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

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 700 Hz (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 700 Hz. In principle, such a deviation could become important in models of quasiperiodic oscillations in low-mass x-ray binaries and could serve as a test of strong gravity (if other parameters are well constrained). 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.