Gravastars must have anisotropic pressures

One of the very small number of serious alternatives to the usual concept of an astrophysical black hole is the 'gravastar' model developed by Mazur and Mottola, and a related phase-transition model due to Laughlin et at. We consider a generalized class of similar models that exhibit continuous pressure-without the presence of infinitesimally thin shells. By considering the usual TOV equation for static solutions with negative central pressure, we find that gravastars cannot be perfect fluids-anisotropic pressures in the 'crust' of a gravastar-like object are unavoidable. The anisotropic TOV equation can then be used to bound the pressure anisotropy. The transverse stresses that support a gravastar permit a higher compactness than is given by the Buchdahl-Bondi bound for perfect-fluid stars. Finally, we comment on the qualitative features of the equation of state that gravastar material must have if it is to do the desired job of preventing horizon formation.