Quarkyonic Matter and Neutron Stars

We consider quarkyonic matter to naturally explain the observed properties of neutron stars. We argue that such matter might exist at densities close to that of nuclear matter, and at the onset, the pressure and the sound velocity in quarkyonic matter increase rapidly. In the limit of large number of quark colors $N_c$, this transition is characterized by a discontinuous change in pressure as a function of baryon number density. We make a simple model of quarkyonic matter and show that generically the sound velocity is a nonmonotonic function of density—it reaches a maximum at relatively low density, decreases, and then increases again to its asymptotic value of $1/\sqrt{3}$.

Figure 1

The schematic shows the momentum distribution of quarks and baryons. The diffuse distribution of quarks in the right-hand upper graph indicates they are confined inside baryons that occupy momentum states with width $\delta k_F=\mathrm{\Delta }$.

Figure 2

The speed of sound in quarkyonic matter (solid curves) and in matter containing only nucleons (dashed curves) is shown. The blue curves are obtained for isospin symmetric nuclear matter containing equal numbers of neutron and protons, and the black curves are for matter containing only neutrons. The speed of sound at the saturation density $n_0=0.16{\mathrm{fm}}^{-3}$, indicated by the arrow, is small and grows rapidly with increasing density.

Figure 3

EOS of quarkyonic matter and neutron matter. The model is discussed in the text.

Figure 4

Mass-radius curve of a quarkyonic star (solid curve) is compared to that of an ordinary neutron star. The EOS in the core is described in the text and the EOS of the outer and inner crust are taken from Refs. [40, 41], respectively. The largest and smallest observed neutron star masses, and the limits on the radii of the neutron stars inferred from the observation of gravitational waves from GW170817, are also shown.