Abstract
In recent years there has been a surge of interest in what has come to be known as the `universal relations' between various global properties of neutron stars. These universal relations are equation of state independent relations between quantities such as the moment of inertia $I$, the tidal deformability or Love number $\lambda$, and the quadrupole $Q$ (I-Love-Q relations), or the relativistic multipole moments (3-hair relations). While I-Love-Q relations have been studied extensively in both general relativity and various alternatives, 3-hair relations have been studied only in general relativity. Recent progress on the definition of the multipole moments of a compact object in the case of scalar-tensor theories allows for the study of 3-hair relations in modified theories of gravity. Specifically, the aim of this work is to study them for scalarized stars in scalar-tensor theories with a massless scalar field that admit spontaneous scalarization. We find that the 3-hair relations between the mass and angular momentum moments that hold in general relativity hold for scalarized stars as well. The scalar moments also exhibit a universal behaviour, which is equation of state independent within one specific theory, but differs between different theories. Combining astrophysical observations one can in principle measure the different properties of scalarized neutron star and tell different theories apart.
Highlights
Neutron stars have been studied extensively in ScalarTensor Theories (STT) of gravity
We find that the 3-hair relations between the mass and angular momentum moments that hold in general relativity hold for scalarized stars as well
Universal or equation of state (EOS) independent relations between global properties of neutron stars have proven to be a versatile tool for inferring the properties of neutron stars
Summary
Neutron stars have been studied extensively in ScalarTensor Theories (STT) of gravity. Since the class of STT discussed above cannot be constrained by the weak field experiments, one has to use observations involving strong field effects, such as the gravitational wave emission of neutron stars located in close binary systems leading to shrinking of their orbits. These observations pose strong constraints on the theory [12] and the latest results lead to tight bounds on the free coupling parameters [23,24].
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