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Neutron stars are expected to have a tight relation between their moment of inertia ($I$), tidal deformability ($lambda$, which is related to the Love number), and rotational mass quadrupole moment ($Q$) that is nearly independent of the unknown equation of state (EoS) of cold dense matter. These and similar relations are often called universal, and they have been used for various applications including analysis of gravitational wave data. We extend these studies using piecewise polytropic representations of dense matter, including for so-called twin stars that have a second branch of stability at high central densities. The second-branch relations are less tight, by a factor of $sim 3$, than the relations found in the first stable branch. We find that the relations on both branches become tighter when we increase the lower limit to the maximum mass for the EoS under consideration. We also propose new empirical relations between $I$, $lambda$, $Q$, and the complex frequency $omega=omega_R+iomega_I$ of the fundamental axial $w$-mode, and find that they are comparably tight to the I-Love-Q correlations.
Gravitational wave observations of GW170817 placed bounds on the tidal deformabilities of compact stars allowing one to probe equations of state for matter at supranuclear densities. Here we design new parametrizations for hybrid hadron-quark equatio
We relate the fundamental quadrupolar fluid mode of isolated non-rotating NSs and the dominant oscillation frequency of neutron star merger remnants. Both frequencies individually are known to correlate with certain stellar parameters like radii or t
Quasi-universal relations connecting the tidal deformability and the quadrupole moment of individual neutron stars are predicted by theoretical computations, but have not been measured experimentally. However, such relations are employed during the i
Axions and axion-like particles are a leading model for the dark matter in the Universe; therefore, dark matter halos may be boson stars in the process of collapsing. We examine a class of static boson stars with a non-minimal coupling to gravity. We
We investigate the nature of low T/W dynamical instabilities in various ranges of the stiffness of the equation of state in differentially rotating stars. Here T is the rotational kinetic energy, while W the gravitational binding energy. We analyze t