Sound velocity and tidal deformability in compact stars

Abstract: The sound velocity $v_s$ and dimensionless tidal deformability $\Lambda$ are analyzed using the pseudo-conformal model we developed before. In contrast to the conclusion obtained in the previous works in the literature, our model with the upper bound of the sound velocity $v_s = 1/\sqrt{3}$, the so-called conformal sound velocity, set in at a { density relevant to compact stars} $\gsim 2 n_0$ where $n_0$ is the normal nuclear matter density, can accommodate {\it all} presently established nuclear matter and compact-star properties including the maximum star-mass constraint $ \simeq 2.3 M_\odot$. This observation is associated with a possible emergence of pseudoconformal structure in compact star matter---in which the trace of energy-momentum tensor is a nearly density-independent nonzero constant---brought in by a topology change at $2.0 \lesssim n_{1/2}/n_0 \lesssim 4.0$ commensurate with a possible change of degrees of freedom from hadrons.