Universal I-Q relations for rapidly rotating neutron and strange stars in scalar-tensor theories

We study how rapid rotation influences the relation between the normalized moment of inertia $\overline{I}$ and quadrupole moment $\overline{Q}$ for scalarized neutron stars. The questions one has to answer are whether the equation of state universality is preserved in this regime and what are the deviations from general relativity. Our results show that the $\overline{I}\ensuremath{-}\overline{Q}$ relation is nearly equation of state independent for scalarized rapidly rotating stars, but the differences with pure Einstein's theory increase compared to the slowly rotating case. In general, smaller negative values of the scalar field coupling parameters $\ensuremath{\beta}$ lead to larger deviations, but these deviations are below the expected accuracy of the future astrophysical observations if one considers values of $\ensuremath{\beta}$ in agreement with the current observational constraints. An important remark is that although the normalized $\overline{I}\ensuremath{-}\overline{Q}$ relation is quite similar for scalar-tensor theories and general relativity, the unnormalized moment of inertia and quadrupole moment can be very different in the two theories. This demonstrates that although the $\overline{I}\ensuremath{-}\overline{Q}$ relations are potentially very useful for some purposes, they might not serve us well when trying to distinguish between different theories of gravity.

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