QUANTUM ASPECTS OF ERGOREGION INSTABILITY

It has been known classically that a star with an ergoregion but no ev ent horizon is unstable to the emission of scalar, electromagnetic, an d gravitational waves. This classical ergoregion instability is charac terized by complex frequency modes. We show how to canonically quantiz e a neutral scalar field in the presence of such unstable modes by con sidering a simple model for a rapidly rotating star. One of the intere sting results is that there exists a physically meaningful mode decomp osition including unstable normal mode solutions whose representation turns out to be a non-Fock-like Hilbert space. A ''particle'' detector model placed in the in-vacuum state also shows that stars with ergore gions give rise to a spontaneous energy radiation to spatial infinity until ergoregions disappear.