w-mode instability of ultracompact relativistic stars

Description/Abstract

In this article we show that the mainly spacetime w modes may become unstable in a rotating ultracompact (R<3M) relativistic star. We provide results for the axial modes of a rotating star in the slow rotation approximation, and demonstrate that an instability may become active in stars with an ergosphere. This suggests a close association with the so-called "ergosphere instability" discussed by other authors. The unstable modes are found to grow on a time scale of seconds to minutes. We discuss briefly whether the instability is likely to have any astrophysical relevance.