Luminosity versus Rotation in a Supermassive Star

Abstract

We determine the effect of rotation on the luminosity of supermassive stars. We apply the Roche model to calculate analytically the emitted radiation from a uniformly rotating, radiation-dominated supermassive configuration. We find that the luminosity at maximum rotation, when mass at the equator orbits at the Kepler period, is reduced by ~36% below the usual Eddington luminosity from the corresponding nonrotating star. A supermassive star is believed to evolve in a quasistationary manner along such a maximally rotating ``mass-shedding'' sequence before reaching the point of dynamical instability; hence this reduced luminosity determines the evolutionary timescale. Our result therefore implies that the lifetime of a supermassive star prior to dynamical collapse is ~56% longer than the value typically estimated by employing the usual Eddington luminosity.