Breakdown of I -Love- Q Universality in Rapidly Rotating Relativistic Stars

It was shown recently that normalized relations between the moment of inertia ( I ), the quadrupole moment ( Q ), and the tidal deformability (Love number) exist and for slowly rotating neutron stars they are almost independent of the equation of state (EOS). We extend the computation of the I – Q relation to models rotating up to the mass-shedding limit and show that the universality of the relations is lost . With increasing rotation rate, the normalized I – Q relation departs significantly from its slow-rotation limit, deviating up to 40% for neutron stars and up to 75% for strange stars. The deviation is also EOS dependent and for a broad set of hadronic and strange matter EOSs the spread due to rotation is comparable to the spread due to the EOS, if one considers sequences with fixed rotational frequency. Still, for a restricted sample of modern realistic EOSs one can parameterize the deviations from universality as a function of rotation only. The previously proposed I –Love– Q relations should thus be used with care, because they lose their universality in astrophysical situations involving compact objects rotating faster than a few hundred Hz.