Ergostar models: where do they reside?

Recently we have shown that dynamically stable ergostar solutions (equilibrium neutron stars that contain an ergoregion) with a compressible and causal equation of state exist [1]. These stars are hypermassive, differentially rotating and highly compact. In this work we make a systematic study of equilibrium models in order to locate the position of ergostars in the parameter space. We adopt four equations of state that differ in the matching density of a maximally stiff core. By constructing a large number of models both with uniform and differential rotation of different degrees we identify the parameters for which ergostars appear. We find that the most favorable conditions for the appearance of dynamically stable ergostars are a significant finite density close to the surface of the star (i.e. similar to self-bound quark stars) and a small degree of differential rotation.