Neutron stars in the large-Nc limit

We study the phase transition from dense baryonic matter to dense quark matter within the large-N-c limit. By using simple constant speed of sound equations of state for the two phases, we derive the scaling with N-c of the critical quark chemical potential mu(crit)(c) for this phase transition. While quark matter is strongly suppressed at large N-c, the phase transition at a large but finite density could nevertheless be important to determine the maximum mass of compact stars. In particular, in the range 3 <= N-c less than or similar to 5.5 the quark phase would take place in compact stars and would lead to the formation of an unstable branch of hybrid stars. As a consequence, the maximum mass is restricted to the range 2.1M(circle dot) < M-max < 3M(circle dot). For larger value of N-c, the phase transition would occur at densities too high to be reached in the core of compact stars. However, the very requirement that it occurs (although at a very large density) translates into interesting constraints on the stiffness of the baryonic phase: its speed of sound must exceed root 1/3. (C) 2017 Elsevier B.V. All rights reserved.