UPPER LIMITS SET BY CAUSALITY ON THE ROTATION AND MASS OF UNIFORMLY ROTATING RELATIVISTIC STARS

Causality alone suffices to set a lower bound on the period of rotatio n of relativistic stars as a function of their maximum observed mass. That is, by assuming a one-parameter equation of state (EOS) that sati sfies upsilon(sound) < c and that allows stars with masses as large as the largest observed neutron star mass, M-sph(max), we find P(ms) > 0 .282 + 0.196(M-sph(max)/M. - 1.442). The limit does not assume that th e EOS agrees with a known low-density form for ordinary matter, and if one adds that assumption, the minimum period is raised by a few perce nt: for a match at n = 0.1 fm(-3), we find P(ms) > 0.295 + 0.203(M-sph (max)/M. - 1.442). The minimizing EOS yields models with a maximally s oft exterior supported by a maximally stiff core. An analogous upper l imit set by causality on the maximum mass of rotating neutron stars re quires a low-density match, and the limit depends on the matching dens ity, epsilon(m). We recompute it, obtaining a slightly revised value, M-rot(max) similar or equal to 6.1(2 x 10(14) g cm(-3)/<epsilon(m)>)(1 /2) M..