Quantum aspects of ergoregion instability

It has been known classically that a star with an ergoregion but no event horizon is unstable to the emission of scalar, electromagnetic, and gravitational waves. This classical ergoregion instability is characterized by complex frequency modes. We show how to canonically quantize a neutral scalar field in the presence of such unstable modes by considering a simple model for a rapidly rotating star. One of the interesting results is that there exists a physically meaningful mode decomposition 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 ergoregions give rise to a spontaneous energy radiation to spatial infinity until ergoregions disappear.