On the maximum mass of a uniformly rotating neutron star

Abstract

The effect of rapid uniform rotation on the upper mass limit for neutron stars is studied under the assumption that the equation of state is subject to only a minimal set of physical constraints. If only the energy condition e > 0, the microstability constraint dp/de > 0, and the causality constraint dp/de < 1 are imposed on the density e and the pressure p above a fiducial value e 0 of e, the maximum mass of a uniformly rotating neutron star is approximately equal to 8.4(e o /10 14 g cm-3)-1/2 M 0. This amounts to an increase of ~24%-25% over the corresponding nonrotating maximum mass, and it is to be compared with the value ~3.2 M 0 of the maximum mass of uniformly rotating configurations associated with the stiffest of the equations of state proposed for actual neutron-star matter. Rotation increases the upper limit on baryon mass by only ~20%, while the limiting moment of inertia is about twice its value for the nonrotating case. Also found are upper limits on the frequency of rotation, the frequency of frame dragging, and the maximum polar and equatorial redshifts. It is estimated that discarding the causality constraint would allow the upper mass limit to increase to ~14(e o /10 14 g cm-3)-1/2 M 0. Rotating configurations near the upper mass limits found here probably exhibit a rotationally induced, bar-mode instability driven by gravitational radiation.