Sound velocity and tidal deformability in compact stars

The sound velocity $v_s$ and dimensionless tidal deformability $\mathrm{\Lambda }$ 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 $v_s=1\mathrm{/}\sqrt{3}$, the so-called conformal sound velocity, set in at a { density relevant to compact stars} $\text{gsim}2n_0$ 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 $\simeq 2.3M_{\odot }$. 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 $2.0\lesssim n_{1\mathrm{/}2}\mathrm{/}{n}_0\lesssim 4.0$ commensurate with a possible change of degrees of freedom from hadrons.