation). A similar situation takes place for AP4 and BSK20 EoS. Such an effect can give rise to more compact stars than in General Relativity. If observationally identified, such objects could constitute a formidable signature for modified gravity at astrophysical level. Another interesting result can be achieved in modified gravity with only a cubic correction. For some EoS, the upper limit of neutron star mass increases and therefore these EoS can describe realistic star configurations (although, in General Relativity, these EoS are excluded by observational constraints).

Neutron stars in Scalar-Tensor-Vector Gravity

General Relativity and Gravitation

Scalar-Tensor-Vector Gravity (STVG), also referred as MOdified Gravity (MOG), is an alternative theory of the gravitational interaction. Its weak field approximation has been successfully used to described Solar System observations, galaxy rotation curves, dynamics of clusters of galaxies, and cosmological data, without the imposition of dark components. The theory was formulated by John Moffat in 2006. In this work, we derive matter-sourced solutions of STVG and construct neutron star models. We aim at exploring STVG predictions about stellar structure in the strong gravity regime. Specifically, we represent spacetime with a static, spherically symmetric manifold, and model the stellar matter content with a perfect fluid energymomentum tensor. We then derive the modified Tolman-Oppenheimer-Volkoff equation in STVG and integrate it for different equations of state. We find that STVG allows heavier neutron stars than General Relativity (GR). Maximum masses depend on a normalized parameter that quantifies the deviation from GR. The theory exhibits unusual predictions for extreme values of this parameter. We conclude that STVG admits suitable spherically symmetric solutions with matter sources, relevant for stellar structure. Since recent determinations of neutron stars masses violate some GR predictions, STVG appears as a viable candidate for a new gravity theory.

Theoretical and observational constraints on the mass-radius relations of neutron stars

arXiv (Cornell University), 2016

We investigate theoretical and observational constraints on the mass-radius relations for neutron stars. For that purpose we consider the model of neutron stars taking into considerations strong, weak, electromagnetic and gravitational interactions in the equation of state and integrate the structure equations within the Hartle-Thorne formalism for rotating configurations. On the basis of the theoretical restrictions imposed by general relativity, mass-shedding and axisymmetric secular instabilities we calculate the upper and lower bounds for the parameters of neutron stars. Our theoretical calculations have been compared and contrasted with the observational constraints and as a result we show that the observational constraints favor stiff equations of state.

Magnetic neutron stars in f(R) gravity

Astrophysics and Space Science, 2014

Neutron stars with strong magnetic fields are considered in the framework of f (R) gravity. In order to describe dense matter in magnetic field, the model with baryon octet interacting through σρω-fields is used. The hyperonization process results in softening the equation of state (EoS) and in decreasing the maximal mass. We investigate the effect of strong magnetic field in models involving quadratic and cubic corrections in the Ricci scalar R to the Hilbert-Einstein action. For large fields, the Mass-Radius relation differs considerably from that of General Relativity only for stars with masses close to the maximal one. Another interesting feature is the possible existence of more compact stable stars with extremely large magnetic fields (∼ 6 × 10 18 G instead of ∼ 4 × 10 18 G as in General Relativity) in the central regions of the stars. Due to cubic terms, a significant increasing of the maximal mass is possible.

Neutron stars: compact objects with relativistic gravity

TURKISH JOURNAL OF PHYSICS, 2016

General properties of neutron stars are briefly reviewed with an emphasis on the indispensability of general relativity in our understanding of these fascinating objects. In Newtonian gravity the pressure within a star merely plays the role of opposing self-gravity. In general relativity all sources of energy and momentum contribute to the gravity. As a result the pressure not only opposes gravity but also enhances it. The latter role of pressure becomes more pronounced with increasing compactness, M/R where M and R are the mass and radius of the star, and sets a critical mass beyond which collapse is inevitable. This critical mass has no Newtonian analogue; it is conceptually different than the Stoner-Landau-Chandrasekhar limit in Newtonian gravity which is attained asymptotically for ultrarelativistic fermions. For white dwarfs the general relativistic critical mass is very close to the Stoner-Landau-Chandrasekhar limit. For neutron stars the maximum mass-so called Oppenheimer-Volkoff limit-is significantly smaller than the Stoner-Landau-Chandrasekhar limit. This follows from the fact that the general relativistic correction to hydrostatic equilibrium within a neutron star is significant throughout the star, including the central part where the mass contained within radial coordinate, m(r), and the Newtonian gravitational acceleration, Gm(r)/r 2 , are small.