Ultracompact (R<3 M) objects in general relativity

The authors investigate the possible existence of ultracompact objects, i.e., configurations with radii less than three times the mass in geometrical units. Within the framework of core envelope models for a comparatively low value of the fiducial (p/ rho c 2 ) at the interface, stable ultracompact objects with causal cores are in fact found to be possible. On the other hand available equations of state valid at higher densities when used throughout the star lead invariably to configurations with radii greater than 3 M. It is also shown that within the framework of core-envelope models stability requirements do not bring down the mass limit.