Orbital and epicyclic frequencies around rapidly rotating compact stars in scalar-tensor theories of gravity

Abstract

We study the orbital and epicyclic frequencies of particles orbiting around rapidly rotating neutron stars and strange stars in a particular scalar-tensor theory of gravity. We find very large deviations of these frequencies, when compared to their corresponding values in general relativity, for the maximum-mass rotating models. In contrast, for models rotating with spin frequency of 700Hz (approximately the largest known rotation rate of neutron stars), the deviations are generally small. Nevertheless, for a very stiff equation of state and a high mass the deviation of one of the epicyclic frequencies from its GR value is appreciable even at a spin frequency of 700Hz. In principle, such a deviation could become important in models of quasi-periodic oscillations in low-mass x-ray binaries and could serve as a test of strong gravity (if other parameters are well constraint). Even though the present paper is concentrated mainly on orbital and epicyclic frequencies, we present here for the first time rapidly rotating, scalarized equilibrium compact stars with realistic hadronic equations of state and strange matter equation of state. We also provide analytical expressions for the exterior spacetime of scalarized neutron stars and their epicyclic frequencies in the nonrotating limit.

Quasiequilibrium sequences of binary neutron stars undergoing dynamical scalarization

“…(See also Refs. [23][24][25] for scalarization of rotating stars.) However, it is important to notice that for the same values of the DEF parameters for which dynamical scalarization can occur, the DEF model may have problems in providing cosmological solutions consistent with our Universe [26][27][28].…”

Scalarization of neutron stars with realistic equations of state

“…We have investigated the effect of scalarization on neutron star models with a wide variety of realistic EOSs, including stars consisting of nucleons (3), of nucleons and hyperons (5), of nucleons and quarks (4), and only of quarks (2), thus extending earlier investigations [10,11,12,14,15,16,18,19,20,21,22] by also considering the classes of hyperon and hybrid stars.…”

“…Doneva and collaborators have extended these investigations to rapidly rotating neutron stars in STT [19,20,21,22], observing that the effect of scalarization is further enhanced. Scalarized neutron stars with a massive scalar field have also been considered [23,24,25], in which case the constraints on the theory are weaker, allowing, in principle, for strongly scalarized configurations with larger deviations from GR [26,27].…”

Geodesic properties in terms of multipole moments in scalar–tensor theories of gravity: Table 1.

“…Values for the mass M , the scalar charge W 0 and the resulting deviations from GR, for non-rotating neutron star models constructed with the APR EOS. The values were taken from Doneva et al (2014) and the models assumed β 0 = −4.5, α 0 = 0 and a value for the scalar field at infinity, φ∞ = 0. precession,…”