Oscillation modes of rapidly rotating neutron stars in scalar-tensor theories of gravity

Abstract

We perform the first study of the oscillation frequencies of rapidly rotating neutron stars in alternative theories of gravity, focusing mainly on the fundamental f -modes. We concentrated on a particular class of alternative theories -the (massive) scalar-tensor theories. The generalization to rapid rotation is important because on one hand the rapid rotation can magnify the deviations from general relativity compared to the static case and on the other hand some of the most efficient emitters of gravitational radiation, such as the binary neutron star merger remnants, are supposed to be rotating close to their Kepler (mass-shedding) limits shortly after their formation. We have constructed several sequences of models starting from the nonrotating case and reaching up to the Kepler limit, with different values of the scalar-tensor theory coupling constant and the scalar field mass. The results show that the deviations from pure Einstein's theory can be significant especially in the case of nonzero scalar field mass. An important property of the oscillation modes of rapidly rotating stars is that they can become secularly unstable due to the emission of gravitational radiation, that is so-called Chandrasekhar-Friedman-Schutz instability. Such unstable modes are efficient emitters of gravitational radiation. Our studies show that the inclusion of nonzero scalar field would decrease the threshold value of the normalized angular momentum where this instability stars to operate, but the growth time of the instability seems to be increased compared to pure general relativity.

Stellar structure models in modified theories of gravity: Lessons and challenges

“…For the latter, they show that while the maximum mass may be raised, for a maximum allowed value of α = 10 4 , by a ∼ 10% in the static case as compared to GR, this is enhanced to a ∼ 16% in the rapidly rotating case, while the increase in the moment of inertia goes from ∼ 41% to ∼ 65%. Yazadjiev et al [381] compute the oscillation fundamental modes of scalar-tensor theories of gravity, finding significant deviations for these frequencies from GR results within the range −6.0 < β < −4.5, particularly for scalar fields with non-vanishing masses. As for Einstein-Gauss-Bonnet-dilaton gravity, Kleihaus et al [382] consider this scenario with an analytical fit to FPS EOS and a simple (polytropic) DI-II [383] EOS, both below the 2M ⊙ threshold.…”

New Class of Quasinormal Modes of Neutron Stars in Scalar-Tensor Gravity

“…Indeed, previous works made different choices, either keeping both the metric and the scalar field fixed (as in Ref. [17,21]), or allowing for scalar field perturbations while keeping the Jordan-frame metric fixed (as in Refs. [19,20]).…”

Axial quasinormal modes of neutron stars in R2 gravity

“…Polar modes, in contrast, couple matter, scalar and tensor perturbations, making them much more interesting and at the same time much more complicated to study. Therefore the fundamental quadrupole mode has so far only been obtained in the Cowling approximation [46][47][48], where all gravitational degrees of freedom are frozen. Full calculations have only been performed for the radial modes of spontaneously scalarized NSs [49], showing the presence of scalar QNMs in massless STT, and thus an enriched spectrum with respect to GR.…”