Rapid uniform rotation of protoneutron stars

Rapid uniform rotation of newborn neutron stars (protoneutron stars) is studied for a range of internal temperatures and entropies per baryon predicted by the existing numerical simulations. Calculations are performed using general relativistic equations of hydrostatic equilibrium of rotating, axially symmetric stars. Stability of rotating configurations with respect to mass shedding and the axially symmetric perturbations is studied. Numerical calculations are performed for a realistic densematter equation of state, under various assumptions concerning neutron star interior (large trapped lepton number, no trapped lepton number, isentropic, isothermal). For configurations with baryon mass well below the maximum one for the non-rotating models, the mass shedding limit depends quite sensitively on the position of the “neutrinosphere” (which has a deformed, spheroidal shape); this dependence weakens with increasing baryon mass. The absolute upper limit on rotation frequency is, to a good approximation, obtained for the maximumbaryonmass of rotating configurations. Empirical formula for the maximum rotation frequency of uniformly rotating protoneutron stars is shown to be quite precise; it actually coincides with that used for cold neutron stars. Evolutionary sequences at fixed baryon mass and angular momentum, which correspond to evolution of protoneutron stars into cold neutron stars are studied, and resulting constraints on the maximum rotation frequency of solitary pulsars are discussed.