Visser, "Acoustic black holes: horizons, ergospheres
and Hawking radiation," Classical and Quantum Gravity, vol.
In Section 2, we present horizons, ergospheres, and ergoregions.
There are four cases for the existence of ergospheres and ergoregions shown in Figure 2.
In the black hole case, ergospheres and ergoregions exist for any density profile (the curve (3a) for the case of two horizons and the curve (3b) for the double-horizon case).
For black holes, ergospheres and ergoregions exist for any density profile.
Caption: Figure 2: Four cases for the existence of ergospheres and ergoregions.
Ergosphere is a surface of a static limit [g.sub.tt] = 0 given by
Each point of the ergosphere belongs to some of confocal ellipsoids (7) covering the whole space as the coordinate surfaces r = const.