The observation of maximally rotating neutron stars (in comparison to nonrotating ones) may provide more information on the behavior of nuclear matter at high densities. We provide a theoretical treatment concerning the effects of the upper bound of the sound speed in dense matter on the bulk properties of maximally rotating (at the mass-shedding limit) neutron stars. In particular, we consider two upper bounds for the speed of sound, $v_s=c$ and $v_s=c/\sqrt{3}$, and the one provided by relativistic kinetic theory. We investigate to what extent the possible predicted upper bounds (from various theories and conjectures) on the speed of sound constrain the ones of various key quantities, including the maximum mass and the corresponding radius, Keplerian frequency, Kerr parameter, and moment of inertia. We mainly focus on the lower proposed limit, $v_s=c/\sqrt{3}$, and we explore in which mass region a rotating neutron star collapses to a black hole. In any case, useful relations of the mentioned bulk properties with the transition density are derived and compared with the corresponding nonrotating cases. We conclude that the proposed limit $v_s=c/\sqrt{3}$ leads to a dramatic decrease on the values of the maximum mass, Kerr parameter, and moment of inertia, preventing a neutron star to reach values which are derived with the consideration of realistic equations of state or from other constraints. Possible measurements of the Kerr parameter and moment of inertia shed light on these issues and help to reveal the speed of sound bound in dense matter.

• Received 6 November 2019
• Accepted 5 February 2020

DOI:https://doi.org/10.1103/PhysRevD.101.043023