Torsional oscillations of neutron stars in scalar-tensor theory of gravity

Abstract

We study torsional oscillations of neutron stars in the scalar-tensor theory of gravity using the relativistic Cowling approximation. We compute unperturbed neutron star models adopting realistic equations of state for the neutron star's core and crust. For scalar-tensor theories that allow for spontaneous scalarization, the crust thickness can be significantly smaller than in general relativity. We derive the perturbation equation describing torsional oscillations in scalar-tensor theory, and we solve the corresponding eigenvalue problem to find the oscillation frequencies. The fundamental mode (overtone) frequencies become smaller (larger) than in general relativity for scalarized stellar models. Torsional oscillation frequencies may yield information on the crust microphysics if microphysics effects are not degenerate with strong-gravity effects, such as those due to scalarization. To address this issue, we consider two different models for the equation of state of the crust and we look at the effects of electron screening. The effect of scalarization on torsional oscillation frequencies turns out to be smaller than uncertainties in the microphysics for all spontaneous scalarization models allowed by binary pulsar observations. Our study shows that the observation of quasiperiodic oscillations following giant flares can be used to constrain the microphysics of neutron star crusts, whether spontaneous scalarization occurs or not.

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

“…[17,21]), or allowing for scalar field perturbations while keeping the Jordan-frame metric fixed (as in Refs. [19,20]). Here we compare our results with those obtained with the latter procedure, which was employed in Ref.…”

“…Consequently, their vibration modes are also modified. Stellar pulsations in STTs have been studied in a variety of contexts, often using strong approximation schemes [17][18][19][20][21]. Here we examine the problem of radial oscillations of neutron stars (NSs) in STTs in its full generality.…”

Invariant quantities of scalar–tensor theories for stellar structure

“…The advantage of introducing such quantities is also of different type: they allow to classify mathematically equivalent theories, since a class of conformally-related frames will yield the same values of invariants. Stars in the scalar-tensor theories framework, relativistic [65][66][67][68][69][70][71][72][73][74][75][76][77] and non-relativistic ones [78][79][80][81][82][83][84][85][86][87][88], were widely studied in literature; however, the invariant theory has not been applied yet to stellar objects. In this paper, we aim at using the results of [61] in order to analyze stellar structure in ST theories in a way independent of the choice of conformal metric.…”

Numerical relativity and high energy physics: Recent developments

“…181- 184 The bottom line is that the oscillation frequencies can differ significantly from their GR counterparts if spontaneous scalarization modifies the equilibrium properties of the star (e.g. the mass-radius relation) by appreciable amounts.…”